The effect of evaporation kinetics on nanoparticle structuring within contact line deposits of volatile drops

Diffusiophoresis-enhanced particle deposition for additive manufacturing

The ability to govern particle assembly in an evaporative-driven additive manufacturing (AM) can realize multi-scale features fundamental to creating printed electronics. However, existing techniques remain challenging and often require templates or contaminating solutes. We explore the control of particle deposition in 3D-printed colloids by diffusiophoresis, a previously unexplored mechanism in multi-scale AM. Diffusiophoresis can introduce spontaneous phoretic particle motion by establishing local solute concentration gradients. We show that diffusiophoresis can play a dominant role in complex evaporative-driven particle assembly, enabling a fundamentally new and versatile control of particle deposition in a multi-scale AM process.

Estimation of surface free energy at microstructured surface to investigate intermediate wetting state for partial wetting model

While partial wetting at nano-/microstructured surfaces can be described using the intermediate wetting state between the Cassie-Baxter and Wenzel states, the limitations of the partial wetting model remain unclear. In this study, we performed surface free energy analysis at a microstructured Si-water interface from both theoretical and experimental viewpoints. We experimentally measured the water contact angle on microstructured Si surfaces with square holes and compared the measured values with theoretical predictions. Furthermore, the surface free energy was analyzed using the effective wetting area estimated from the measured contact angle and electrochemical impedance spectroscopy results. We verified the validity of the partial wetting model for fabricated Si surfaces with a hole aperture a less than 230 μm and a hole height h of 12 μm, and for a < 400 μm, h = 40 μm. The model was found to be applicable to microstructured Si surfaces with a/h < 10.

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.

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.

Controlling disorder in self-assembled colloidal monolayers via evaporative processes

Monolayers of assembled nano-objects with a controlled degree of disorder hold interest in many optical applications, including photovoltaics, light emission, sensing, and structural coloration. Controlled disorder can be achieved through either top-down or bottom-up approaches, but the latter is more suited to large-scale, low-cost fabrication. Disordered colloidal monolayers can be assembled through evaporatively driven convective assembly, a bottom-up process with a wide range of parameters impacting particle placement. Motivated by the photonic applications of such monolayers, in this review we discuss the quantification of monolayer disorder, and the assembly methods that have been used to produce them. We review the impact of particle and solvent properties, as well as the use of substrate patterning, to create the desired spatial distributions of particles.

Colloidal Deposits via Capillary Bridge Evaporation and Particle Sorting Thereof

Evaporating droplets of colloidal suspensions leave behind particle deposits which could be effectively controlled via manipulating the surrounding conditions and particles and liquid properties. While previous studies extensively focused on sessile and pendant droplets, the present work investigates the evaporation dynamics of capillary bridges of colloidal suspensions formed between two parallel plates. We vary the wettability of the plates and the particle size and composition of the colloidal suspensions, keeping the same spacing between the plates. We employ side visualization, optical microscopy, fluorescence microscopy, and scanning electron microscopy and develop computational and theoretical models to collect the data. A computational model based on diffusion-limited evaporation is used to characterize the timescale of the evaporation of the capillary bridge. The model predictions are in good agreement with the present and prior experimental measurements. We discuss about the deposits of monodispersed particle suspension formed by the interplay of pinning of the contact line and evaporation dynamics. Multiple rings on the plates are observed due to the stick-slip motion of the contact line. The larger particles tend to form asymmetric deposits, with most particles concentrated on the bottom plates due to a considerably stronger gravitational pull than the hydrodynamic drag. This deposition is explained by estimating the competing forces on the particles during the evaporation. A regime map is proposed for classifying deposits on the particle size wettability plane. Lastly, we demonstrate size-based particle sorting of bidispersed colloidal suspensions in this framework. We describe two mechanisms: gravity-assisted and geometry-assisted sorting, which can be designed to sort particles efficiently. A regime map depicting the regions of influence of each mechanism is presented.

Evaporation Dynamics of a Sessile Droplet of Binary Mixture Laden with Nanoparticles

We investigate the evaporation dynamics of a sessile droplet of ethanol-water binary mixtures of different compositions laden with alumina nanoparticles and compare with the no-loading condition at different substrate temperatures. Shadowgraphy and infrared imaging methods are used, and the experimental images are postprocessed using a machine learning technique. We found that the loading and no-loading cases display distinct wetting and contact angle dynamics. Although the wetting diameter of a droplet decreases monotonically in the absence of loading, the droplet with 0.6 wt % nanoparticle loading remains pinned for the majority of its lifetime. The temporal variation of the normalized droplet volume in the no-loading case has two distinct slopes, with ethanol and water phases dominating the early and late stages of evaporation, respectively. The normalized droplet volume with 0.6 wt % loading displays a nearly linear behavior because of the increase in the heat transfer rate. Our results from infrared imaging reveal that a nanofluid droplet displays far richer thermal patterns than a droplet without nanoparticle loading. In nanoparticle-laden droplets, the pinning effect, as well as the resulting thermo-capillary and thermo-solutal convection, causes more intense internal mixing and a faster evaporation rate. Finally, a theoretical model is also developed that satisfactorily predicts the evaporation dynamics of binary nanofluid droplets.

Dependency of Contact Angles on Three-Phase Contact Line: A Review

The wetted area of a sessile droplet on a practical substrate is limited by the three-phase contact line and characterized by contact angle, contact radius and drop height. Although, contact angles of droplets have been studied for more than two hundred years, there are still some unanswered questions. In the last two decades, it was experimentally proven that the advancing and receding contact angles, and the contact angle hysteresis of rough and chemically heterogeneous surfaces, are determined by interactions of the liquid and the solid at the three-phase contact line alone, and the interfacial area within the contact perimeter is irrelevant. However, confusion and misunderstanding still exist in this field regarding the relationship between contact angle and surface roughness and chemical heterogeneity. An extensive review was published on the debate for the dependence of apparent contact angles on drop contact area or the three-phase contact line in 2014. Following this old review, several new articles were published on the same subject. This article presents a review of the novel articles (mostly published after 2014 to present) on the dependency of contact angles on the three-phase contact line, after a short summary is given for this long-lasting debate. Recently, some improvements have been made; for example, a relationship of the apparent contact angle with the properties of the three-phase line was obtained by replacing the solid–vapor interfacial tension term, γSV, with a string tension term containing the edge energy, γSLV, and curvature of the triple contact line, km, terms. In addition, a novel Gibbsian thermodynamics composite system was developed for a liquid drop resting on a heterogeneous multiphase and also on a homogeneous rough solid substrate at equilibrium conditions, and this approach led to the same conclusions given above. Moreover, some publications on the line energy concept along the three-phase contact line, and on the “modified” Cassie equations were also examined in this review.

The Impact of Nanofluids on Droplet/Spray Cooling of a Heated Surface: A Critical Review

Cooling by impinging droplets has been the subject of several studies for decades and still is, and, in the last few years, the potential heat transfer enhancement obtained thanks to nanofluids’ use has received increased interest. Indeed, the use of high thermal conductivity fluids, such as nanofluids’, is considered today as a possible way to strongly enhance this heat transfer process. This enhancement is related to several physical mechanisms. It is linked to the nanofluids’ rheology, their degree of stabilization, and how the presence of the nanoparticles impact the droplet/substrate dynamics. Although there are several articles on droplet impact dynamics and nanofluid heat transfer enhancement, there is a lack of review studies that couple these two topics. As such, this review aims to provide an analysis of the available literature dedicated to the dynamics between a single nanofluid droplet and a hot substrate, and the consequent enhancement or reduction of heat transfer. Finally, we also conduct a review of the available publications on nanofluids spray cooling. Although using nanofluids in spray cooling may seem a promising option, the few works present in the literature are not yet conclusive, and the mechanism of enhancement needs to be clarified.

Evaporation and crystallization process for sessile saline droplets during depressurization

The evaporation and crystallization process for sessile saline droplets during depressurization is experimentally studied. The relationship between ambient pressure and the crystallization pattern is primarily discussed. When the ambient pressure is low, salt particles are easily formed at the droplet contact line. In contrast, when the ambient pressure is similar to atmospheric pressure, it is more likely for cubic crystals to be formed inside the droplet. By analysing the contact angle fluctuation during crystallization, the experimental results show that the growth of a cubic salt crystal under high ambient pressure or low salt concentration leads to a greater deformation of the liquid-gas interface and a larger contact angle fluctuation. Finally, the Peclet number Pe is introduced to reflect the ratio of the rate of ion advection to the rate of diffusion. The Pe number is larger at lower ambient pressure, which means that the external mass transfer and convection effect is more significant under low pressure, with salt particles easily formed at the droplet contact line. The effect of concentration diffusion inside the droplet increases at higher ambient pressure, thereby, making it easy for cubic crystals to be formed inside the droplet.

Nanomaterial Patterning in 3D Printing

The synergistic integration of nanomaterials with 3D printing technologies can enable the creation of architecture and devices with an unprecedented level of functional integration. In particular, a multiscale 3D printing approach can seamlessly interweave nanomaterials with diverse classes of materials to impart, program, or modulate a wide range of functional properties in an otherwise passive 3D printed object. However, achieving such multiscale integration is challenging as it requires the ability to pattern, organize, or assemble nanomaterials in a 3D printing process. This review highlights the latest advances in the integration of nanomaterials with 3D printing, achieved by leveraging mechanical, electrical, magnetic, optical, or thermal phenomena. Ultimately, it is envisioned that such approaches can enable the creation of multifunctional constructs and devices that cannot be fabricated with conventional manufacturing approaches.

On the Effect of Substrate Viscoelasticity on the Evaporation Kinetics and Deposition Patterns of Nanosuspension Drops

This study investigates the evaporation of sessile pure water and nanosuspension drops on viscoelastic polydimethylsiloxane (PDMS) films. We varied the viscoelasticity of the PDMS films by controlling the curing ratio and categorized them into three types: stiff (10:1, 20:1, 40:1), soft (60:1, 80:1), and very soft (100:1, 120:1, 140:1, 160:1). On stiff surfaces, pure water drops initially evaporate in a constant contact radius (CCR) mode, followed by a constant contact angle mode, and finally in a mixed mode of evaporation. Nanosuspension drops follow the same trend as water drops but with a difference toward the end of their lifetimes, when a short second CCR mode is observed. Complete evaporation of nanosuspension drops on stiff substrates leads to particle deposition patterns similar to a coffee ring with cracks and deposition tails. On soft surfaces, the initial spreading is followed by a pseudo-CCR mode. Complete evaporation of nanosuspension drops on soft substrates leads to deposits in the form of a uniform ring with a sharp ox-horn profile. Unexpectedly, the initial spreading is followed by a mixed mode on very soft substrates, on which wetting ridges (WRs) pulled up by the vertical component of surface tension are clearly observed in the vicinity of the contact line (CL). As the evaporation proceeds, the decreasing contact angle breaks the force balance in the horizontal direction at the CL and gives rise to a net horizontal force, which causes the CL to recede, transferring the horizontal force to the WR. Because of the viscoelastic nature of the very soft substrate, this horizontal force acting on the WR cannot be completely countered by the bulk of the substrate underneath. As a result, the WR moves horizontally in a viscous-flow way, which also enables the CL to be continuously anchored to the ridge and to recede relative to the bulk of the substrate. Consequently, a mixed mode of evaporation occurs. Complete evaporation of nanosuspension drops on very soft substrates leads to finger-like deposits.

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.

Interfacial Targeting of Sessile Droplets Using Electrospray

We report on the use of electrospray atomization to deliver nanoparticles and surfactant directly to the surface of sessile droplets. The particles delivered to the target droplet remained adsorbed at its interface since they arrived solvent-free. Upon complete evaporation, the interface of the target drop was mapped to the underlying substrate, forming a nanoparticle deposit. The use of electrospray permitted the exploration of the interfacial particle transport and the role of surfactants in governing particle motion and deposit structure. When no surfactant was present in the sprayed solution, there was no observable convection of the interfacial particles. When Tween 80, a high-molecular-weight surfactant, was added to the sprayed solution, the surface flow was similarly suppressed. Only when small surfactants (e.g., sodium dodecyl sulfate) were present in the sprayed solution was Marangoni flow, directed toward the droplet apex, induced at the interface. This flow drove the interfacial particles to the apex of the target droplet, creating a particle-dense region at the center of the final deposit. We found that small surfactants were capable of desorbing from the interface at a sufficiently high rate relative to the evaporation time scale of the target droplet. Once inside the drop, the desorbed surfactant was convected to the contact line where it accumulated, inducing a surface tension gradient and a solutal Marangoni flow. Numerical modeling using the lattice Boltzmann-Brownian dynamics method confirmed this mechanism of particle transport and its relationship to deposit structure. The use of sacrificial targets combined with electrospray may provide a unique capability for building colloidal monolayers with organized structure in a scalable way.

Mechanisms of pattern formation from dried sessile drops

The formation of patterns after the evaporation of colloidal droplets deposited on a solid surface is an everyday natural phenomenon. During the past two decades, this topic has gained broader audience due to its numerous applications in biomedicine, nanotechnology, printing, coating, etc. This paper presents a detailed review of the experimental studies related to the formation of various deposition patterns from dried droplets of complex fluids (i.e., nanofluids, polymers). First, this review presents the fundamentals of sessile droplet evaporation including evaporation modes and internal flow fields. Then, the most observed dried patterns are presented and the mechanisms behind them are discussed. The review ends with the categorisation and exhaustive investigation of a wide range of factors affecting pattern formation.

Recent advances in droplet wetting and evaporation

Wetting and evaporation of a simple sessile droplet is a very complex problem involving strongly coupled physics.

Surfactant effects on droplet dynamics and deposition patterns: a lattice gas model

A coarse-grained lattice gas model is developed to study pattern forming processes in drying drops containing surfactant. By performing Monte Carlo simulations of the model, the coupled dynamics of surfactant and liquid evaporation and the resulting oscillatory dynamics at the contact line are elucidated. We show that the coupled drop dynamics and the resulting final deposition patterns can be altered by adsorption kinetics. For slow adsorption rates, surfactant molecules recirculate along with colloidal particles and the area covered by the surfactant on the surface grows from the contact line as the initial concentration of the surfactant increases. This leads to coffee-ring patterns with wide rim areas upon drying or to multi-ring patterns depending on the surfactant concentration. For fast adsorption rates, a surfactant skin covers the entire surface area during the early phase of evaporation. This suppresses the coffee ring effect, and uniform patterns are obtained independent of surfactant concentration. The results suggest that the distribution of surfactant on the surface is critical in determining final deposition patterns and that understanding of the skin-forming process of the surfactant on the surface can help in manipulating the delicate pattern forming process of particles in evaporating drops.

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.

Influence of Local Heating on Marangoni Flows and Evaporation Kinetics of Pure Water Drops

The effect of localized heating on the evaporation of pure sessile water drops was probed experimentally by a combination of infrared thermography and optical imaging. In particular, we studied the effect of three different heating powers and two different locations, directly below the center and edge of the drop. In all cases, four distinct stages were identified according to the emerging thermal patterns. In particular, depending on heating location, recirculating vortices emerge that either remain pinned or move azimuthally within the drop. Eventually, these vortices oscillate in different modes depending on heating location. Infrared data allowed extraction of temperature distribution on each drop surface. In turn, the flow velocity in each case was calculated and was found to be higher for edge heating, due to the one-directional nature of the heating. Additionally, calculation of the dimensionless Marangoni and Rayleigh numbers yielded the prevalence of Marangoni convection. Heating the water drops also affected the evaporation kinetics by promoting the "stick-slip" regime. Moreover, both the total number of depinning events and the pinning strength were found to be highly dependent on heating location. Lastly, we report a higher than predicted relationship between evaporation rate and heating temperature, due to the added influence of the recirculating flows on temperature distribution and hence evaporation flux.

Thermo-Driven Evaporation Self-Assembly and Dynamic Analysis of Homocentric Carbon Nanotube Rings

MWCNTs self-assemble into various homocentric rings in a thermo-driven self-assembly system. Closely packed and scatteredly packed MWCNT rings self-assemble on a Si-SiO₂ substrate, whereas on a Au substrate smoothly packed MWCNT rings, rings with waviness, and rings with shuttle-like holes are seen to self-assemble. The dynamic self-assembly process includes convection flow and swirling flow.

Effects of Substrate Heating and Wettability on Evaporation Dynamics and Deposition Patterns for a Sessile Water Droplet Containing Colloidal Particles

Effects of substrate temperature, substrate wettability, and particle concentration are experimentally investigated for evaporation of a sessile water droplet containing colloidal particles. Time-varying droplet shapes and temperature of the liquid-gas interface are measured using high-speed visualization and infrared thermography, respectively. The motion of the particles inside the evaporating droplet is qualitatively visualized by an optical microscope and the profile of the final particle deposit is measured by an optical profilometer. On a nonheated hydrophilic substrate, a ring-like deposit forms after the evaporation, as reported extensively in the literature, while on a heated hydrophilic substrate, a thinner ring with an inner deposit is reported in the present work. The latter is attributed to Marangoni convection, and recorded motion of the particles as well as measured temperature gradient across the liquid-gas interface confirms this hypothesis. The thinning of the ring scales with the substrate temperature and is reasoned to stronger Marangoni convection at larger substrate temperature. In the case of a nonheated hydrophobic substrate, an inner deposit forms due to very early depinning of the contact line. On the other hand, in the case of a heated hydrophobic substrate, the substrate heating as well as larger particle concentration helps in the pinning of the contact line, which results in a thin ring with an inner deposit. We propose a regime map for predicting three types of deposits-namely, ring, thin ring with inner deposit, and inner deposit-for varying substrate temperature, substrate wettability, and particle concentration. A first-order model corroborates the liquid-gas interface temperature measurements and variation in the measured ring profile with the substrate temperature.

Kinetics of Evaporation of Pinned Nanofluid Volatile Droplets at Subatmospheric Pressures

We examine the effects of nanoparticle addition at low concentration on the evaporation kinetics of droplets in the constant radius mode. The evaporative behavior of deionized water and Al2O3 nanoparticle laden water on an aluminum substrate was observed at atmospheric and at different subatmospheric pressures. The two fluids exhibit the same evaporative behavior, independent of the droplet volume or the subatmospheric pressure. Moreover, the linear relationship between evaporation rate and droplet radius, initially proposed by Picknett and Bexon nearly four decades ago for droplets evaporating in the constant radius mode, is satisfied for both liquids. In addition, we have established a unified correlation solely function of fluid properties that extends this relationship to any subatmospheric pressure and fluid tested. We conclude that the addition of a small quantity of nanoparticles to the base fluid does not modify the kinetics of evaporation for pinned volatile droplets.

"Biodrop" Evaporation and Ring-Stain Deposits: The Significance of DNA Length

Small sessile drops of water containing either long or short strands of DNA ("biodrops") were deposited on silicon substrates and allowed to evaporate. Initially, the triple line (TL) of both types of droplet remained pinned but later receded. The TL recession mode continued at constant speed until almost the end of drop lifetime for the biodrops with short DNA strands, whereas those containing long DNA strands entered a regime of significantly lower TL recession. We propose a tentative explanation of our observations based on free energy barriers to unpinning and increases in the viscosity of the base liquid due to the presence of DNA molecules. In addition, the structure of DNA deposits after evaporation was investigated by AFM. DNA self-assembly in a series of perpendicular and parallel orientations was observed near the contact line for the long-strand DNA, while, with the short-stranded DNA, smoother ring-stains with some nanostructuring but no striations were evident. At the interior of the deposits, dendritic and faceted crystals were formed from short and long strands, respectively, due to diffusion and nucleation limited processes, respectively. We suggest that the above results related to the biodrop drying and nanostructuring are indicative of the importance of DNA length, i.e., longer DNA chains consisting of linearly bonded shorter, rod-like DNA strands.

Controlled Uniform Coating from the Interplay of Marangoni Flows and Surface-Adsorbed Macromolecules

Surface coatings and patterning technologies are essential for various physicochemical applications. In this Letter, we describe key parameters to achieve uniform particle coatings from binary solutions. First, multiple sequential Marangoni flows, set by solute and surfactant simultaneously, prevent nonuniform particle distributions and continuously mix suspended materials during droplet evaporation. Second, we show the importance of particle-surface interactions that can be established by surface-adsorbed macromolecules. To achieve a uniform deposit in a binary mixture, a small concentration of surfactant and surface-adsorbed polymer (0.05 wt% each) is sufficient, which offers a new physicochemical avenue for control of coatings.

Impact of the collective diffusion of charged nanoparticles in the convective/capillary deposition directed by receding contact lines

The motion of electrically charged particles under crowding conditions and subjected to evaporation-driven capillary flow might be ruled by collective diffusion. The concentration gradient developed inside an evaporating drop of colloidal suspension may reduce by diffusion the number of particles transported toward the contact line by convection. Unlike self-diffusion coefficient, the cooperative diffusion coefficient of interacting particles becomes more pronounced in crowded environments. In this work, we examined experimentally the role of the collective diffusion of charge-stabilized nanoparticles in colloidal patterning. To decouple the sustained evaporation from the contact line motion, we conducted evaporating menisci experiments with driven receding contact lines at low capillary number. This allowed us to explore convective assembly at fixed and low bulk concentration, which enabled to develop high concentration gradients. At fixed velocity of receding contact line, we explored a variety of substrate-particle systems where the particle-particle electrostatic interaction was changed (via p H) as well as the substrate receding contact angle and the relative humidity. We found that the particle deposition directed by receding contact lines may be controlled by the interplay between evaporative convection and collective diffusion, particularly at low particle concentration.

Control of stain geometry by drop evaporation of surfactant containing dispersions

Control of stain geometry by drop evaporation of surfactant containing dispersions is an important topic of interest because it plays a crucial role in many applications such as forming templates on solid surfaces, in ink-jet printing, spraying of pesticides, micro/nano material fabrication, thin film coatings, biochemical assays, deposition of DNA/RNA micro-arrays, and manufacture of novel optical and electronic materials. This paper presents a review of the published articles on the diffusive drop evaporation of pure liquids (water), the surfactant stains obtained from evaporating drops that do not contain dispersed particles and deposits obtained from drops containing polymer colloids and carbon based particles such as carbon nanotubes, graphite and fullerenes. Experimental results of specific systems and modeling attempts are discussed. This review also has some special subtopics such as suppression of coffee-rings by surfactant addition and "stick-slip" behavior of evaporating nanosuspension drops. In general, the drop evaporation process of a surfactant/particle/substrate system is very complex since dissolved surfactants adsorb on both the insoluble organic/inorganic micro/nanoparticles in the drop, on the air/solution interface and on the substrate surface in different extends. Meanwhile, surfactant adsorbed particles interact with the substrate giving a specific contact angle, and free surfactants create a solutal Marangoni flow in the drop which controls the location of the particle deposition together with the rate of evaporation. In some cases, the presence of a surfactant monolayer at the air/solution interface alters the rate of evaporation. At present, the magnitude of each effect cannot be predicted adequately in advance and consequently they should be carefully studied for any system in order to control the shape and size of the final deposit.

Effect of particle geometry on triple line motion of nano-fluid drops and deposit nano-structuring

We illustrate the importance of particle geometry on droplet contact line pinning, 'coffee-stain' formation and nano-structuring within the resulting rings. We present the fundamentals of pure liquid droplet evaporation and then discuss the effect of particles on the evaporation process. The resulting coffee-stain patterns and particle structuring within them are presented and discussed. In the second part, we turn our attention to the effect of particle geometry on the evaporation process. A wide range of particle shapes, categorised according to aspect ratio, from the simple shape of a sphere to the highly irregular shapes of platelets and tubes is discussed. Particle geometry effect on evaporation behaviour was quantified in terms of change in contact angle and contact radius for the stick-slip cases. Consequently the hysteretic energy barrier pinning the droplets was estimated, showing an increasing trend with particle aspect ratio. The three-phase contact line (TL) motion kinetics are complemented with analysis of the nano-structuring behaviour of each shape, leading to the identification of the two main parameters affecting nanoparticle self-assembly behaviour at the wedge. Flow velocity and wedge constraints were found to have antagonist effects on particle deposition, although these varied with particle shape. This description should help in understanding the drying behaviour of more complex fluids. Furthermore, knowing the fundamentals of this simple and inexpensive surface patterning technique should permit its tailoring to the needs of many potential applications.

Effect of substrate temperature on pattern formation of nanoparticles from volatile drops

This study investigates pattern formation during evaporation of water-based nanofluid sessile droplets placed on a smooth silicon surface at various temperatures. An infrared thermography technique was employed to observe the temperature distribution along the air-liquid interface of evaporating droplets. In addition, an optical interferometry technique is used to quantify and characterize the deposited patterns. Depending on the substrate temperature, three distinctive deposition patterns are observed: a nearly uniform coverage pattern, a "dual-ring" pattern, and multiple rings corresponding to "stick-slip" pattern. At all substrate temperatures, the internal flow within the drop builds a ringlike cluster of the solute on the top region of drying droplets, which is found essential for the formation of the secondary ring deposition onto the substrate for the deposits with the "dual-ring" pattern. The size of the secondary ring is found to be dependent on the substrate temperature. For the deposits with the rather uniform coverage pattern, the ringlike cluster of the solute does not deposit as a distinct secondary ring; instead, it is deformed by the contact line depinning. In the case of the "stick-slip" pattern, the internal flow behavior is complex and found to be vigorous with rapid circulating flow which appears near the edge of the drop.

Sessile nanofluid droplet drying

Nanofluid droplet evaporation has gained much audience nowadays due to its wide applications in painting, coating, surface patterning, particle deposition, etc. This paper reviews the drying progress and deposition formation from the evaporative sessile droplets with the suspended insoluble solutes, especially nanoparticles. The main content covers the evaporation fundamental, the particle self-assembly, and deposition patterns in sessile nanofluid droplet. Both experimental and theoretical studies are presented. The effects of the type, concentration and size of nanoparticles on the spreading and evaporative dynamics are elucidated at first, serving the basis for the understanding of particle motion and deposition process which are introduced afterward. Stressing on particle assembly and production of desirable residue patterns, we express abundant experimental interventions, various types of deposits, and the effects on nanoparticle deposition. The review ends with the introduction of theoretical investigations, including the Navier-Stokes equations in terms of solutions, the Diffusion Limited Aggregation approach, the Kinetic Monte Carlo method, and the Dynamical Density Functional Theory. Nanoparticles have shown great influences in spreading, evaporation rate, evaporation regime, fluid flow and pattern formation of sessile droplets. Under different experimental conditions, various deposition patterns can be formed. The existing theoretical approaches are able to predict fluid dynamics, particle motion and deposition patterns in the particular cases. On the basis of further understanding of the effects of fluid dynamics and particle motion, the desirable patterns can be obtained with appropriate experimental regulations.

Fabrication of hierarchically porous materials and nanowires through coffee ring effect

We report a versatile method for the fabrication of nanowires and hierarchical porous materials from a wide variety of ceramic materials such as CaCO3, ZnO, CuO, Co3O4, Co-doped ZnO, and Ag2O. The method consists of evaporation of CO2-enriched water microdroplets (diameter ∼3 μm) deposited from an aerosol onto heated substrates (T = 120 °C). A variety of porous scaffolds with 1-3 μm sized pores can be generated by tuning the process conditions. Subsequent sintering of the scaffolds is shown to generate nanosized pores in the walls of the porous scaffold creating a dual hierarchy of pore sizes (∼50 nm and 1-3 μm). We propose a mechanism for the formation of scaffolds based on the coffee-ring effect during the evaporation of microdroplets. Ostwald-ripening of CaCO3 scaffolds prepared without sintering yields scaffold structures consisting of two-dimensional crystals of CaCO3 that are one unit cell thick. The favorable application of CaCO3 scaffolds for the enhancement of bone healing around titanium implants with improved biocompatibility is also demonstrated.