Molecular origin of contact line stick-slip motion during droplet evaporation

Liquid-liquid surfactant partitioning drives dewetting of oil from hydrophobic surfaces

HYPOTHESIS

Sessile droplets solubilizing in surfactant solution are frequently encountered in practice, but the factors governing their non-equilibrium dynamics are not well understood. Here, we investigate mechanisms by which solubilizing, sessile oil droplets in aqueous surfactant solution dewet from hydrophobic substrates and spread on hydrophilic substrates.

EXPERIMENTS

We quantify the dependence of droplet contact line dynamics on drop size and oil, surfactant, and substrate chemistries. We consider halogenated alkane oils as well as aromatic oils and focus on common nonionic nonylphenol ethoxylate surfactants. We correlate these results with measurements of the interfacial tensions.

FINDINGS

Counter-intuitively, under a range of conditions, we observe complete dewetting of oil from hydrophobic substrates but spreading on hydrophilic substrates. The timescales needed to reach a steady-state contact angle vary widely, with some droplets examined taking over a day. We find that surfactant surface adsorption governs the contact angle on shorter timescales, while partitioning of surfactant from water to oil, and oil solubilization into the water, act on longer timescales to facilitate the complete dewetting. Understanding of the role played by surfactant and oil transport presents opportunities for tailoring sessile droplet behaviors and controlling droplet dynamics under conditions that would previously not have been considered.

Interfacial Features Govern Nanoscale Jumping Droplets

The solid surfaces with different profile levels impact the liquid-solid contact nature and hence wetting characteristics, showing a vital role in liquid droplets' mobility and dynamic behaviors. Therefore, engineering nanostructured features ultimately enables tuning and controlling the dynamic motion of droplets. In this study, we demonstrate an approach to manipulate nanodroplets' motion behaviors in contact with a surface through tailoring the surface morphological profile. By tracking the trajectories of water molecules at the interface, we find that the motions of a nanodroplet subjected to different levels of lateral force reveal various modes that are identified as creeping, rolling, and jumping motions. Interestingly, the elastic deformation of the droplet and sudden changes in the receding contact angle provide the mechanistic origin for droplet jumping. Guided by computational simulations, a regime map delineating the droplet motion modes with surface profile levels and applied forces is constructed, providing a design strategy for controlling droplet motions via surface engineering.

Early-Stage Liquid Infiltration in Nanoconfinements

Liquid infiltration is one of the commonly adapted flow mechanisms in microscale/nanoscale heat-transfer applications. The theoretical modeling of dynamic infiltration profile in the microscale/nanoscale requires a deep study, because the acting forces are entirely different from those of a large-scale system. Herein, a model equation is developed from the fundamental force balance at the microscale/nanoscale level, to capture the dynamic infiltration flow profile. Molecular kinetic theory (MKT) is used to predict the dynamic contact angle. Molecular dynamics (MD) simulations are performed to study the capillary infiltration in two different geometries. The infiltration length is computed from the simulation results. The model is also evaluated over surfaces having different surface wettability. The generated model provides a better estimation of the infiltration length, compared to the well-established models. The developed model is expected to aid in the designing of microscale/nanoscale devices where liquid infiltration plays a key role.

Suppression of wetting transition on evaporative fakir droplets by using slippery superhydrophobic surfaces with low depinning force

This study experimentally investigated the evaporation and wetting transition behavior of fakir droplets on five different microstructured surfaces. Diamond-like carbon was introduced as the substrate, and the influence of varying the width, height, and pitch of the micropillars was assessed. The experimental results showed that the interfacial properties of the surfaces change the evaporation behavior and the starting point of the wetting transition. An important result of this study is the demonstration of a slippery superhydrophobic surface with low depinning force that suppresses the transition from the Cassie-Baxter state to the Wenzel state for microdroplets less than 0.37 mm in diameter, without employing large pillar height or multiscale roughness. By selecting an appropriate pillar pitch and employing tapered micropillars with small pillar widths, the solid-liquid contact at the three-phase contact line was reduced and low depinning forces were obtained. The underlying mechanism by which slippery superhydrophobic surfaces suppress wetting transitions is also discussed. The accuracy of the theoretical models for predicting the critical transition parameters was assessed, and a numerical model was developed in the surface evolver to compute the penetration of the droplet bottom meniscus within the micropillars.

Thermal capillary waves on bounded nanoscale thin films

The effect of confining walls on the fluctuation of a nanoscale thin film's free surface is studied using stochastic thin-film equations (STFEs). Two canonical boundary conditions are employed to reveal the influence of the confinement: (1) an imposed contact angle and (2) a pinned contact line. A linear stability analysis provides the wave eigenmodes, after which thermal-capillary-wave theory predicts the wave fluctuation amplitudes. Molecular dynamics (MD) simulations are performed to test the predictions, and a Langevin diffusion model is proposed to capture oscillations of the contact lines observed in MD simulations. Good agreement between the theoretical predictions and the MD simulation results is recovered, and it is discovered that confinement can influence the entire film. Notably, a constraint on the length scale of wave modes is found to affect fluctuation amplitudes from our theoretical model, especially for 3D films. This opens up challenges and future lines of inquiry.

Competition between thermal and surfactant-induced Marangoni flow in evaporating sessile droplets

HYPOTHESIS

Thermal Marangoni flow in evaporating sessile water droplets is much weaker in experiments than predicted theoretically. Often this is attributed to surfactant contamination, but there have not been any in-depth analyses that consider the full fluid and surfactant dynamics. It is expected that more insight into this problem can be gained by using numerical models to analyze the interplay between thermal Marangoni flow and surfactant dynamics in terms of dimensionless parameters.

SIMULATIONS

Two numerical models are implemented: one dynamic model based on lubrication theory and one quasi-stationary model, that allows for arbitrary contact angles.

FINDINGS

It is found that insoluble surfactants can suppress the thermal Marangoni flow if their concentration is sufficiently large and evaporation and diffusion are sufficiently slow. Soluble surfactants, however, either reduce or increase the interfacial velocity, depending on their sorption kinetics. Furthermore, insoluble surfactant concentrations that cause an order 0.1% surface tension reduction are sufficient to reduce the spatially averaged tangential flow velocity at the interface by a factor 100. For larger contact angles and smaller droplets this required concentration is larger (typically <1% surface tension reduction). The numerical models are mutually validated by comparing their results in cases where both are valid.

Surface morphology effects on clathrate hydrate wettability

HYPOTHESIS

Clathrate hydrates preferentially form at interfaces; hence, wetting properties play an important role in their formation, growth, and agglomeration. Experimental evidence suggests that the hydrate preparation process can strongly affect contact angle measurements, leading to the different results reported in the literature. These differences hamper technological progress. We hypothesize that changes in hydrate surface morphologies are responsible for the wide variation of contact angles reported in the literature.

EXPERIMENTS

Experimental testing of our hypothesis is problematic due to the preparation history of hydrates on their surface properties, and the difficulties in advanced surface characterization. Thus, we employ molecular dynamics simulations, which allow us to systematically change the interfacial features and the system composition. Implementing advanced algorithms, we quantify fundamental thermodynamic properties to validate our observations.

FINDINGS

We achieve excellent agreement with experimental observations for both atomically smooth and rough hydrate surfaces. Our results suggest that contact line pinning forces, enhanced by surface heterogeneity, are accountable for altering water contact angles, thus explaining the differences among reported experimental data. Our analysis and molecular level insights help interpret adhesion force measurements and yield a better understanding of the agglomeration between hydrate particles, providing a microscopic tool for advancing flow assurance applications.

Chitin-amyloid synergism and their use as sustainable structural adhesives

Structural adhesives are relevant to many engineering applications, especially those requiring load-bearing joints with high lap shear strength. Typical adhesives are synthesized from acrylics, epoxies, or urethanes, which may pose a burden to sustainability and the environment. In nature, the interfacial interactions between chitin and proteins are used for structural purposes and as a bio-cement, resulting in materials with properties unmatched by their man-made counterparts. Herein, we show that related supramolecular interactions can be harnessed to develop high strength green adhesives based on chitin nanocrystals (ChNCs), isolated from shrimp shells, and hen egg white lysozyme (HEWL) used in its monomeric or amyloid forms. Consolidation of the bicomponent suspensions, placed between glass substrates, results in long-range ordered superstructures. The formation of these structures is evaluated by surface energy considerations, followed by scanning electron, atomic force, and polarized microscopies of the consolidated materials. For 0.8 mg of bio-adhesive (lysozyme, ChNCs or their composites), lap shear loads of over 300 N are reached. Such remarkable adhesion reaches maximum values at protein-to-ChNC ratios below 1 : 4, reflecting the synergy established between the components (ca. 25% higher load compared to ChNCs, the strongest single component). We put the observed adhesive performance in perspective by comparing the lap-shear performance with current research on green supramolecular adhesives using natural biopolymers. The results are discussed in the context of current efforts to standardize the measurement of adhesive strength and bond preparation. The latter is key to formalizing the metrology and materials chemistry of bio-based adhesives. The proposed all-green system is expected to expand current developments in the design of bio-based adhesives.

Molecular View of the Distortion and Pinning Force of a Receding Contact Line: Impact of the Nanocavity Geometry

We present a molecular view using many-body dissipative particle dynamics simulations to unravel the pinning phenomenon of a liquid film receding over a solid substrate with a nanocavity. We find that the pinning force and distortion of the pinned contact line vary across different nanocavity shapes. We show that the mechanism of a caterpillar motion, which has previously been proposed for advancing precursor films, persists in a partially pinned receding contact line. Our results also demonstrate a localized clamping effect, which is originated from the variation of the dynamic contact angle along the pinned contact line. The simulation results suggest that the clamping effect can be controlled by the geometry of the nanocavity and hydrophilicity of the underlying substrate.

Absorption of surfactant-laden droplets into porous media: A numerical study

HYPOTHESIS

Droplets can absorb into permeable substrates due to capillarity. It is hypothesized that the contact line dynamics influence this process and that an unpinned contact line results in slower absorption than a pinned contact line, since the contact area between the droplet and the substrate will decrease over time for the former. Furthermore, it is expected that surfactants can be used to accelerate the absorption.

SIMULATIONS

Lubrication theory is employed to model the droplet and Darcy's law is combined with the conservation law of mass to describe the absorption dynamics. For the surfactant transport, several convection-diffusion-adsorption equations are solved.

FINDINGS

It is found that moving contact lines result in a parabola-shaped wetted area and a slower absorption and a deeper penetration depth than pinned contact lines. The evolution of the penetration depth was quantitatively validated by comparison with two experimental studies from literature. Surfactants were shown to accelerate the absorption process, but only if their adsorption kinetics are slow compared to the absorption. Otherwise, all surfactant adsorbs onto the pore walls before reaching the wetting front, resulting in the same absorption rate as without surfactants. This behavior agrees with both experimental and analytical literature.

The evaporation of surfactant-laden droplets: A comparison between contact line models

HYPOTHESIS

There are two different sharp-interface models for moving contact lines: slip models and precursor film models. While both models predict a mostly constant contact angle during the evaporation of pure droplets, it is expected that they behave differently when surfactants are present, because of the inherent dissimilarities in their respective interface definitions.

SIMULATIONS

Both contact line models are numerically implemented using lubrication theory to analyze evaporating droplets. A convection-diffusion equation is implemented for insoluble surfactants. For pure droplets the models are compared with experiments performed by Nguyen et al. (2012).

FINDINGS

The two contact line models show results comparable to the experiments with pure droplets. If insoluble surfactants are present, the slip model increasingly shows pinning-like behavior as the initial surfactant concentration is increased. This 'quasi-pinning' is found to be consistent with experimental results in literature. The precursor film model, in contrast, shows no significant change when surfactants are added. This lack of change is a result of surfactant flowing from the droplet into the precursor film and vice versa. While suggesting potential solutions to this unphysical behavior, it is concluded that in the context of surfactants, slip models are preferable over precursor film models given the current state of the art.

Size dependent influence of contact line pinning on wetting of nano-textured/patterned silica surfaces

Wetting behavior on a heterogeneous surface undergoes contact angle hysteresis as the droplet stabilized at a metastable state with a contact angle significantly different from its equilibrium value due to contact line pinning. However, there is a lack of consensus on how to calculate the influence of pinning forces. In general, the pinning effect can be characterized as (i) microscopic behavior when a droplet is pinned and the contact angle increases/decreases as the droplet volume increases/decreases and (ii) macroscopic behavior as the pinning effects decrease and ultimately, disappear with the increase of the droplet size. The current work studied both behaviors using molecular dynamics (MD) simulation with more than 300 different size water droplets on silica surfaces with three different patterns across two different wetting conditions. Results showed that the contact angle increases linearly with increasing droplet volume through the microscopic behavior, while the droplet is pinned on top of a certain number of patterns. When we normalized the droplet size with the corresponding pattern size, we observed a "wetting similarity" that linear microscopic contact angle variations over different size heterogeneities continuously line up. This shows that the pinning force remains constant and the resulting pinning effects are scalable by the size ratio between the droplet and pattern, independent of the size-scale. The slope of these microscopic linear variations decreases with an increase in the droplet size as observed through the macroscopic behavior. We further found a universal behavior in the variation of the corresponding pinning forces, independent of the wetting condition. In macroscopic behavior, pinning effects become negligible and the contact angle reaches the equilibrium value of the corresponding surface when the diameter of the free-standing droplet is approximately equal to 24 times the size of the surface structure. We found that the pinning effect is scalable with the droplet volume, not the size of the droplet base.

Evaporation of Initially Heated Sessile Droplets and the Resultant Dried Colloidal Deposits on Substrates Held at Ambient Temperature

The present study experimentally and numerically investigates the evaporation and resultant patterns of dried deposits of aqueous colloidal sessile droplets when the droplets are initially elevated to a high temperature before being placed on a substrate held at ambient temperature. The system is then released for natural evaporation without applying any external perturbation. Infrared thermography and optical profilometry are used as essential tools for interfacial temperature measurements and quantification of coffee-ring dimensions, respectively. Initially, a significant temperature gradient exists along the liquid-gas interface as soon as the droplet is deposited on the substrate, which triggers a Marangoni stress-induced recirculation flow directed from the top of the droplet toward the contact line along the liquid-gas interface. Thus, the flow is in the reverse direction to that seen in the conventional substrate heating case. Interestingly, this temperature gradient decays rapidly within the first 10% of the total evaporation time and the droplet-substrate system reaches thermal equilibrium with ambient thereafter. Despite the fast decay of the temperature gradient, the coffee-ring dimensions significantly diminish, leading to an inner deposit. A reduction of 50-70% in the coffee-ring dimensions is recorded by elevating the initial droplet temperature from 25 to 75 °C for suspended particle concentration varying between 0.05 and 1.0% v/v. This suppression of the coffee-ring effect is attributed to the fact that the initial Marangoni stress-induced recirculation flow continues until the last stage of evaporation, even after the interfacial temperature gradient vanishes. This is essentially a consequence of liquid inertia. Finally, a finite-element-based two-dimensional modeling in axisymmetric geometry is found to capture the measurements with reasonable fidelity and the hypothesis considered in the present study corroborates well with a first approximation qualitative scaling analysis. Overall, together with a new experimental condition, the present investigation discloses a distinct nature of Marangoni stress-induced flow in a drying droplet and its role in influencing the associated colloidal deposits, which was not explored previously. The insights gained from this study are useful to advance technical applications such as spray cooling, inkjet printing, bioassays, etc.

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.

Microscopic Origin of Capillary Force Balance at Contact Line

We investigate the underlying mechanism of capillary force balance at the contact line. In particular, we offer a novel approach to describe and quantify the capillary force on the liquid in coexistence with its vapor phase, which is crucial in wetting and spreading dynamics. Its relation with the interface tension is elucidated. The proposed model is verified by our molecular dynamics simulations over a wide contact angle range. Differences in capillary forces are observed in evaporating droplets on homogeneous and decorated surfaces. Our findings not only provide a theoretical insight into capillary forces at the contact line, but also validate Young's equation based on a mechanical interpretation.

Statics and dynamics of polymeric droplets on chemically homogeneous and heterogeneous substrates

We present a molecular dynamics study of the motion of cylindrical polymer droplets on striped surfaces. We first consider the equilibrium properties of droplets on different surfaces, we show that for small stripes the Cassie-Baxter equation gives a good approximation of the equilibrium contact angle. As the stripe width becomes nonnegligible compared to the dimension of the droplet, it has to deform significantly to minimize its free energy; this results in a smaller value of the contact angle than the continuum model predicts. We then evaluate the slip length and thus the damping coefficient as a function of the stripe width. For very small stripes, the heterogeneous surface behaves as an effective surface, with the same damping as a homogeneous surface with the same contact angle. However, as the stripe width increases, damping at the surface increases until reaching a plateau. Afterwards, we study the dynamics of droplets under a bulk force. We show that if the stripes are large enough the droplets are pinned until a critical force. The critical force increases linearly with stripe width. For large enough forces, the average velocity increases linearly with the force, we show that it can then be predicted by a model depending only on droplet size, contact angle, viscosity, and slip length. We show that the velocity of the droplet varies sinusoidally as a function of its position on the substrate. However, for bulk forces just above the depinning force we observe a characteristic stick-slip motion, with successive pinnings and depinnings.

Temperature regulation of the contact angle of water droplets on the solid surfaces

We investigate theoretically the stability of the wetting property, i.e., the contact angle values, as a function of the temperature. We find that the estimated temperature coefficient of the contact angle for the water droplets on an ordered water monolayer on a 100 surface of face-center cubic (FCC) is about one order of magnitude larger than that on a hydrophobic hexagonal surface in the temperature range between 290 K and 350 K, using molecular dynamics simulations. As temperature rises, the number of hydrogen bonds between the ordered water monolayer and the water droplet will increase, which therefore enhances the hydrophilicity of the ordered water monolayer at the FCC model surface. Our work thus provides an easily controllable and reversible way to control the degree of hydrophobicity of various solid surfaces exhibiting a similar wetting property of water droplets on the ordered water monolayer as such particular FCC (100) surfaces.

Pinning-Depinning Mechanism of the Contact Line during Evaporation of Nanodroplets on Heated Heterogeneous Surfaces: A Molecular Dynamics Simulation

Droplet evaporation on heterogeneous or patterned surfaces has numerous potential applications, for example, inkjet printing. The effect of surface heterogeneities on the evaporation of a nanometer-sized cylindrical droplet on a solid surface is studied using molecular dynamics simulations of Lennard-Jones particles. Different heterogeneities of the surface were achieved through alternating stripes of equal width but two chemical types, which lead to different contact angles. The evaporation induced by the heated substrate instead of the isothermal evaporation is investigated. It is found that the whole evaporation process is generally dominated by the nonuniform evaporation effect. However, at the initial moment, the volume expansion and local evaporation effects play important roles. From the nanoscale point of view, the slow movement of the contact line during the pinning process is observed, which is different from the macroscopic stationary pinning. Particularly, we found that the speed of the contact line may be not only affected by the intrinsic energy barrier between the two adjacent stripes ( ũ) but also relevant to the evaporation rate. Generally speaking, the larger the intrinsic energy barrier, the slower the movement of the contact line. At the specified temperature, when ũ is less than a critical energy barrier ( ũ*), the speed of the contact line would increase with the evaporate rate. When ũ > ũ*, the speed of the contact line is determined only by ũ and no longer affected by the evaporation rate at different stages (the first stick and the second stick).

Double-Emulsion Drop Evaporation and Formation of a Daughter Droplet

In this study, we present experimental and theoretical analyses of evaporating a double-emulsion drop resting on a substrate. Multistage evaporation of the outer and inner droplet is witnessed. The complete evaporation of the outer drop and the initialization of the inner drop evaporation demonstrate an interesting transition dynamics. After the apparent completion of evaporation of the inner phase of a double-emulsion drop, surprisingly, formation of a daughter droplet is observed. We further investigated to hypothesize this phenomenon and achieved the formation of the daughter droplet for a single-phase drop as well. While engineering the "daughter drop formation" phenomena, we also proposed a way to obtain prolonged fixed contact line evaporation for a single-phase drop.

Patterning Bubbles by the Stick-Slip Motion of the Advancing Triple Phase Line on Nanostructures

The stick-slip motion of the triple phase contact line (TCL) has wide applications in inkjet printing, surface coatings, functional material assembly, and device fabrication. Here, for the first time, we report that on an alumina substrate with nanostructures, the stick-slip motion of the advancing TCL during spreading of an emulsion droplet can serve as an effective nanopatterning process. Air enclosed in the substrate nanostructures can be exchanged with liquid during the "stick" phase, resulting in the formation of bubbles arranged in a ring pattern. The process takes place in two stages: rings of air form first and then, as the volume of air increases, they separate into air bubbles as a result of the Plateau Rayleigh instability. During the first stage, the rings form due to the stick-slip of the advancing TCL and are ascribed to hydrogen-bonding interactions. Ultimate bubble size is dependent on the substrate pore dimensions. The process was simulated using finite-element analysis to elucidate the mechanism associated with subsequent bubble formation. The simulations corroborate well with the experimental results. This stick-slip motion of the advancing TCL provides new insights into the phenomena associated with droplet spreading and wetting, and the ability to control the formation of patterned bubbles will be promising in applications ranging from microfluidics to printing of functional materials and devices based on bubble templates and applications requiring submerged hydrophobic surface.

Controlling breath figure patterns on PDMS by concentration variation of ethanol-methanol binary vapors

In this paper, the self-assembly of condensed droplets on smooth and constrained surfaces under saturated vapor atmosphere of ethanol and methanol binary system is reported. Hexagonally ordered array of pores are obtained on smooth surfaces with saturated vapors of binary liquids without the assistance of any additives. The results show that the addition of a small amount of ethanol to methanol plays a role very similar to that of surface active agents in inducing the formation of a regular droplet array. The effect of constraints on a self-assembled droplet pattern such as the movement of the contact line and the depinning of the contact line is also investigated. It is observed that the pore size, pore shape, pore depth and ring diameter are influenced by the atmosphere of binary vapors in addition to the commonly held attribution to the surface tension of the solvent. Contact angle studies of the patterned substrates show hydrophobicity with high adhesiveness and transitions between the Wenzel and Cassie impregnating state over the entire concentration region.

Pinning-Depinning Mechanisms of the Contact Line during Evaporation of Microdroplets on Rough Surfaces: A Lattice Boltzmann Simulation

In this study, pinning and depinning of the contact line during droplet evaporation on the rough surfaces with randomly distributed structures is theoretically analyzed and numerically investigated. A fast Fourier transformation (FFT) method is used to generate the rough surfaces, whose skewness ( Sk), kurtosis ( K), and root-mean-square ( Rq) are obtained from real surfaces. A thermal multiphase LB model is proposed to simulate the isothermal pinning and depinning processes. The evaporation processes are recorded with the variations in contact angle, contact radius, and drop shape. It is found that the drops sitting on rough surfaces show different behavior from those on smoother surfaces. The former shows a pinned contact line during almost the whole lifetime. By contrast, the latter experiences a stick-slip-jump behavior until the drop disappears. At mesoscopic scale, the pinning of the contact line is actually a slow motion rather than a complete immobilization at the sharp edges. The dynamic equilibrium is achieved by the self-adjustment of the contact line according to each edge.

Dynamics of Contact Line Pinning and Depinning of Droplets Evaporating on Microribs

The contact line dynamics of evaporating droplets deposited on a set of parallel microribs is analyzed with the use of a recently developed entropic lattice Boltzmann model for two-phase flow. Upon deposition, part of the droplet penetrates into the space between ribs because of capillary action, whereas the remaining liquid of the droplet remains pinned on top of the microribs. In the first stage, evaporation continues until the droplet undergoes a series of pinning-depinning events, showing alternatively the constant contact radius and constant contact angle modes. While the droplet is pinned, evaporation results in a contact angle reduction, whereas the contact radius remains constant. At a critical contact angle, the contact line depins, the contact radius reduces, and the droplet rearranges to a larger apparent contact angle. This pinning-depinning behavior goes on until the liquid above the microribs is evaporated. By computing the Gibbs free energy taking into account the interfacial energy, pressure terms, and viscous dissipation due to drop internal flow, we found that the mechanism that causes the unpinning of the contact line results from an excess in Gibbs free energy. The spacing distance and the rib height play an important role in controlling the pinning-depinning cycling, the critical contact angle, and the excess Gibbs free energy. However, we found that neither the critical contact angle nor the maximum excess Gibbs free energy depends on the rib width. We show that the different terms, that is, pressure term, viscous dissipation, and interfacial energy, contributing to the excess Gibbs free energy, can be varied differently by varying different geometrical properties of the microribs. It is demonstrated that, by varying the spacing distance between the ribs, the energy barrier is controlled by the interfacial energy while the contribution of the viscous dissipation is dominant if either rib height or width is changed. Main finding of this is study is that, for microrib patterned surfaces, the energy barrier required for the contact line to depin can be enlarged by increasing the spacing or the rib height, which can be important for practical applications.

Solvent Retention and Crack Evolution in Dropcast PEDOT:PSS and Dependence on Surface Wetting

The drying of nanocolloidal polymers is governed by the interplay among surface tension, evaporation, and contact-line pinning, among other phenomena. Here, we describe the sequential evolution of poly-3,4-ethylenedioxythiophene:poly(styrene sulfonate) (PEDOT:PSS) through two distinct regimes evidenced by annular or radial cracking and show that the cracking dynamics and solvent-retention postdrying and postcracking are mediated by wetting to the substrate surface. The corresponding changes in the PEDOT:PSS morphology are also observed to relate to the radial or cracking dynamics. It is suggested that the wetting-dependent effect offers a route to control morphology, understand solvent retention, and reduce cracking in polymer latex films. This study highlights the importance of substrate choice, an underexplored area of investigation in the study of colloidal materials.

Thermodynamic formulation of the barrier for heterogeneous pinned nucleation: Implication to the crossover scenarios associated with barrierless and homogeneous nucleation

The effect of contact line pinning on nucleation is reported using continuum thermodynamics. Based on the principle of the free-energy maximization, closed-form expressions in the dimensionless form for the free-energy of the three-phase metastable system and the thermodynamic barrier are formulated with respect to the system geometry and the substrate wettability. The condition of maximality limits the dynamic contact angle within the cluster-phase-phobic regime. The dimensionless nucleation barrier or the potency factor can be divided into two components related to the system geometry and the pinning effect. Depending on the relative value of the equilibrium and the critical dynamic contact angle, the contact line pinning can either have favorable or adverse effects. Associated pinning-depinning transition can also lead to the crossovers related to barrierless and homogeneous nucleation. Contact line tension is found to have a considerable effect during these transitional scenarios. Complete wetting transition associated with barrierless nucleation can take place due to the presence of tensile (negative) line tension. On the other hand, complete drying transition related to homogeneous nucleation can occur when line tension is compressive (positive) in nature. The pinning has a favorable effect only when the substrate wettability is within the cluster-phase-philic regime. There can be favorable, adverse, or no pinning effects when the substrate wettability is within the cluster-phase-phobic regime. Although the contact line is pinned, the minimum value of the potency factor is obtained when equilibrium and dynamic contact angles are equal.

Hand-Held Femtogram Detection of Hazardous Picric Acid with Hydrophobic Ag Nanopillar SERS Substrates and Mechanism of Elasto-Capillarity

Picric acid (PA) is a severe environmental and security risk due to its unstable, toxic, and explosive properties. It is also challenging to detect in trace amounts and in situ because of its highly acidic and anionic character. Here, we assess sensing of PA under nonlaboratory conditions using surface-enhanced Raman scattering (SERS) silver nanopillar substrates and hand-held Raman spectroscopy equipment. The advancing elasto-capillarity effects are explained by molecular dynamics simulations. We obtain a SERS PA detection limit on the order of 20 ppt, corresponding attomole amounts, which together with the simple analysis methodology demonstrates that the presented approach is highly competitive for ultrasensitive analysis in the field.

Pinning-Depinning Mechanism of the Contact Line during Evaporation on Chemically Patterned Surfaces: A Lattice Boltzmann Study

In this paper, the pinning and depinning mechanism of the contact line during droplet evaporation on chemically stripe-patterned surfaces is numerically investigated using a thermal multiphase lattice Boltzmann (LB) model with liquid-vapor phase change. A local force balance in the context of diffuse interfaces is introduced to explain the equilibrium states of droplets on chemically patterned surfaces. It is shown that when the contact line is pinned on a hydrophobic-hydrophilic boundary, different contact angles can be interpreted as the variation of the length of the contact line occupied by each component. The stick-slip-jump behavior of evaporating droplets on chemically patterned surfaces is well captured by the LB simulations. Particularly, a slow movement of the contact line is clearly observed during the stick (pinning) mode, which shows that the pinning of the contact line during droplet evaporation on chemically stripe-patterned surfaces is actually a dynamic pinning process and the dynamic equilibrium is achieved by the self-adjustment of the contact lines occupied by each component. Moreover, it is shown that when the surface tension varies with the temperature, the Marangoni effect has an important influence on the depinning of the contact line, which occurs when the horizontal component (toward the center of the droplet) of the force caused by the Marangoni stress overcomes the unbalanced Young's force toward the outside.

Stick-Jump (SJ) Evaporation of Strongly Pinned Nanoliter Volume Sessile Water Droplets on Quick Drying, Micropatterned Surfaces

We present an experimental study of stick-jump (SJ) evaporation of strongly pinned nanoliter volume sessile water droplets drying on micropatterned surfaces. The evaporation is studied on surfaces composed of photolithographically micropatterned negative photoresist (SU-8). The micropatterning of the SU-8 enables circular, smooth, trough-like features to be formed which causes a very strong pinning of the three phase (liquid-vapor-solid) contact line of an evaporating droplet. This is ideal for studying SJ evaporation as it contains sequential constant contact radius (CCR) evaporation phases during droplet evaporation. The evaporation was studied in nonconfined conditions, and forced convection was not used. Micropatterned concentric circles were defined having an initial radius of 1000 μm decreasing by a spacing ranging from 500 to 50 μm. The droplet evaporates, successively pinning and depinning from circle to circle. For each pinning radius, the droplet contact angle and volume are observed to decrease quasi-linearly with time. The experimental average evaporation rates were found to decrease with decreasing pining radii. In contrast, the experimental average evaporation flux is found to increase with decreasing droplet radii. The data also demonstrate the influence of the initial contact angle on evaporation rate and flux. The data indicate that the total evaporation time of a droplet depends on the specific micropattern spacing and that the total evaporation time on micropatterned surfaces is always less than on flat, homogeneous surfaces. Although the surface patterning is observed to have little effect on the average droplet flux-indicating that the underlying evaporation physics is not significantly changed by the patterning-the total evaporation time is considerably modified by patterning, up to a factor or almost 2 compared to evaporation on a flat, homogeneous surface. The closely spaced concentric circle pinning maintains a large droplet radius and small contact angle from jump to jump; the result is a large evaporation rate leading to faster evaporation.

Controlling particle deposit morphologies in drying nano-particle laden sessile droplets using substrate oscillations

Sessile water droplets containing nano-silica particles are allowed to evaporate in the presence of driven substrate oscillations at chosen frequencies.