Dynamically controlled deposition of colloidal nanoparticle suspension in evaporating drops using laser radiation

Moving Meniscus-Assisted Template-Free Optothermofluidic Nanoparticle Patterning and Its Application in Optothermoconvective Particle Trapping

Moving meniscus-assisted vertical lifting is a commonly employed particle assembly technique to realize large-area particle patterning for the easy fabrication of colloidal photonic crystals and sensors. Though great success has been achieved for large-area patterning, inscribing desired patterns over the target substrate with precise control over the morphology remains a challenge. The target substrates need to be functionalized (physically or chemically) to realize desired patterns, which increases the complexity and limits their applicability to specific particle-liquid combinations. We demonstrate a new approach for the precise patterning of gold nanoparticles (Au NPs, diameter ∼60 nm) over solid substrates by the synergy of light-induced Marangoni flow and vertical lifting process (moving meniscus), without the requirement of photomasks or templates. The core idea relies on the particle accumulation due to light-induced Marangoni flow near the liquid meniscus in contact with a solid surface (due to plasmonic absorption of the particles) and the controlled lifting of the substrate. We present both the simulation and experimental results of the developed patterning technique. Various patterns such as continuous lines, intermittent lines with varying lengths, patterns with continuously varying widths, cross patterns, etc. are successfully inscribed. Dynamic control over the three-dimensional morphology of the deposited patterns is achieved by varying the lifting velocity, laser irradiation time, and lifting direction during the inscription process. Finally, we show the applicability of the developed plasmonically active surface for the large-area parallel manipulation of nonabsorbing microparticles based on optothermoconvective flow. The major advantage of the developed method compared to the existing light-controlled patterning techniques is its ability to inscribe patterns over large distances (up to several centimeters). We expect that the results presented in this paper will benefit different applications requiring precise particle patterning, such as optical elements, sensors, plasmonic substrates, microfluidic master templates, and electronic circuits.

Formation and Application of Polymer Spherulite-like Patterns of Halloysite Nanotubes by Evaporation-Induced Self-Assembly

Halloysite nanotubes (HNTs) are one-dimensional clay nanomaterials featuring distinct tubular structures and unique surface charges. HNTs can readily form ordered assembly structures under specific conditions, which shows significant potential applications in optical and biological fields. In this study, sodium hexametaphosphate (SHMP) was employed as a stabilizer to prepare polymer spherulite-like patterns via the evaporation-induced self-assembly (EISA) technique. The incorporation of SHMP enhanced the repulsion force among the nanotubes and the surface potential, which facilitated the orderly deposition of HNTs. The influence of HNT concentration, SHMP concentration, drying temperature, and substrate on the polymer spherulites-like pattern has been investigated in detail. The optimal conditions were 10 wt % HNT dispersion, 0.6 wt % SHMP concentration, 30 °C as drying temperature, and glass substrates. In addition, by changing the droplet volume and shape of the three-phase contact line, patterns of different sizes and shapes can be achieved. Bovine serum albumin or metal salt compounds were incorporated into the dispersion of SHMP-modified HNTs, which altered the charge and the self-assembled patterns with different area ratios. Thus, this technology can be utilized for the analysis and comparison of protein and metal ion concentration accurately. This study creates the correlation between the structural parameters and the preparation process involved in creating polymer spherulite-like patterns of modified HNTs and offers fresh insights into potential applications for the self-assembly of HNT droplets in the realms of anticounterfeiting and solution concentration analysis.

Simultaneously Suppressing the Coffee Ring Effect of Solutes with Different Sizes

Suppressing the coffee ring effect (CRE), which improves the uniformity of deposition, has attracted great attention. Usually, a realistic system contains solutes of various sizes. Large particles preferentially settle onto the substrate under gravity, separated from small particles even when CRE is suppressed, which generates nonuniformity in another way. This hinders small particles from filling the gaps at the deposition-substrate interface, leaving a frail deposition. Here, the CRE of polydispersed solutes is simultaneously suppressed, and a more uniform deposition is achieved by suspending the drop together with adding trace amounts of cations. The gaps tend to be filled, which makes the deposition bind more tightly. Analysis shows that gravity coordinates with the interactions that mediate the attraction between particles and the substrate, resulting in the coinstantaneous adsorption of all particles. This work adds another dimension to the suppression of CRE, improving the uniformity of deposition in complex systems and paving the way for the development of techniques in diverse manufacturing industries.

The control of dry-out patterns using bubble-containing droplets

HYPOTHESIS

For an evaporating nanofluid droplet that contains a bubble inside, we suspect the bubble boundary remains pinned during evaporation whereas the droplet perimeter recedes. Thus, the dry-out patterns are mainly determined by the presence of the bubble and their morphology can be tuned by the size and location of the added bubble.

EXPERIMENTS

Bubbles with varying base diameters and lifetimes are added into evaporating droplets that contain nanoparticles with different types, sizes, concentrations, shapes, and wettability. The geometric dimensions of the dry-out patterns are measured.

FINDINGS

For a droplet containing a long-lifetime bubble, a complete ring-like deposit forms, and its diameter and thickness increases and decreases with the bubble base diameter, respectively. The ring completeness, i.e., the ratio of actual ring length to its imaginary perimeter, decreases with the decrease in bubble lifetime. The pinning of droplet receding contact line by particles near the bubble perimeter has been found to be the key factor leading to ring-like deposits. This study introduces a strategy of producing ring-like deposit and allows a control of the ring morphology in a simple, cheap, and impurity-free fashion, which is applicable to various applications associated with evaporative self-assembly.

Thermal Marangoni trapping driven by laser absorption in evaporating droplets for particle deposition

Controlling the deposition of particles is of great importance in many applications. In this work, we study particle deposition driven by Marangoni flows, triggered by laser absorption inside an evaporating droplet. When the laser is turned on, thermal gradients are generated and produce a toroidal Marangoni flow that concentrates the particles around the laser beam and ultimately controls the final deposition. We experimentally characterize the radius of the Marangoni flows as a function of the laser parameters. Counter-intuitively, the radius of the Marangoni region appears to remain constant and is not proportional to the thickness of the drop which decreases due to evaporation. We develop a model to predict the size of the Marangoni region that combines evaporative flows and laser-induced Marangoni flows. The experimental data are in good agreement with the predictions, allowing us to estimate the particle overconcentration factor resulting from the laser heating effects. The addition of surfactants to the solution allows the coupling of solutal Marangoni flows with thermal ones to achieve a final micron-scale deposit located at the laser spot. These results pave the way for new methods with high tunability provided by spatio-temporal light control for surface patterning applications.

Applications, fluid mechanics, and colloidal science of carbon-nanotube-based 3D printable inks

Additive manufacturing, also known as 3D printing (3DP), is a novel and developing technology, which has a wide range of industrial and scientific applications. This technology has continuously progressed over the past several decades, with improvement in productivity, resolution of the printed features, achievement of more and more complex shapes and topographies, scalability of the printed components and devices, and discovery of new printing materials with multi-functional capabilities. Among these newly developed printing materials, carbon-nanotubes (CNT) based inks, with their remarkable mechanical, electrical, and thermal properties, have emerged as an extremely attractive option. Various formulae of CNT-based ink have been developed, including CNT-nano-particle inks, CNT-polymer inks, and CNT-based non-nanocomposite inks (i.e., CNT ink that is not in a form where CNT particles are suspended in a polymer matrix). Various types of sensors as well as soft and smart electronic devices with a multitude of applications have been fabricated with CNT-based inks by employing different 3DP methods including syringe printing (SP), aerosol-jet printing (AJP), fused deposition modeling (FDM), and stereolithography (SLA). Despite such progress, there is inadequate literature on the various fluid mechanics and colloidal science aspects associated with the printability and property-tunability of nanoparticulate inks, specifically CNT-based inks. This review article, therefore, will focus on the formulation, dispersion, and the associated fluid mechanics and the colloidal science of 3D printable CNT-based inks. This article will first focus on the different examples where 3DP has been employed for printing CNT-based inks for a multitude of applications. Following that, we shall highlight the various key fluid mechanics and colloidal science issues that are central and vital to printing with such inks. Finally, the article will point out the open existing challenges and scope of future work on this topic.

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.

Patterning of Metallic Nanoparticles over Solid Surfaces from Sessile Droplets by Thermoplasmonically Controlled Liquid Flow

Optically controlled assembly of suspended particles from evaporating sessile droplets is an emerging method to realize on-demand patterning of particles over solid substrates. Most of the reported strategies rely either on additives or surface texturing to modulate particle deposition. Though dynamic control over the assembly of microparticles is possible, limited success has been achieved in nanoparticle patterning, especially in the case of metallic nanoparticles. This work demonstrates a simple light-directed patterning of gold (Au) nanoparticles based on the thermoplasmonically controlled liquid flow. Excitation at the plasmonic wavelength (532 nm) generates the required temperature gradient, resulting in the particle assembly at the irradiation zone in response to the thermocapillary flow created inside the droplet. Particle streak velocimetry experiments and analysis confirm the existence of a strong thermocapillary flow, which counteracts the naturally occurring evaporative convection flows. By modulating the illumination conditions, we could achieve patterns with various morphologies, including center deposit, off-center deposit, multi-spot deposit, and lines. We successfully applied the developed strategy for realizing closely packed hybrid particle assembly containing different particles: Au and polystyrene particles (PS). We performed optical microscopy, 3D profilometry, and SEM analysis to characterize the particle deposit. We analyzed the periodicity of Au-PS hybrid assembly using fast Fourier transform and radial distribution function analysis. PS particles formed a hexagonal close-packed arrangement at the irradiation zone, with Au NPs residing inside the voids. We believe that the presented strategy could significantly enhance the applicability of the evaporative lithography from sessile droplets for the programmable patterning of metallic nanoparticles.

Effect of Particle Concentration on Surfactant-Induced Alteration of the Contact Line Deposition in Evaporating Sessile Droplets

Controlling and suppressing the so-called "coffee-ring effect" (CRE) is an issue of cardinal importance and intense interest in many industries and scientific fields. Here, the combined effect of the particle and surfactant concentration on the CRE is investigated by gradually adding Triton X-100 surfactant to colloidal suspensions of SiO₂ nanoparticles in ethanol for various particle concentrations. First, the effect of particle concentration on the contact line dynamics during the evaporation of a sessile droplet is investigated. It is shown that increasing the particle concentration leads to an increase in pinning time and ring width, whereas the droplet's initial and dynamic contact angle remains unchanged. Afterward, the effect of different concentrations of surfactant is studied for different particle concentrations. It is concluded that the surfactant concentration at which the CRE is suppressed is dependent on the initial particle concentration of the colloid, and it increases as the particle concentration increases. Furthermore, as adding surfactant with a concentration lower than this critical concentration results in an unsuppressed CRE, it is shown that surpassing this concentration will result in a depletion of particles in the contact line. Moreover, it is demonstrated that this critical surfactant concentration has no significant effect on the droplet's geometry and the total evaporation time.

Applying droplets and films in evaporative lithography

This review covers experimental results of evaporative lithography and analyzes existing mathematical models of this method. Evaporating droplets and films are used in different fields, such as cooling of heated surfaces of electronic devices, diagnostics in health care, creation of transparent conductive coatings on flexible substrates, and surface patterning. A method called evaporative lithography emerged after the connection between the coffee ring effect taking place in drying colloidal droplets and naturally occurring inhomogeneous vapor flux densities from liquid-vapor interfaces was established. Essential control of the colloidal particle deposit patterns is achieved in this method by producing ambient conditions that induce a nonuniform evaporation profile from the colloidal liquid surface. Evaporative lithography is part of a wider field known as "evaporative-induced self-assembly" (EISA). EISA involves methods based on contact line processes, methods employing particle interaction effects, and evaporative lithography. As a rule, evaporative lithography is a flexible and single-stage process with such advantages as simplicity, low price, and the possibility of application to almost any substrate without pretreatment. Since there is no mechanical impact on the template in evaporative lithography, the template integrity is preserved in the process. The method is also useful for creating materials with localized functions, such as slipperiness and self-healing. For these reasons, evaporative lithography attracts increasing attention and has a number of noticeable achievements at present. We also analyze limitations of the approach and ways of its further development.

Multiscale Characteristic in Symmetric/Asymmetric Solar-Driven Nanofluid Droplet Evaporation

Driven by nanoparticle plasmonic heating, sessile droplet evaporation presents challenges on the coupling mechanisms between time-spatial heat source distribution and flow/temperature fields in a droplet. Here, symmetric/asymmetric solar-driven droplet evaporation is investigated. An infrared camera captures droplet surface temperatures in the micrometer scale after correction. An optical three-dimensional profiler quantifies nanoparticle deposition in the nanoscale. We show that droplet surface temperatures do display a nonmonotonic variation trend. Based on measurements, we are able to decouple the droplet into a contact line region (CLR) and a bulk volume region (BVR). The CLR volume is two to three magnitudes smaller than the droplet volume. The temperature gradient is significant in CLR, but flat temperature exists in BVR. Radial flow in BVR transports nanoparticles from the droplet body to the contact line, while Marangoni flow in CLR stabilizes nanoparticles there. Light energy is also decoupled based on its wavelength band. It is found that CLR dominates the visible energy absorption, but BVR has a weak contribution. Top light heating causes symmetry temperatures and a coffee-ring profile along the circumference. However, side heating yields higher temperatures and more nanoparticles deposition on the sunny side than on the night side. The above findings are valid when the initial droplet volume and incident irradiation flux are changed on the hydrophilic wall. The hydrophilic wall and hydrophobic wall maintain the evaporation modes of constant contact diameter and "stick-slip", respectively. The present paper enhances the understanding of light-induced droplet evaporation from the multiscale point of view.

Shape-driven arrest of coffee stain effect drives the fabrication of carbon-nanotube-graphene-oxide inks for printing embedded structures and temperature sensors

Carbon nanotube (CNT) based binder-free, syringe-printable inks, with graphene oxide (GO) being used as the dispersant, have been designed and developed. We discovered that the printability of the ink is directly attributed to the uniform deposition of the GO-CNT agglomerates, as opposed to the 'coffee-staining' despite these aggregates being micron-sized. The ellipsoidal nature of the micron-scale GO-CNT agglomerates/particles enables these particles to severely perturb the air-water interface, triggering a large long-range capillary interaction that causes the uniform deposition by overcoming the "coffee-stain"-forming forces from the evaporation-mediated flows. We evaluated the properties of this ink and identified a temperature-dependent resistance with a negative temperature coefficient of resistance (TCR) α ranging from ∼-10^-3 to -10^-2/°C depending on ink compositions. Finally, the printing is conducted on flat and curved surfaces, for developing polymer-ink embedded structures that might serve as precursors to syringe-printable CNT-based nanocomposites, and for fabricating sensor-like patterns that for certain ink compositions demonstrate α∼-10^-3/°C with a large averaged resistance drop (per unit temperature) of -3.5 Ω°C^-1.

Convection inside condensing and evaporating droplets of aqueous solution

We experimentally study the fluid convection inside a condensing droplet of aqueous NaCl solution and compare it with that of an evaporating droplet. The droplets are sandwiched between two horizontal hydrophobic surfaces and surrounded by a reservoir with solution of different concentration. Condensation and evaporation of the droplets occur due to the vapor pressure difference between the droplet and the reservoir solution. The micro-PIV technique has been used to study the velocity field inside the droplets. Buoyancy driven Rayleigh convection is observed inside both the condensing and evaporating droplets. In the condensing droplet, water condenses on the liquid-air interface creating a low density region near the interface. There is upward movement of fluid along the condensing interface towards the top region of the droplet which recirculates back from the center region of the droplet in the downward direction. In contrast, the fluid moves in the downward direction along the interface in the case of an evaporating droplet with an upward plume like flow at the center region of the droplet. Both evaporating and condensing droplets show a recirculating loop inside the droplets of opposite direction.

A review on suppression and utilization of the coffee-ring effect

Evaporation of sessile droplets containing non-volatile solutes dispersed in a volatile solvent leaves behind ring-like solid stains. As the volatile species evaporates, pinning of the contact line gives rise to capillary flows that transport non-volatile solutes to the contact line. This phenomenon, called the coffee-ring effect, compromises the overall performance of industrially relevant manufacturing processes involving evaporation such as printing, biochemical analysis, manufacturing of nano-structured materials through colloidal and macromolecular patterning. Various approaches have been developed to suppress this phenomenon, which is otherwise difficult to avoid. The coffee-ring effect has also been leveraged to prepare new materials through convection induced assembly. This review underlines not only the strategies developed to suppress the coffee-ring effect but also sheds light on approaches to arrive at novel processes and materials. Working principles and applicability of these strategies are discussed together with a critical comparison.

Evaporative Optical Marangoni Assembly: Tailoring the Three-Dimensional Morphology of Individual Deposits of Nanoparticles from Sessile Drops

We have recently devised the evaporative optical Marangoni assembly (eOMA), a novel and versatile interfacial flow-based method for directing the deposition of colloidal nanoparticles (NPs) on solid substrates from evaporating sessile drops along desired patterns using shaped UV light. Here, we focus on a fixed UV spot irradiation resulting in a cylinder-like deposit of assembled particles and show how the geometrical features of the single deposit can be tailored in three dimensions by simply adjusting the optical conditions or the sample composition, in a quantitative and reproducible manner. Sessile drops containing cationic NPs and a photosensitive surfactant at various concentrations are allowed to evaporate under a single UV beam with a diameter much smaller than that of the drop. After complete evaporation, the geometrical characteristics of the NP deposits are precisely assessed using optical profilometry. We show that both the volume and the radial size of the light-directed NP deposit can be adjusted by varying the diameter or the intensity of the UV beam or alternatively by changing the concentration of the photosensitive surfactant. Notably, in all these cases, the deposits display an almost constant median height corresponding to a few layers of particles. Moreover, both the radial and the axial extent of the patterns are tuned by changing the NP concentration. These results are explained by the correlation among the strength of Marangoni flow, the particle trapping efficiency, and the volume of the deposit, and by the role of evaporation-driven flow in strongly controlling the deposit height. Finally, we extend the versatility of eOMA by demonstrating that NPs down to 30 nm in diameter can be effectively patterned on glass or polymeric substrates.

Approaches to self-assembly of colloidal monolayers: A guide for nanotechnologists

Self-assembly of quasi-spherical colloidal particles in two-dimensional (2D) arrangements is essential for a wide range of applications from optoelectronics to surface engineering, from chemical and biological sensing to light harvesting and environmental remediation. Several self-assembly approaches have flourished throughout the years, with specific features in terms of complexity of the implementation, sensitivity to process parameters, characteristics of the final colloidal assembly. Selecting the proper method for a given application amidst the vast literature in this field can be a challenging task. In this review, we present an extensive classification and comparison of the different techniques adopted for 2D self-assembly in order to provide useful guidelines for scientists approaching this field. After an overview of the main applications of 2D colloidal assemblies, we describe the main mechanisms underlying their formation and introduce the mathematical tools commonly used to analyse their final morphology. Subsequently, we examine in detail each class of self-assembly techniques, with an explanation of the physical processes intervening in crystallization and a thorough investigation of the technical peculiarities of the different practical implementations. We point out the specific characteristics of the set-ups and apparatuses developed for self-assembly in terms of complexity, requirements, reproducibility, robustness, sensitivity to process parameters and morphology of the final colloidal pattern. Such an analysis will help the reader to individuate more easily the approach more suitable for a given application and will draw the attention towards the importance of the details of each implementation for the final results.