Evaporation stains: suppressing the coffee-ring effect by contact angle hysteresis

Deposition patterns formed by the evaporation of linear diblock copolymer solution nanodroplets on solid surfaces

The evaporation-induced deposition pattern of the linear diblock copolymer solution has attracted attention in recent years. Given its critical applications, we study deposition patterns of the linear diblock copolymer solution nanodroplet on a solid surface (the wall) by molecular dynamics simulations. This study focuses on the influence of the nonbonded interaction strength, including the interaction between the wall and polymer blocks (ɛAW and ɛBW), the interaction between the solvent and the wall (ɛSW), and the interaction between polymer blocks (ɛAB). Conditions leading to diverse deposition patterns are explored, including the coffee-ring and the volcano-like structures. The formation of the coffee-ring structure is attributed to receding interfaces, the heterogeneity inside the droplet, and the self-assembly of polymer chains. This study contributes to the establishment of guidelines for designing deposition patterns of the linear diblock copolymer solution nanodroplet, which facilitates practical applications such as inkjet printing.

Influence of Sol-Gel State in Smectite Aqueous Dispersions on Drying Patterns of Droplets

The sol-gel state of smectite clay dispersions varies with the volume fraction of clay and electrolyte concentration. In this study, it was elucidated that the drying patterns of droplets from four types of smectite clay dispersions vary according to their sol-gel states. Droplets in the sol state exhibited a ring-shaped pattern, while those in the gel state showed a bump-shaped pattern. Near the boundary between the sol and gel states, patterns featuring both ring and bump structures were observed regardless of whether the droplets were on the sol or gel side. When guest particles or molecules were introduced into the clay dispersion, they dispersed uniformly within the system, and the drying pattern depended on the sol-gel state of the droplets. These findings suggest that the presence or absence of convection within the droplets during drying governs the drying pattern.

Highly sensitive plasmonic paper substrate fabricated via amphiphilic polymer self-assembly in microdroplet for detection of emerging pharmaceutical pollutants

We report an innovative and facile approach to fabricating an ultrasensitive plasmonic paper substrate for surface-enhanced Raman spectroscopy (SERS). The approach exploits the self-assembling capability of poly(styrene-b-2-vinyl pyridine) block copolymers to form a thin film at the air-liquid interface within the single microdroplet scale for the first time and the subsequent in situ growth of silver nanoparticles (AgNPs). The concentration of the block copolymer was found to play an essential role in stabilizing the droplets during the mass transfer phase and formation of silver nanoparticles, thus influencing the SERS signals. SEM analysis of the morphology of the plasmonic paper substrates revealed the formation of spherical AgNPs evenly distributed across the surface of the formed copolymer film with a size distribution of 47.5 nm. The resultant enhancement factor was calculated to be 1.2 × 107, and the detection limit of rhodamine 6G was as low as 48.9 pM. The nanohybridized plasmonic paper was successfully applied to detect two emerging pollutants-sildenafil and flibanserin-with LODs as low as 1.48 nM and 3.45 nM, respectively. Thus, this study offers new prospects for designing an affordable and readily available, yet highly sensitive, paper-based SERS substrate with the potential for development as a lab-on-a-chip device.

Structural Colors from Amyloid-Based Liquid Crystals

The helical periodicity and layered structure in cholesteric liquid crystals (CLCs) may be tuned to generate structural color according to the Bragg's law of diffraction. A wide range of natural-based materials such as condensed DNA, collagen, chitin, cellulose, and chiral biopolymers exhibit cholesteric phases with left-handed helixes and ensued structural colors. Here, the possibility of using amyloid CLCs is reported to prepare films with iridescent color reflection and opposite handedness. Right-handed CLCs assembled by left-handed amyloid fibrils are dried into layered structures with variable pitch controlled by the addition of glucose. Circularly polarized light with the same handedness of amyloid CLCs helix is reflected in the Bragg regime. Varying the drying speed leads to the switching between films with a rainbow-like color gradient and large area uniform color. It is confirmed that the origin of the colors derives from the layered structures of the amyloid CLCs, given the negligeable birefringence of the films, calculated from optical rotatory dispersion. These findings provide a facile approach to constructing biosourced cholesteric materials and introduce an original class of proteinaceous materials for the generation of structural colors from right-handed circularly polarized light.

Effect of Dispersing Solvents for an Ionomer on the Performance of Copper Catalyst Layers for CO2 Electrolysis to Multicarbon Products

To explore the effects of solvent-ionomer interactions in catalyst inks on the structure and performance of Cu catalyst layers (CLs) for CO2 electrolysis, we used a "like for like" rationale to select acetone and methanol as dispersion solvents with a distinct affinity for the ionomer backbone or sulfonated ionic heads, respectively, of the perfluorinated sulfonic acid (PFSA) ionomer Aquivion. First, we characterized the morphology and wettability of Aquivion films drop-cast from acetone- and methanol-based inks on flat Cu foils and glassy carbons. On a flat surface, the ionomer films cast from the Aquivion and acetone mixture were more continuous and hydrophobic than films cast from methanol-based inks. Our study's second stage compared the performance of Cu nanoparticle CLs prepared with acetone and methanol on gas diffusion electrodes (GDEs) in a flow cell electrolyzer. The effects of the ionomer-solvent interaction led to a more uniform and flooding-tolerant GDE when acetone was the dispersion solvent (acetone-CL) than when we used methanol (methanol-CL). As a result, acetone-CL yielded a higher selectivity for CO2 electrolysis to C2+ products at high current density, up to 25% greater than methanol-CL at 500 mA cm-2. Ethylene was the primary product for both CLs, with a Faradaic efficiency for ethylene of 47.4 ± 4.0% on the acetone-CL and that of 37.6 ± 5.5% on the methanol-CL at a current density of 300 mA cm-2. We attribute the enhanced C2+ selectivity of the acetone-CL to this electrode's better resistance to electrolyte flooding, with zero seepage observed at tested current densities. Our findings reveal the critical role of solvent-ionomer interaction in determining the film structure and hydrophobicity, providing new insights into the CL design for enhanced multicarbon production in high current densities in CO2 electrolysis processes.

In Situ Interfacial-Assembly Perovskite Quantum Dot via Marangoni and Capillary Convection Manipulation for Robust Luminescence

In solid substrates, colloidal solutions produce irregular deposits on the surface by Marangoni flow and capillary flow during evaporation. Reportedly, perovskite quantum dots (PQDs) as a colloidal solution have irregular surfaces based on a similar principle as the coffee ring effect in QD systems when droplets evaporate from the substrate. Given that this issue is due to the direction of Marangoni and capillary flows, the substrate is tilted to change the direction of the flows. The appropriate angle is determined by controlling the angle of the substrate so that the two flows circulate similarly; this method is called "assembly-coating". Herein, we compare the PL intensity before and after the thermal evaporation of the thin films prepared by conventional and assembly-coating. Moreover, by characterizing the diode device (hole-only space charge limited current) for each coating process, the charge carrier characteristics are investigated in detail. Therefore, we suggest a facile strategy to obtain a uniform surface and thermal evaporative stability using colloidal solutions. This strategy is effective in designing surface uniformity and light-emitting layers for colloidal solution deposition and assembly.

State-and-rate friction in contact-line dynamics

In order to probe the dynamics of contact-line motion, we study the macroscopic properties of sessile drops deposited on and then aspirated from carefully prepared horizontal surfaces. By measuring the contact angle and drop width simultaneously during droplet removal, we determine the changes in the shape of the drop as it depins and recedes. Our data indicate that there is a force which opposes the motion of the contact line that depends both on the amount of time that the drop has been in contact with the surface and on the withdrawal rate. For water on silanized glass, we capture the experimentally observed behavior with an overdamped dynamical model of contact-line motion in which the phenomenological drag coefficient and the assumed equilibrium contact angle are the only inputs. In this case, the damping coefficient decreases with increasing velocity of the contact line. For other liquid-substrate pairs, the observed contact-line motion suggests that a maximum static friction force is important in addition to damping. The dependence on time of contact and withdrawal rate, reminiscent of rate-and-state friction between solid surfaces, is qualitatively consistent across three substrate-liquid pairs.

Rational design of wettability-patterned microchips for high-performance attomolar surface-enhanced Raman detection

Recent progress in wettability-patterned microchips has facilitated the development of ultra-trace detection in multiple biomedical and food safety fields. The existence of a superhydrophilic trap can realize targeted deposition of the analyte. However, the wetting transition from the Cassie-Baxter state to the Wenzel state usually occurs during evaporation and leads to a larger deposition footprint, which has a strong impact on the detection sensitivity and uniformity. In this paper, we report an integrated design, fabrication, and evaporation strategy to avoid the transition for high-performance attomolar surface-enhanced Raman scattering (SERS) detection. An improved force balance model was proposed to design the microstructures of wettability-patterned microchips, which were fabricated by nanosecond laser direct writing and surface fluorination. The microchips were composed of superhydrophobic micro-grooves and superhydrophilic traps, by which the targeted deposition of Au nanoparticles and rhodamine 6G (R6G) onto a minimal area of ∼70 × 70 μm² was realized after a two-step heated evaporation. Accordingly, the detection limit was down to the attomolar level (5 × 10^-18 M) with SERS enhancement factors (EFs) exceeding 10¹⁰. More importantly, the Raman signals showed good uniformity (RSD of 11.5%) for the concentration of 2 × 10^-17 M. A good linear relationship was obtained in the quantitative concentration range of 10^-12 M to 5 × 10^-18 M with a high correlation coefficient (R2) of 0.996. These wettability-patterned microchips exhibit high performance (that is, both good sensitivity and good uniformity) in the detection of ultra-trace molecules in aqueous solutions, avoiding the need for expensive equipment and considerable skill in operations. The proposed strategy could also be applied to other microfluidic devices for rapid and simple analyte pre-concentration.

Triple-line dynamics of a soft colloid-laden drop on a hydrophobic surface

Evaporation of fluid from a pinned drop placed on solid surface proceeds via constant contact radius (CCR) mode, with a continuous reduction in the contact angle. The reduction of contact angle leads to an imbalance of interfacial tensions at the three-phase contact line. When the unbalanced force is sufficiently strong, the drop slips from the pinned contact line and slides inward. Depinning of the drop alters the mode of evaporation to constant contact angle (CCA) mode till it repins onto the surface. The change in evaporation mode from CCR to CCA is usually achieved by tuning the pinning energy barrier by controlling the surface properties of the substrate. Here, we demonstrate that the evaporation mode can be controlled by solely tailoring the surface tension of the drop, which is achieved in microgel particle-laden sessile drops that show spontaneous adsorption of microgels to the air/water interface, leading to a decrease in the interfacial tension. We show that droplets containing a sufficient number of microgels evaporate predominantly in CCR mode even on a hydrophobic surface, and the contact line remains pinned throughout the evaporation of the drop. Interestingly, the contact line dynamics can be controlled by tuning the softness of the microgels and the particle concentration in the drops.

Ion Addition by Electrolysis to Improve the Quantitative Analysis of Bacteria with MALDI-TOF MS

Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) is commonly applied to the identification of bacteria but rarely used for quantitative detection due to the inhomogeneous crystallization of the matrix leading to the unsatisfactory linear relationship between the sample amount and the mass spectrum signals. Herein, we proposed a noninterference ion addition (NIA) method by electrolysis to improve homogeneous crystallization during the evaporation progress of sample droplets on the target plates. The active metal wire was inserted in the droplet as the anode electrode, and metal ions were released through electrolysis. The directional migration of metal ions under the electric field can hinder the migration of matrix molecules to the boundary and homogenize the matrix crystals by forming spherical crystals. Simultaneously, trace cationic surfactant was added to the droplet for pinning the contact surface to define the circle crystallization region. The metal ions from the anode electrode wire were deposited on the surface of the target plates which served as the cathode. Therefore, ion addition has no interference effect on ionization during MALDI-MS detection. This NIA method benefits the homogeneous crystallization and so improves the quantitative analysis. NIA is suitable for biological samples with different matrices, and bacterial samples could be quantitatively analyzed.

Colorimetric Chemosensor Based on Fe3O4 Magnetic Molecularly Imprinted Nanoparticles for Highly Selective and Sensitive Detection of Norfloxacin in Milk

Long-term use of norfloxacin (NOR) will cause NOR residues in foods and harm human bodies. The determination of NOR residues is important for guaranteeing food safety. In this study, a simple, selective, and label-free colorimetric chemosensor for in situ NOR detection was developed based on Fe₃O₄ magnetic molecularly imprinted nanoparticles (Fe₃O₄ MMIP NPs). The Fe₃O₄ MMIP NPs showed good peroxidase-like catalytic activity to 3,3',5,5'-tetramethylbenzidine (TMB) and selective adsorption ability to NOR. The colorimetric chemosensor was constructed based on the Fe₃O₄ MMIP NPs-H₂O₂-TMB reaction system. The absorbance differences were proportional to the concentrations of NOR in the range of 10-300 ng/mL with a limit of detection at 9 ng/mL. The colorimetric chemosensor was successfully applied to detect NOR residue in milk. The recovery range was 78.2-95.81%, with a relative standard deviation of 2.1-9.88%. Together, the proposed colorimetric chemosensor provides a reliable strategy for the detection of NOR residues in foods.

Effect of Nanoparticle Addition on Evaporation of Jet Fuel Liquid Films and Nanoparticle Deposition Patterns during Evaporation

Jet fuel-based nanofluid fuel has been proposed for improving the energy density and utilization efficiency of jet fuel that is widely applied in aircraft powered by aviation turbine engines. To recognize the evaporation behavior of the formed liquid film as a jet fuel-based nanofluid sprayed onto the engine wall or blades, this paper presents the evaporation and deposition characteristics of the jet fuel-based nanofluid liquid film adhering on the hydrophilic substrate. The changes in contact line, contact angle, volume, and deposition pattern during liquid film evaporation under different substrate temperatures, different nanoparticle concentrations, and different kinds of nanoparticle additions were investigated. The effect of nano-Al addition on the evaporation kinetics and deposition pattern of the nano-Al/jet fuel (nAl/JF) nanofluid fuel liquid film was explored. Repeated pinning and de-pinning of contact lines during evaporation occurred, resulting in the formation of concentric multi-ring deposition patterns. The addition of nano-Al increased the evaporation rate and shortened the evaporation lifetime, demonstrating a promotion effect on jet fuel liquid film evaporation. The existence of an energy barrier shows that the movement of three-phase contact lines on the hydrophilic solid surface presented not a continuous sliding behavior but a "stick-slip" behavior, and there were multiple jumps in contact lines and contact angles. Finally, a comparison was made with the deposition pattern of jet fuel liquid films with different graphite and Fe nanoparticle additions during evaporation. The mechanism of deposition phenomena was deeply revealed by the analysis of capillary flow and Marangoni recirculation.

A supraparticle-based biomimetic cascade catalyst for continuous flow reaction

Robust millimeter-sized spherical particles with controlled compositions and microstructures hold promises of important practical applications especially in relation to continuous flow cascade catalysis. However, the efficient fabrication methods for producing such particles remain scare. Here, we demonstrate a liquid marble approach to fabricate robust mm-sized porous supraparticles (SPs) through the bottom-up assembly of silica nanoparticles in the presence of strength additive or surface interactions, without the need for the specific liquid-repellent surfaces used by the existing methods. As the proof of the concept, our method was exemplified by fabricating biomimetic cascade catalysts through assembly of two types of well-defined catalytically active nanoparticles. The obtained SP-based cascade catalysts work well in industrially preferred fixed-bed reactors, exhibiting excellent catalysis efficiency, controlled reaction kinetics, high enantioselectivity (99% ee) and outstanding stability (200~500 h) in the cascades of ketone hydrogenation-kinetic resolution and amine racemization-kinetic resolution. The excellent catalytic performances are attributed to the structural features, reconciling close proximity of different catalytic sites and their sufficient spatial isolation.

Robust millimeter-sized spherical particles with controlled compositions and microstructures hold promises of important practical applications. Here the authors develop a liquid marble method to facilely fabricate robust millimeter-sized supraparticles with controlled microstructures through the bottom-up assembly of silica nanoparticles.

M. T, Cho G, Lee J. Control of the Drying Patterns for Complex Colloidal Solutions and Their Applications

The uneven deposition at the edges of an evaporating droplet, termed the coffee-ring effect, has been extensively studied during the past few decades to better understand the underlying cause, namely the flow dynamics, and the subsequent patterns formed after drying. The non-uniform evaporation rate across the colloidal droplet hampers the formation of a uniform and homogeneous film in printed electronics, rechargeable batteries, etc., and often causes device failures. This review aims to highlight the diverse range of techniques used to alleviate the coffee-ring effect, from classic methods such as adding chemical additives, applying external sources, and manipulating geometrical configurations to recently developed advancements, specifically using bubbles, humidity, confined systems, etc., which do not involve modification of surface, particle or liquid properties. Each of these methodologies mitigates the edge deposition via multi-body interactions, for example, particle-liquid, particle-particle, particle-solid interfaces and particle-flow interactions. The mechanisms behind each of these approaches help to find methods to inhibit the non-uniform film formation, and the corresponding applications have been discussed together with a critical comparison in detail. This review could pave the way for developing inks and processes to apply in functional coatings and printed electronic devices with improved efficiency and device yield.

Multiple Scattering and Random Quasi-Phase-Matching in Disordered Assemblies of LiNbO3 Nanocubes

Nonlinear disordered photonic media (NDPM), composed of a random configuration of noncentrosymmetric crystals, offer a versatile platform to tailor nonlinear optical effects. The second-harmonic generation (SHG) and its random quasi-phase-matching (RQPM) in the multiple scattering regime are still poorly explored. In this work, we bottom-up assemble NDPM in two different geometries by using LiNbO₃ nanocubes as building blocks and investigate both the multiple scattering and the nonlinear properties. We produce disordered slabs with a continuously variable thickness and microspheres with different diameters, which display a remarkable strong light scattering, evidenced by a subwavelength transport mean free path (). We first provide explicit evidence that the SHG power scales linearly with both the thickness of the slab and the volume of the microspheres. These observations generalize the characteristic linear scaling of RQPM power with the volume to the multiple scattering regime and to different sample geometries. Our structures represent a promising platform to investigate the interplay between disorder and optical nonlinear effects.

Self-assembly of highly ordered micro- and nanoparticle deposits

The evaporation of particle-laden sessile droplets is associated with capillary-driven outward flow and leaves nonuniform coffee-ring-like particle patterns due to far-from-equilibrium effects. Traditionally, the surface energies of the drop and solid phases are tuned, or external forces are applied to suppress the coffee-ring; however, achieving a uniform and repeatable particle deposition is extremely challenging. Here, we report a simple, scalable, and noninvasive technique that yields uniform and exceptionally ordered particle deposits on a microscale surface area by placing the droplet on a near neutral-wet shadow mold attached to a hydrophilic substrate. The simplicity of the method, no external forces, and no tuning materials' physiochemical properties make the present generic approach an excellent candidate for a wide range of sensitive applications. We demonstrate the utility of this method for fabricating ordered mono- and multilayer patternable coatings, producing nanofilters with controlled pore size, and creating reproducible functionalized nanosensors.

Deep-Learning-Assisted Stratification of Amyloid Beta Mutants Using Drying Droplet Patterns

The development of simple and accurate methods to predict mutations in proteins remains an unsolved challenge in modern biochemistry. It is discovered that critical information about primary and secondary peptide structures can be inferred from the stains left behind by their drying droplets. To analyze the complex stain patterns, deep-learning neuronal networks are challenged with polarized light microscopy images derived from the drying droplet deposits of a range of amyloid beta (1-42) (Aβ₄₂ ) peptides. These peptides differ in a single amino acid residue and represent hereditary mutants of Alzheimer's disease. Stain patterns are not only reproducible but also result in comprehensive stratification of eight amyloid beta (Aβ) variants with predictive accuracies above 99%. Similarly, peptide stains of a range of distinct Aβ₄₂ peptide conformations are identified with accuracies above 99%. The results suggest that a method as simple as drying a droplet of a peptide solution onto a solid surface may serve as an indicator of minute, yet structurally meaningful differences in peptides' primary and secondary structures. Scalable and accurate detection schemes for stratification of conformational and structural protein alterations are critically needed to unravel pathological signatures in many human diseases such as Alzheimer's and Parkinson's disease.

Fabrication and characterization of chitosan nanoparticles using the coffee-ring effect for photodynamic therapy

BACKGROUND AND OBJECTIVES

Biocompatible nanoparticles have been increasingly used in a variety of medical applications, including photodynamic therapy. Although the impact of synthesis parameters and purification methods is reported in previous studies, it is still challenging to produce a reliable protocol for the fabrication, purification, and characterization of nanoparticles in the 200-300 nm range that are highly monodisperse for biomedical applications.

STUDY DESIGN/MATERIALS AND METHODS

We investigated the synthesis of chitosan nanoparticles in the 200-300 nm range by evaluating the chitosan to sodium tripolyphosphate (TPP) mass ratio and acetic acid concentration of the chitosan solution. Chitosan nanoparticles were also crosslinked to rose bengal and incubated with human breast cancer cells (MCF-7) to test photodynamic activity using a green laser (λ = 532 nm, power = 90 mW).

RESULTS

We established a simple protocol to fabricate and purify biocompatible nanoparticles with the most frequent size occurring between 200 and 300 nm. This was achieved using a chitosan to TPP mass ratio of 5:1 in 1% v/v acetic acid at a pH of 5.5. The protocol involved the formation of nanoparticle coffee rings that showed the particle shape to be spherical in the first approximation. Photodynamic treatment with rose bengal-nanoparticles killed ~98% of cancer cells.

CONCLUSION

A simple protocol was established to prepare and purify spherical and biocompatible chitosan nanoparticles with a peak size of ~200 nm. These have remarkable antitumor activity when coupled with photodynamic treatment.

From coffee stains to uniform deposits: Significance of the contact-line mobility

HYPOTHESIS

Contact-line motion upon drying of a sessile droplet strongly affects the solute transport and solvent evaporation profile. Hence, it should have a strong impact on the deposit formation and might be responsible for volcano-like, dome-like and flat deposit morphologies.

EXPERIMENTS

A method based on a thin-film interference was used to track the drop height profile and contact line motion during the drying. A diverse set of drying scenarios was obtained by using inks with different solvent compositions and by adjusting the substrate wetting properties. The experimental data was compared to the predictions of a phenomenological model.

FINDINGS

We highlight the essential role of contact-line mobility on the deposit morphology of solution-based inks. A pinned contact line produces exclusively ring-like deposits under normal conditions. On the contrary, drops with a mobile contact line can produce ring-, flat- or dome-like morphology. The developed phenomenological model shows that the deposit morphology depends on solvent evaporation profile, evolution of the drop radius relative to its contact angle, and the ratio between initial and maximal (gelling) solute concentration. These parameters can be adjusted by the ink solvent composition and substrate wetting behaviour, which provides a way for deposition of uniform and flat deposits via inkjet printing.

Magnetophoretic Control of Diamagnetic Particles Inside an Evaporating Droplet

The present study reports the magnetophoresis of diamagnetic particles in an evaporating ferrofluid droplet. Both solid and ring magnet arrangements are used to investigate the effect of magnetic field distribution. The distance of the magnet from the droplet is varied to study the effect of magnetic field strength. The magnetic field distribution is computed using COMSOL multiphysics software. Magnetometer measurements have been carried out to validate the simulation results. The motion of particles and the drying pattern of evaporating ferrofluid droplets are visualized using the confocal microscopy technique. Both bright-field and fluorescence imagings have been carried out to observe the differential deposition of the fluorescent particle (microparticle) and magnetic nanoparticles in the absence and presence of a magnetic field. The velocity of diamagnetic particles as a function of magnetic field distribution and strength has been studied using the micro-PIV technique. In the absence of the magnetic field, a ring-shaped deposition pattern is observed. The mixture of microparticles (diamagnetic) and nanoparticles (magnetic) is deposited between the outer and inner edges of the ring. The diamagnetic particles occupy the inner and outer edges of the ring. Magnetic particles travel toward the higher magnetic field zone and diamagnetic particles move toward the smaller magnetic field zone when a magnetic field is applied by a solid magnet placed over the droplet. This can be attributed to the negative magnetic force originating from the difference between the susceptibility of magnetic and nonmagnetic particles. The negative magnetic force on the microparticle increases as the magnetic field intensity increases, causing the microparticle to convect faster toward the contact line. The deposition behavior can be reversed or suppressed using a ring magnet in place of a solid magnet. In this case, the negative magnetic force is stronger at the contact line region of the droplet and decreases as it approaches the center region of the droplet. The deposition behavior of diamagnetic particle depends on the balance between the Marangoni force and the magnetophoretic force. Overall, the present study demonstrates the capability of the controlled deposition of diamagnetic polystyrene particles by suitable arrangement of the solid and ring magnet.

Inkjet-Printed Electron Transport Layers for Perovskite Solar Cells

Inkjet printing emerged as an alternative deposition method to spin coating in the field of perovskite solar cells (PSCs) with the potential of scalable, low-cost, and no-waste manufacturing. In this study, the materials TiO₂, SrTiO₃, and SnO₂ were inkjet-printed as electron transport layers (ETLs), and the PSC performance based on these ETLs was optimized by adjusting the ink preparation methods and printing processes. For the mesoporous ETLs inkjet-printed from TiO₂ and SrTiO₃ nanoparticle inks, the selection of solvents for dispersing nanoparticles was found to be important and a cosolvent system is beneficial for the film formation. Meanwhile, to overcome the low current density and severe hysteresis in SrTiO₃-based devices, mixed mesoporous SrTiO₃/TiO₂ ETLs were also investigated. In addition, inkjet-printed SnO₂ thin films were fabricated by using a cosolvent system and the effect of the SnO₂ ink concentrations on the device performance was investigated. In comparison with PSCs based on TiO₂ and SrTiO₃ ETLs, the SnO₂-based devices offer an optimal power conversion efficiency (PCE) of 17.37% in combination with a low hysteresis. This work expands the range of suitable ETL materials for inkjet-printed PSCs and promotes the commercial applications of inkjet printing techniques in PSC manufacturing.

Interfacial assembly of nanorods: smectic alignment and multilayer stacking

Large-scale spatial arrangement and orientation ordering of nanorod assembly on substrates are critical for nanodevice fabrication. However, complicated processes and templates or surface modification of nanorods are often required. In this work, we demonstrate, by dissipative particle dynamics simulations, that various ordered structures of adsorbed nanorods on smooth substrates can be simply achieved by non-affinity adsorption. The structures of interfacial assembly, including monolayers with a nematic-like arrangement and multilayer stacking with a smectic-like arrangement, depend on the nanorod concentration and the solvent size. As the nanorod concentration increases, the adsorbed layer becomes densely packed and the arrangement of nanorods changes from nematic-like to smectic. The assembly process driven by entropy is a two-dimensional layer-by-layer growth. Multilayer stacking with a smectic-like arrangement takes place at dilute concentrations of nanorods for large solvents such as pentamers, but at concentrated concentrations, it takes place for small solvents such as monomers. Moreover, nanorod bundles appear in the bulk phase for large solvents at dilute concentrations. The proposed strategy for interfacial assembly is caused by the free volume released for solvents, which is independent of the chemical compositions of substrates and nanorods.

Steering droplets on substrates using moving steps in wettability

Droplets move on substrates with a spatio-temporal wettability pattern as generated, for example, on light-switchable surfaces. To study such cases, we implement the boundary-element method to solve the governing Stokes equations for the fluid flow field inside and on the surface of a droplet and supplement it by the Cox-Voinov law for the dynamics of the contact line. Our approach reproduces the relaxation of an axisymmetric droplet in experiments, which we initiate by instantaneously switching the uniform wettability of a substrate quantified by the equilibrium contact angle. In a step profile of wettability the droplet moves towards higher wettability. Using a feedback loop to keep the distance or offset between step and droplet center constant, induces a constant velocity with which the droplet surfs on the wettability step. We analyze the velocity in terms of droplet offset and step width for typical wetting parameters. Moving instead the wettability step with constant speed, we determine the maximally possible droplet velocities under various conditions. The observed droplet speeds agree with the values from the feedback study for the same positive droplet offset.

Evaporation-driven colloidal cluster assembly using droplets on superhydrophobic fractal-like structures

Microparticles can be considered building units for functional systems, but their assembly into larger structures typically involves complex methods. In this work, we show that a large variety of macro-agglomerate clusters ("supra-particles") can be obtained, by systematically varying the initial particle concentration in an evaporating droplet, spanning more than 3 decades. The key is the use of robust superhydrophobic substrates: in this study we make use of a recently discovered kind of patterned surface with fractal-like microstructures which dramatically reduce the contact of the droplet with the solid substrate. Our results show a clear transition from quasi-2D to 3D clusters as a function of the initial particle concentration, and a clear transition from unstable to stable 3D spheroids as a function of the evaporation rate. The origin of such shape transitions can respectively be found in the dynamic wetting of the fractal-like structure, but also in the enhanced mechanical stability of the particle agglomerate as its particle packing fraction increases.

Toward Controlling Evaporative Deposition: Effects of Substrate, Solvent, and Solute

Understanding evaporative deposition from a colloidal suspension and on-demand control over it are important due to its industrial and biomedical applications. In particular, it is known that interactions among substrate, solute, and solvent have important consequences on evaporative depositions; however, how these are affecting the deposition patterns and at which conditions these interactions are prominent need detailed investigations. Here we report that the total time of deposition (t_d) and the geometric shape of the droplet (L_c = initial footprint diameter/height) have a significant role in determining the evaporative deposition patterns. We have identified four zones based on t_d and L_c, and found that with longer deposition time (high t_d) and larger available space for particle motion within a liquid droplet (high L_c), deposition patterns were governed by the interactions among the substrate, solute, and solvent. We also experimentally demonstrated that the pinned contact line is indispensable for the "coffee ring" effect by comparing the deposition on surfaces with and without hysteresis. The effect of the Marangoni flow is also discussed, and it is shown that by controlling Marangoni flow, one can manipulate the droplet deposition from uniform disk-like to coffee ring with a central deposition.

Size-dependent behavior and failure of young's equation for wetting of two-component nanodroplets

HYPOTHESIS

For macroscopic systems, the interfacial properties are size-independent and Young's equation is generally valid for smooth substrates. For nanoscale systems, however, size-dependence and failure of Young's equation may emerge.

EXPERIMENTS

The wetting behavior of a nanodroplet containing two miscible liquids on a smooth substrate is explored by many-body dissipative particle dynamics simulations. The size-dependent surface tension of nanofilms is investigated as well.

FINDINGS

It is found that Young's equation is valid for nanodroplets of pure fluids but fails for two-component nanodroplets. The actual contact angle is always larger than the Young's contact angle, and their difference is getting smaller as the composition approaches pure fluids or the compatibility of the mixture is increased. The failure of Young's equation is closely associated with the size-dependent behavior in two-component nanodroplets and nanofilms. As the nanodroplet size is increased, the actual contact angle is found to decline but approaches a constant expected in macroscopic systems. Similarly, as the nanofilm thickness is increased, surface tension decreases and reaches its macroscopic value. The change of surface tension is attributed to the size-dependent surface composition, which is responsible for the failure of Young's equation.

Nanoscale-Specific Reaction in a Precursor Film: Mixing Sodium Carbonate, Calcium Chloride, and an Organic Thiol to Produce Crystals of Calcium sulfate

The ultrathin precursor film surrounding droplets of liquid on a solid surface is used here as a confined reaction medium in order to drive a reaction that would not occur in bulk fluid. Sodium carbonate and calcium chloride mixed together in the presence of the organic thiol dithiothreitol (DTT) produced crystals of gypsum, or calcium sulfate, instead of the otherwise expected calcium carbonate. The possible sources of sulfate in the system are contaminants in the DTT or the oxidation product of the DTT sulfhydryl. The amount of gypsum produced implies that contaminants do not account for the total sulfate present in the system, suggesting that the DTT could be oxidized. The reaction quotient may be skewed in favor of this unexpected reaction by a combination of efficient removal of sulfate by precipitation and the concentration of DTT at the leading edge of the precursor film through the coffee-ring effect during a brief drying step.

Formation of Deposition Patterns Induced by the Evaporation of the Restricted Liquid

Evaporation-induced self-assembly of colloids or suspensions has received increasing attention. Given its critical applications in many fields of science and industry, we report deposition patterns constructed by the evaporation of the restricted aqueous suspension with polystyrene particles at different substrate temperatures and geometric container dimensions. With the temperature increases, the deposition patterns transition from honeycomb to multiring to island, which is attributed to the competition between the particle deposition rate U_P and the contact line velocity U_CL, and the dimension of the geometric container has an effect on the characteristics of patterns. In this paper, the formation of an ordered multiring pattern is mainly focused on as a result of U_P keeping up with U_CL such that the entire contact line can be pinned, that is, the periodic stick-slip motion of the contact line and the particle sedimentation. Moreover, based on the Onsager principle, we develop a theoretical model to reveal the physical mechanisms behind the multiring phenomena. The position and spacing of rings are measured, which shows that the theoretical prediction agrees well with experiments. We also find that the ring spacing decays exponentially from center to edge experimentally and theoretically. This may not only help us to understand the formation of the deposition patterns but also assist future design and control in practical applications.

Absorption induced ordered ring and inner network structures on a nanoporous substrate

Interaction of colloidal droplets with a porous medium is the key issue for many industrial applications, such as direct-ink-write printing on flexible wearable clothing. In this work, we find a novel pattern of an ordered ring with inner network from a colloidal droplet resting on the nanoporous substrate. Experimental results show that the outward flow caused by the lateral absorption is responsible for the ring structures. The mutual competition between the inward dewetting and the outward flow determines the formation of the inner network pattern. The capillary immersion forces dominate the self-assembly of particles and promote the ordered arrays of the structures.

Compartmentalized Arrays of Matrix Droplets for Quantitative Mass Spectrometry Imaging of Adsorbed Peptides

Mass spectrometry imaging (MSI) based on matrix-assisted laser desorption/ionization (MALDI) provides information on the identification and spatial distribution of biomolecules. Quantitative analysis, however, has been challenging largely due to heterogeneity in both the size of the matrix crystals and the extraction area. In this work, we present a compartmentalized elastomeric stamp for quantitative MALDI-MSI of adsorbed peptides. Filling the compartments with matrix solution and stamping onto a planar substrate extract and concentrate analytes adsorbed in each compartment into a single analyte-matrix cocrystal over the entire stamped area. Walls between compartments help preserve spatial information on the adsorbates. The mass intensity of the cocrystals directly correlates with the surface coverage of analytes, which enables not only quantitative analysis but estimation of an equilibrium constant for the adsorption. We demonstrate via MALDI-MSI relative quantitation of peptides adsorbed along a microchannel with varying surface coverages.

Development of paper-based microfluidic device for the determination of nitrite in meat

This study employed the use of a microfluidic paper-based analytical device (µPAD) to determine the concentration of nitrite in pork and enhanced the limit of detection by analyzing the coffee-ring effect. The µPAD was fabricated by designing and embedding wax channels onto the cellulose-based filter paper through printing and subjecting the paper to heat treatment to allow wax penetration. Nitrite concentration was determined by monitoring the colorimetric reaction that occurred between nitrite and the added Griess reagent. The limit of detection of this device for nitrite in pork was determined to be 19.2 mg kg^-1 by analyzing the inner-chamber reaction, while it could be as low as 1.1 mg kg^-1 if the coffee-ring region was analyzed. The overall analysis could be completed within 15 min. This µPAD-based method has potential applications to routinely screen the nitrite concentration of meat products and ensure food safety and consumer health.

Determination of norfloxacin residues in foods by exploiting the coffee-ring effect and paper-based microfluidics device coupling with smartphone-based detection

By utilizing the coffee-ring effect and microfluidic paper-based analytical devices (µPADs), this study improved the sensitivity of the determination of norfloxacin in four different food matrices. Micro-PADs in this study were fabricated by designing and embedding wax channels onto cellulose-based filter paper through printing and subjecting the paper to heat to allow the wax to penetrate the paper. Determination of norfloxacin concentration in food samples was achieved by monitoring the colorimetric reaction that occurred between norfloxacin and the added iron (III) nitrate nonahydrate in 5 mM ammonia in each reaction chamber. A transition metal hydroxide was formed through this reaction that resulted in the formation of a solid precipitate to enable the antibiotic to bind to the iron molecule via coordination chemistry. This metal ion-antibiotic complex generated a visible color change. Following the colorimetric reaction, images were taken and subsequently analyzed via ImageJ to determine the relative pixel intensity that was used to infer norfloxacin concentration. The analytical sensitivity of this device was determined to be as low as 50 ppm when analyzing the inner-ring reaction, and as low as 5 ppm when analyzing the outer coffee ring thereby allowing for an alternative cheaper, faster, and more user-friendly method to detect norfloxacin than the conventional methods. PRACTICAL APPLICATION: This novel paper-based microfluidic device can achieve the detection of antibiotic residues in agrifoods in a faster, cheaper, and more user-friendly manner.

Role of surfactant in controlling the deposition pattern of a particle-laden droplet: Fundamentals and strategies

Evaporation of particle-laden droplets has attracted wide attention propelled by the vast applications from disease diagnostics, bio-medicines, agriculture, inkjet printing to coating. Surfactant plays a vital role in controlling the deposition patterns of dried droplets, thanks to its extensive influences on particle transport through adsorbing at particle surface and droplet interfaces as well as suppressing or facilitating multiple flows. In order to accurately control the subtle morphology of a deposition, it is of significance to systematically elaborate the microscopic functions of surfactant, and bridge them to the various phenomena of a droplet. In this review, we first elucidate the effects of surfactant on the flow paradigms of capillary flow, solutal Marangoni flow, thermal Marangoni flow, and the mixed flow patterns as capillary force, thermal and solutal surface tensions are in competence or collaboration. Second, surfactant adsorption at particle surface and droplet interfaces modifying short-range and long-range forces such as electrostatic force, van der Waals force, capillary attraction, and hydrophobic bonding among particles and between particles and interfaces are introduced by the underlying mechanisms and approaches. Two phase diagrams are developed to respectively illustrate the roles of capillary force among particles, and the electrostatic interaction between particles and solid-liquid interface in modifying the deposition profiles. This review could build a fundamental framework of knowledge for evaporating particle-laden surfactant solution droplets, and may shed light on strategies to manipulate particle deposition in abundant fluidic-based techniques.

Ultrasensitive SERS-Based Plasmonic Sensor with Analyte Enrichment System Produced by Direct Laser Writing

We report an easy-to-implement device for surface-enhanced Raman scattering (SERS)-based detection of various analytes dissolved in water droplets at trace concentrations. The device combines an analyte-enrichment system and SERS-active sensor site, both produced via inexpensive and high-performance direct femtosecond (fs)-laser printing. Fabricated on a surface of water-repellent polytetrafluoroethylene substrate as an arrangement of micropillars, the analyte-enrichment system supports evaporating water droplet in the Cassie-Baxter superhydrophobic state, thus ensuring delivery of the dissolved analyte molecules towards the hydrophilic SERS-active site. The efficient pre-concentration of the analyte onto the sensor site based on densely arranged spiky plasmonic nanotextures results in its subsequent label-free identification by means of SERS spectroscopy. Using the proposed device, we demonstrate reliable SERS-based fingerprinting of various analytes, including common organic dyes and medical drugs at ppb concentrations. The proposed device is believed to find applications in various areas, including label-free environmental monitoring, medical diagnostics, and forensics.

Inkjet Printing of Magnetic Particles Toward Anisotropic Magnetic Properties

Unique properties of one-dimensional assemblies of particles have attracted great attention during the past decades, particularly with respect to the potential for anisotropic magnetism. Patterned films can be created using inkjet printing; however, drying of particle-laden colloidal droplets on solid surfaces is usually accompanied by the well-known coffee-ring effect, deteriorating both the uniformity and resolution of the printed configurations. This study examines the effect of externally applied magnetic field on particle deposition patterns. Ferromagnetic Gd5Si4 particles were formulated in terpineol oil and directly deposited via magnetic field-assisted inkjet printing on a photopaper to generate patterned films with suppressed coffee-ring effect. The particle deposition morphology is determined by both solvent imbibition and particle-magnetic field interactions. Three characteristic times are considered, namely, the critical time for solvent imbibition into the substrate (tim), the time it takes for particles to form chains in the presence of the magnetic field (tch), and the time in which the particles reach the substrate in the direction normal to the substrate (tpz). The characteristic time ratios (tpz/tim) and (tpz/tch) determine the final deposition morphology in the presence of magnetic field. The ability to control particle deposition and assembly, thus tuning the magnetic anisotropic properties of nanostructured materials is a promising approach for many engineering applications.

Self-Assembly of Ordered Microparticle Monolayers from Drying a Droplet on a Liquid Substrate

Drying droplets on solid substrates has always formed a nonuniform and disordered "coffee ring" stain, which has a great negative effect on the application of inject printing and colloidal assembly. We obtain a macrouniform and micro-ordered pattern through evaporation of a colloidal droplet resting on a liquid substrate. The evaporative convection and the capillary forces were responsible for the formation of the ordered structures, which assembled into a monolayer pattern at the liquid-air interface under the action of the weak capillary flow and shrinkage of the triple line. The central bump deposits with disordered particle stacking on the liquid-liquid interface could be attributed to the fast meeting of the descending particles (gravitational sedimentation) and ascending liquid-liquid interface; they would scatter on the ordered monolayer structure and form the final uniform pattern.

Stepped Annealed Inkjet-Printed InGaZnO Thin-Film Transistors

The preparation of thin-film transistors (TFTs) using ink-jet printing technology can reduce the complexity and material wastage of traditional TFT fabrication technologies. We prepared channel inks suitable for printing with different molar ratios of their constituent elements. Through the spin-coated and etching method, two different types of TFTs designated as depletion and enhancement mode were obtained simply by controlling the molar ratios of the InGaZnO channel elements. To overcome the problem of patterned films being prone to fracture during high-temperature annealing, a stepped annealing method is proposed to remove organic molecules from the channel layer and to improve the properties of the patterned films. The different interfaces between the insulation layers, channel layers, and drain/source electrodes were processed by argon plasma. This was done to improve the printing accuracy of the patterned InGaZnO channel layers, drain, and source electrodes, as well as to optimize the printing thickness of channel layers, reduce the defect density, and, ultimately, enhance the electrical performance of printed TFT devices.

Tuning Solute-Redistribution Dynamics for Scalable Fabrication of Colloidal Quantum-Dot Optoelectronics

Solution-processed colloidal quantum dots (CQDs) are attractive materials for the realization of low-cost and efficient optoelectronic devices. Although impressive CQD-solar-cell performance has been achieved, the fabrication of CQD films is still limited to laboratory-scale small areas because of the complicated deposition of CQD inks. Large-area, uniform deposition of lead sulfide (PbS) CQD inks is successfully realized for photovoltaic device applications by engineering the solute redistribution of CQD droplets. It is shown experimentally and theoretically that the solute-redistribution dynamics of CQD droplets are highly dependent on the movement of the contact line and on the evaporation kinetics of the solvent. By lowering the friction constant of the contact line and increasing the evaporation rate of the droplets, a uniform deposition of CQD ink in length and width over large areas is realized. By utilizing a spray-coating process, large-area (up to 100 cm² ) CQD films are fabricated with 3-7% thickness variation on various substrates including glass, indium tin oxide glass, and polyethylene terephthalate. Furthermore, scalable fabrication of CQD solar cells is demonstrated with 100 cm² CQD films which exhibits a notably high efficiency of 8.10%.

Active matter alters the growth dynamics of coffee rings

How particles are deposited at the edge of evaporating droplets, i.e. the coffee ring effect, plays a crucial role in phenomena as diverse as thin-film deposition, self-assembly, and biofilm formation. Recently, microorganisms have been shown to passively exploit and alter these deposition dynamics to increase their survival chances under harshening conditions. Here, we show that, as the droplet evaporation rate slows down, bacterial mobility starts playing a major role in determining the growth dynamics of the edge of drying droplets. Such motility-induced dynamics can influence several biophysical phenomena, from the formation of biofilms to the spreading of pathogens in humid environments and on surfaces subject to periodic drying. Analogous dynamics in other active matter systems can be exploited for technological applications in printing, coating, and self-assembly, where the standard coffee-ring effect is often a nuisance.

Probing the Colloidal Particle Dynamics in Drying Sessile Droplets

Particle deposition and assembly in the vicinity of contact lines of evaporative sessile droplets have been intensively investigated during the past decade. Yet little is known about particle arrangement in the contact-line region initiated by the self-assembled particles at the air-liquid interface and how the particle pinning behaves differently compared with that when particles are transported from the bulk of the sessile droplet to the three-phase contact line. We utilized the dual-droplet inkjet printing process to elucidate the versatility in particle deposition and assembly generated near the contact-line region and demonstrated the influence of such printing technique on particle pinning at the contact line after solvent evaporation. Wetting droplets containing sulfate-functionalized polystyrene (sulfate-PS) nanoparticles were jetted over the supporting droplets with carboxyl-PS nanoparticles, where the interplay between the solvent evaporation and particle transport dictates the final morphology of particle deposition. Depending on the particle size and concentration used in the supporting droplet, different morphologies of particle depositions near the periphery of the supporting droplet have been obtained such as stratified rings, blended rings, and rings of particles mainly from the air-liquid interface. Three characteristic times are considered in this study, namely, total time for solvent evaporation ( t_evp), time required for the colloidal particles in the supporting droplet to reach the contact line and form the first layers of deposition ( t_ps), and time needed for the particles at the interface to reach the contact line ( t_pw). The ratios of characteristic times ( t_ps/ t_evp) and ( t_ps/ t_pw) determine the final particle assembly near the contact-line region. The ability to control such particle deposition and assembly could have a direct implication on developing facile, cost-effective technologies essential for patterning heterogeneous structured coatings and devices.

Pattern Formation in Drying Sessile and Pendant Droplet: Interactions of Gravity Settling, Interface Shrinkage, and Capillary Flow

We reported the interactions of the gravitational sedimentation, interface shrinkage, and outward capillary flow in drying droplets. This coupling effect is the inference we draw from deposition patterns of both sessile and pendant droplets, which contain particles of different sizes, evaporating on a patterned substrate. The deposition difference between sessile and pendant droplets containing microparticles indicated that gravitational sedimentation has a significant influence on the deposition morphology. The phase diagram shows that the particle deposition process can be divided into two stages: in the first stage, the competition between the interface shrinkage and the gravitational sedimentation determines whether the particles can be captured by the liquid-air interface; in the second stage, the capillary flow takes the particles inside the droplet toward the edge. The deposition morphology is the result of competition and cooperation interactions of the free setting, interface shrinkage, and outward capillary flow.