Wetting and evaporation of salt-water nanodroplets: A molecular dynamics investigation

A Novel Electrophoretic Technique to Improve Metasurface Sensing of Low Concentration Particles in Solution

A novel electrophoretic technique to improve metasurface sensing capabilities of charged particles in solution is presented. The proposed technique may improve the ability of metasurfaces to sense charged particles in solution in a manner not possible using the current standard of particle deposition (which allows particles to sediment randomly on a metasurface under evaporation) by inducing an external, nonuniform electric field through the metasurface apertures. Such a technique may be useful in various sensing applications, such as in biological, polymer, or environmental sciences, where low concentration particles in solution are of interest. The electrophoretic technique was simulated and experimentally tested using latex nanoparticles in solution. The results suggest that, using this technique, one may theoretically increase the particle density within the metasurface regions of greatest sensitivity by nearly 1900% in comparison to random sedimentation due to evaporation. Such an increase in particle density within the regions of greatest sensitivity may facilitate more precise material property measurements and enhance identification and detection capabilities of metasurfaces to particles in solution which constitute only a few hundred parts per million by mass. It was experimentally determined that the electrophoretic technique enhanced metasurface sensing capabilities of 333 parts per million by mass latex nanoparticle solutions by nearly 1700%.

Gradient-Charged Hydrogels for Highly Efficient Solar Steam Generation and Desalination

Solar-driven interface desalination (SDID) is a promising and sustainable technology that produces freshwater from brine. Ionic hydrogels are effective evaporators, providing enhanced interaction with water and ions due to the charged groups on hydrophilic polymer networks. In this study, we developed a hydrogel-based solar steam generator with a gradient-charged (GC) structure for desalination. The gradient-charged groups' distribution on the hydrogels creates gradients of free water and osmotic pressure, realizing rapid water supplement as well as desalination of concentrated brine. Consequently, the GC hydrogel demonstrated an exceptional water evaporation rate, achieving a value as high as 2.61 kg m-2 h-1 in pure water and 1.72 kg m-2 h-1 on treating with 20 wt % NaCl solution under one sun illumination. Following the substantial solar-driven evaporation, impurities, including salt and other pollutants, were removed, thereby ensuring the purity of the condensed water. Overall, the GC hydrogel-based evaporator is a promising solution for SDID to achieve effective and sustainable water desalination.

Enhanced efficiency of water desalination in nanostructured thin-film membranes with polymer grafted nanoparticles

Polyamide composite (PA-TFC) membranes are the state-of-the-art ubiquitous platforms to desalinate water at scale. We have developed a novel, transformative platform where the performance of such membranes is significantly and controllably improved by depositing thin films of polymethylacrylate [PMA] grafted silica nanoparticles (PGNPs) through the venerable Langmuir-Blodgett method. Our key practically important finding is that these constructs can have unprecedented selectivity values (i.e., ∼250-3000 bar^-1, >99.0% salt rejection) at reduced feed water pressure (i.e., reduced cost) while maintaining acceptable water permeance A (= 2-5 L m^-2 h^-1 Bar^-1) with as little as 5-7 PGNP layers. We also observe that the transport of solvent and solute are governed by different mechanisms, unlike gas transport, leading to independent control of A and selectivity. Since these membranes can be formulated using simple and low cost self-assembly methods, our work opens a new direction towards development of affordable, scalable water desalination methods.

Impact of nanodroplets on cone-textured surfaces

Molecular dynamics simulations have been performed to study the dynamics of nanodroplets impacting on a flat superhydrophobic surface and surfaces covered with nanocone structures. We present a panorama of nanodroplet behaviors for a wide range of impact velocities and different cone geometrics, and develop a model to predict whether a nanodroplet impacting onto cone-textured surfaces will touch the underlying substrate during impact. The advantages and disadvantages of applying nanocone structures to the solid surface are revealed by the investigations into restitution coefficient and contact time. The effects of nanocone structures on droplet bouncing dynamics are probed using momentum analysis rather than conventional energy analysis. We further demonstrate that a single Weber number is inadequate for unifying the dynamics of macroscale and nanoscale droplets on cone-textured surfaces, and propose a combined dimensionless number to address it. The extensive findings of this study carry noteworthy implications for engineering applications, such as nanoprinting and nanomedicine on functional patterned surfaces, providing fundamental support for these technologies.

Mechanism, Kinetics, and Potential of Mean Force of Evaporation of Water from Aqueous Sodium Chloride Solutions of Varying Concentrations

The mechanism, kinetics, and potential of mean force of evaporation of water from aqueous NaCl solutions are investigated through both unbiased molecular dynamics simulations and also biased simulations using the umbrella sampling method. The results are obtained for aqueous solutions of three different NaCl concentrations ranging from 0.6 to 6.0 m and also for pure water. The rate of evaporation is found to decrease in the presence of ions. It is found that the process of evaporation of a surface water molecule from ionic solutions can be triggered through its collision with another water or chloride ion. Such collisions provide the additional kinetic energy that is required for evaporation. However, when the collision takes place with a Cl^- ion, the evaporation of the escaping water also involves a collision with water in the vicinity of the ion at the same time along with the ion-water collision. These two collisions together provide the required kinetic energy for escape of the evaporating water molecule. Thus, the mechanism of evaporation process of ionic solutions can be more complex than that of pure water. The potential of mean force (PMF) of evaporation is found to be positive and it increases with increasing ion concentration. Also, no barrier in the PMF is found to be present for the condensation of water from vapor phase to the surfaces of the solutions. A detailed analysis of the unsuccessful evaporation attempts by surface water molecules is also made in the current study.

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.

Confinement Dynamics of Nanodroplets between Two Surfaces: Effects of Wettability and Electric Field

The electrowetting effect and related applications of tiny droplets have aroused widespread research interest. In this work, we report molecular dynamics simulations of confinement dynamics of nanodroplets under different droplet-surface interactions and surface distances under an external electric field. So far, the effect of the surface-droplet interactions on electric field-induced dynamics behaviors of droplets in confined spaces has not been extensively studied. Our results show that in the absence of electric field there is a critical value of surface wettability for the shape transition of droplets. Above this value, the droplet is divided into small droplets adhered on the bottom and top surfaces; below this value, the droplets are detached from the surfaces. When an external electric field is applied parallel to the surfaces, the droplet spreads on the surface along the direction of the electric field. It was found that the surface separation significantly influences the transition of the droplet shape. The steady morphology of the droplets under the electric field depends on the surface-droplet interaction and surface separation. We explore the underlying mechanism causing the morphological transition through analyzing the molecular interactions, the number of interracial molecules and the interaction force between the droplets and surfaces. These results provide basic insights into the molecular interactions of nanodroplets under different confined environments, and clues for applications of confined nanodroplets under the control of electric field.

Evaporation Dynamics of Sessile Saline Microdroplets in Oil

The occurrence of concentration and temperature gradients in saline microdroplets evaporating directly in air makes them unsuitable for nucleation studies where homogeneous composition is required. This can be addressed by immersing the droplet in oil under regulated humidity and reducing the volume to the picoliter range. However, the evaporation dynamics of such a system is not well understood. In this work, we present evaporation models applicable for arrays of sessile microdroplets with dissolved solute submerged in a thin layer of oil. Our model accounts for the variable diffusion distance due to the presence of the oil film separating the droplet and air, the variation of the solution density and water activity due to the evolving solute concentration, and the diffusive interaction between neighboring droplets. Our model shows excellent agreement with experimental data for both pure water and NaCl solution. With this model, we demonstrate that assuming a constant evaporation rate and neglecting the diffusive interactions can lead to severe inaccuracies in the measurement of droplet concentration, particularly during nucleation experiments. Given the significance of droplet evaporation in a wide array of scientific and industrial applications, the models and insights presented herein would be of great value to many fields of interest.

Highly stable gold nanolayer membrane for efficient solar water evaporation under a harsh environment

Interfacial solar water evaporation has attracted tremendous attention for sunlight harvesting for water purification. However, salt formation and stability of the photothermal materials (PTMs) remain a challenge that need addressing before bringing this technology to real-world applications. In this work, a nanoscale thin film of gold (Au) on a polytetrafluoroethylene (PTFE) membrane has been prepared using a magnetic sputtering technique. The fabricated membrane displays a robust mechanical strength and chemical stability arising from the adhesiveness of the thin film Au nanolayer on the PTFE membrane as well as the chemical inertness of the noble metal PTM. The Au nanolayer/PTFE membrane with cellulose sponge substrate resulted in an evaporation rate of 0.88 kg m^-2 h^-1 under 1 sun intensity. Remarkable salt ion rejection of 99.9% has been obtained, meeting the required standard for drinking water. Moreover, the membrane exhibited excellent stability and reusability in natural seawater and high salinity brine (150 g/L) and even in severe conditions (acidic, basic, and oxidized). No noticeable salt formation was observed on the evaporator surface after the tests. These findings reveal promising prospects for using a magnetron sputtering technique to fabricate a stable photothermal membrane for seawater and high salinity brine desalination.

Evaporation of Water Nanodroplets on Heated Surfaces: Does Nano Matter?

While experiments and continuum models have provided a relatively good understanding of the evaporation of macroscopic water droplets, elucidating how sessile nanodroplets evaporate is an open question critical for advancing nanotechnological applications where nanodroplets can play an essential role. Here, using molecular dynamics simulations, we find that evaporating nanodroplets, in contrast to their macroscopic counterparts, are not always in thermal equilibrium with the substrate and that the vapor concentration on the nanodroplet surface does not reach a steady state. As a result, the evaporative behavior of nanodroplets is significantly different. Regardless of hydrophobicity, nanodroplets do not follow conventional evaporation modes but instead exhibit dynamic wetting behavior characterized by huge, non-equilibrium, isovolumetric fluctuations in the contact angle and contact radius. For hydrophilic nanodroplets, the evaporation rate, controlled by the vapor concentration, decays exponentially over time. Hydrophobic nanodroplets follow stretched exponential kinetics arising from the slower thermalization with the substrate. The evaporative half-lifetime of the nanodroplets is directly related to the thermalization time scale and therefore increases monotonically with the hydrophobicity of the substrate. Finally, the evaporative flux profile along the nanodroplet surface is highly nonuniform but does not diverge at the contact line as the macroscopic continuum models predict.

Wetting characteristics of polymer adhesives with different chain bending stiffness

Polymer adhesives are widely used in daily applications and in industry owing to their flexibility and overall non-toxicity, particularly in interfacial adhesion. The spreading of polymer adhesives on adherend is one of the essential considerations for the interfacial adhesion of polymer adhesives, which is strongly related to their wetting behaviors. While relationships between polymer microstructure and adhesion have been investigated in previous studies, it remains challenging to unveil the effect of polymer microstructure on wettability. To address this issue, here we utilize coarse-grained molecular dynamics (CGMD) simulations to systematically elucidate how the wettability of a polymer adhesive droplet on a surface depends on bending stiffness. The wetting dynamics and the contact angle are studied to show the evolution of morphology of droplets during the wetting process. The results indicate the wettability is weakened by the increase of bending stiffness of polymer chain. Detailed thermodynamic property analysis is further conducted, revealing that the adhesion between the polymer droplet and substrate deteriorates due to the decline of wettability. Interestingly, we observe such deterioration becomes more significant by both increasing the temperature and decreasing the bending stiffness. Our study sheds light on the dependence of chain bending stiffness and temperature on the wetting behavior of polymer adhesive droplets, and offers insights, which, upon experimental validation can then be used for the design of adhesives or hydrogels.

3D Photothermal Cryogels for Solar-Driven Desalination

This paper reports the fabrication of photothermal cryogels for freshwater production via the solar-driven evaporation of seawater. Photothermal cryogels were prepared via in situ oxidative polymerization of pyrrole with ammonium persulfate on preformed poly(sodium acrylate) (PSA) cryogels. We found that the pyrrole concentration used in the fabrication process has a significant effect on the final PSA/PPy cryogels (PPCs), causing the as-formed polypyrrole (PPy) layer on the PPC to evolve from nanoparticles to lamellar sheets and to consolidated thin films. PPC fabricated using the lowest pyrrole concentration (i.e., PPC10) displays the best solar-evaporation efficiency compared to the other samples, which is further improved by switching the operative mode from floating to standing. Specifically, in the latter case, the apparent solar evaporation rate and solar-to-vapor conversion efficiency reach 1.41 kg m-2 h-1 and 96.9%, respectively, due to the contribution of evaporation from the exposed lateral surfaces. The distillate obtained from the condensed vapor, generated via solar evaporation of a synthetic seawater through PPC10, shows an at least 99.99% reduction of Na while all the other elements are reduced to a subppm level. We attribute the superior solar evaporation and desalination performance of PPC10 to its (i) higher photoabsorption efficiency, (ii) higher heat localization effect, (iii) open porous structure that facilitates vapor removal, (iv) rough pore surface that increases the surface area for light absorption and water evaporation, and (v) higher water-absorption capacity to ensure efficient water replenishment to the evaporative sites. It is anticipated that the gained know-how from this study would offer insightful guidelines to better designs of polymer-based 3D photothermal materials for solar evaporation as well as for other emerging solar-related applications.

Off-Lattice Monte-Carlo Approach for Studying Nucleation and Evaporation Phenomena at the Molecular Scale

Droplet nucleation and evaporation are ubiquitous in nature and many technological applications, such as phase-change cooling and boiling heat transfer. So far, the description of these phenomena at the molecular scale has posed challenges for modelling with most of the models being implemented on a lattice. Here, we propose an off-lattice Monte-Carlo approach combined with a grid that can be used for the investigation of droplet formation and evaporation. We provide the details of the model, its implementation as Python code, and results illustrating its dependence on various parameters. The method can be easily extended for any force-field (e.g., coarse-grained, all-atom models, and external fields, such as gravity and electric field). Thus, we anticipate that the proposed model will offer opportunities for a wide range of studies in various research areas involving droplet formation and evaporation and will also form the basis for further method developments for the molecular modelling of such phenomena.

Differences between Colloidal and Crystalline Evaporative Deposits

Evaporative deposits from drops are widely studied due to their numerous applications in low-effort self-assembly, including for inkjet printing, microscale separations, and sensing/diagnostics. This phenomenon has been broadly explored for drops containing suspended colloidal particles but has been less quantified for drops with dissolved solutes. When a drop of solute/solvent mixture is evaporated on a substrate, nonvolatile solutes become supersaturated as the solvent evaporates, which then leads to crystal nucleation at the substrate-drop contact line. Emerging crystals alter the local wettability and fundamentally alter the dynamics of evaporation, which, in turn, influences the resultant evaporative deposit. Here we investigate the role of interactions between the substrate, crystals, and solution by comparing the evaporative deposition of three different salts as solutes against an evaporating colloidal solution. We show that nucleation effects can cause crystalline deposits to have a temperature relationship that is opposite to that of colloidal deposits and demonstrate how a balance between the contact-line pinning force and nucleation controls the deposit size.

Crystallization-Driven Flows within Evaporating Aqueous Saline Droplets

Using micro-PIV (particle image velocimetry), we observe for the first time, the direct correlation between crystallization and hydrodynamics in evaporating microliter saline (1 M NaCl) sessile drops. The relationship is demonstrated by a remarkable jet of liquid along the base of the drops, induced by, and directed at the point of nucleation and subsequent crystal growth. Prior to nucleation, the flow is more uniformly outward with the magnitude of the velocity decreasing with time. From calculations and the flow measurements in the two observed stages of evaporation (prior to nucleation and during crystallization), this jet can be explained on the basis of competition between solutal Marangoni convection and mass conservation flow. The jet of fluid leads to vortices on either side of the crystal in which the salt concentration is reduced, providing a potential explanation as to why NaCl deposits as a sequence of discrete crystals rather than as a continuous ring for such drops.

Interaction Between the Cyclopentane Hydrate Particle and Water Droplet in Hydrocarbon Oil

The presence of immiscible water drops in bulk hydrocarbon is likely to bridge hydrate particles to cause hydrate agglomeration, leading to potential pipeline blockage. This can become a major challenge for flow assurance in offshore petroleum transportation. To avoid hydrate aggregation, the attachment between hydrate and water drops should be avoided. In this study, we used our home-designed integrated thin film drainage apparatus to investigate the interactions between a hydrate particle and a water drop inside model oil (i.e., mixture of cyclopentane and toluene with a volumetric ratio of 1:1). Our experiments showed that asphaltenes, a natural component in crude oil, were an effective inhibitor for the attachment between water drops and hydrate particles. Without asphaltenes in the system, the water drop adhered to the hydrate particle immediately after the two surfaces contacted. By adding 0.03 g/L asphaltenes into the oil phase, the attachment was delayed by 0.7 s when the applied preload force was set to around 0.05 mN. By increasing the asphaltenes addition to 0.05 g/L, the attachment between the hydrate and water drop was prevented even when the contact time lasted up to 25 s. This phenomenon could be explained by the adsorption of an asphaltenes layer along the interface between the aqueous drop and hydrocarbon. Measurements of the dynamic interfacial tension and crumping ratio confirmed the presence of the adsorption layer. The addition of 0.6 mol/L NaCl or 0.3 mol/L CaCl₂ in the aqueous drop could further enhance the strength of the adsorption layer. Results of this research provide understanding of the benefits of asphaltenes and salt in preventing hydrate agglomeration.

The nanoscale Leidenfrost effect

Nanoscale evaporation of liquids plays a key role in several applications including cooling, drag reduction and liquid transport. This research investigates the Leidenfrost effect at the nanoscale as a function of substrate material, droplet size and temperature using molecular dynamics models. Water droplets ranging from 4 nm to 20 nm were simulated over gold and silicon substrates at 293 K, 373 K, 473 K, and 573 K. A significant increase in the kinetic energy (>5000 kcal mol^-1) was observed for molecules in the vicinity of the substrates, indicating the presence of a vapor barrier layer between substrate and liquid. Higher droplet velocities were tracked for hydrophobic gold substrates as compared to hydrophilic silicon substrates indicating the influence of the surface wettability on the Leidenfrost effect. Droplets over silicon substrates had a higher number of fluctuations (peaks and valleys) as compared to gold due to the cyclic behavior of vapor formation. An increase in the interfacial kinetic energies and translatory velocities (>10 m s^-1) were observed as the droplet sizes reduced confirming the Leidenfrost effect at 373 K. This research provides understanding of the Leidenfrost effect at the nanoscale which can impact several applications in heat transfer and droplet propulsion.

Morphology modulation in evaporative drying mediated crystallization of sodium chloride solution droplet with surfactant

We report the evaporative drying of an aqueous droplet containing a dilute solution of sodium chloride (NaCl) on a hydrophobic substrate made of cross-linked poly-dimethyl siloxane (PDMS). The salt concentration Cn was varied between 0.08 molar (M) and 2.0 M. The contact line of the evaporating droplets shows significant initial retraction for all Cn, before they get pinned. While the final morphology comprises a few small NaCl crystals deposited around the pinned contact line, in droplets with a low Cn (<0.5 M), it transforms to a single large salt crystal when Cn > 0.7 M with no peripheral deposition. We further show that the deposition morphology drastically changes when an anionic surfactant, sodium dodecyl sulfate (SDS), is added into the salt-solutions. Even in the surfactant-laden droplets, the final deposition morphology changes significantly as a function of Cn. It transforms from a thick SDS ring surrounding a fractal-like deposit of NaCl crystallites at lower Cn to a peripheral deposit of NaCl crystals at higher Cn due to competition between micelle formation and crystallization. However, the crystallographic orientation of the deposited NaCl crystals remains unaltered irrespective of the presence of surfactant.

Dynamics of Nanodroplets on Vibrating Surfaces

We report the results of molecular dynamics investigations into the behavior of nanoscale water droplets on surfaces subjected to cyclic-frequency normal vibration. Our results show, for the first time, a range of vibration-induced phenomena, including the existence of the following different regimes: evaporation, droplet oscillation, and droplet lift-off. We also describe the effect of different surface wettabilities on evaporation. The outcomes of this work can be utilized in the design of future nanoengineered technologies that employ surface/bulk acoustic waves, such as water-based cooling systems for high-heat-generating processor chips, by tuning the vibration frequency and amplitude, as well as the surface wettability, to obtain the desired performance.

Acoustothermal Atomization of Water Nanofilms

We report nonequilibrium molecular simulations of the vibration-induced heating of nanoscale-thick water layers on a metal substrate. In addition to experimentally confirmed acoustothermal evaporation, we observe hitherto unmapped nucleate and film boiling regimes, accompanied by the generation of unprecedented heat fluxes [∼O(10^{9})  W/m^{2}]. We develop a universal scaling parameter to classify the heat-transfer regimes and to predict the thickness of the residual nonevaporating liquid layer. The results find broad application to systems involving drying, coatings, and sprays.

Molecular investigation of the wettability of rough surfaces using molecular dynamics simulation

In the present study, a computational investigation on the effect of surface roughness on the wettability behavior of water nanodroplets has been performed via molecular dynamics simulation.

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.

Ion-Specific Effects on the Elongation Dynamics of a Nanosized Water Droplet in Applied Electric Fields

We report an all-atom molecular dynamics study of the structures and dynamics of salty water droplets on a silicon surface under the influence of applied electric field. Our simulation results support ion-specific effects on the elongation dynamics of salty nanodroplets, induced by the field. This feature has not been explored up to now in monovalent salts. Nevertheless, the importance of ion-specific effects is widely confirmed in biological and colloidal systems. In particular, the increase of salt concentration enhances the effect of the nature of ions on the wetting properties of droplets. In the presence of electric field (0.05 V Å^-1), a complete spreading is implemented in a short time for different droplets at a concentration of 1 M, and the droplet morphology is stable, observed at long time scales. However, a higher salt concentration of 4 M considerably suppresses the spreading process owing to the increase of surface tension. It was found that the NaCl droplet shows deformation oscillations along the external field, but cannot fully wet the substrate surface. By contrast, the CsCl droplet reaches complete elongation rapidly and adopts a steady strip shape. The KCl droplet undergoes frequent transitions between breakup and connection. Additionally, the droplets can be elongated only when the electric field strength exceeds a threshold value. The dipole orientation of interfacial water and the ionic diffusion exhibit ion-specific dependences, but the hydrogen bond network is scarcely disturbed, excluding a concentration-dependent effect.

Water desalination using nano screw pumps with a considerable processing rate

The nano screw pump is used for water desalination while maintaining a considerable, fast water flow.

Effect of a Single Nanoparticle on the Contact Line Motion

In this paper, we use a single nanoparticle (NP) to achieve active control of the droplet contact line. When the droplet is out of equilibrium, the resulting excess free energy provides the driving force for the depinning of the contact line and the NP. There are three ways to increase the energy barriers to be surmounted and to realize the pinning of the contact line, namely, the enhancement of the interactions between the NP and the substrate, the increase in substrate hydrophilicity, and the reduction in the NP hydrophilicity. On this basis, we obtained three styles of contact line motion including complete slipping, alternate pinning-depinning, and complete pinning and theoretically interpreted them. The basic theory presented in this paper can be applied to explain and regulate the dynamics of the contact line involved in many physical processes such as evaporation and spreading.

Molecular Dynamics Study of the Hydrophilic-to-Hydrophobic Switching in the Wettability of a Gold Surface Corrugated with Spherical Cavities

This paper reports a large scale molecular dynamics (MD) simulation study of the wettability of a gold surface engraved with (hemi)spherical cavities. By increasing the depth of cavities, the contact angle (CA) of a water droplet on the surface was varied from a hydrophilic (69°) to a hydrophobic value (>109°). The nonmonotonic behavior of the CA vs the depth of the cavities was consistent with the Cassie-Baxter theory, as found in the experiment by Abdelsalam et al. (Abdelsalam, M. E.; Bartlett, P. N.; Kelf, T.; Baumberg, J. Wetting of Regularly Structured Gold Surfaces. Langmuir 2005, 21, 1753-1757). Depending on the depth of cavities, however, the droplet existed not only in the Cassie-Baxter state, but also in the Wenzel or an intermediate state, where the cavities were penetrated partially by the droplet.

Wettability on Inner and Outer Surface of Single Carbon Nanotubes

The surface wettability of a liquid on the inner and outer surface of single carbon nanotubes (CNTs) was experimentally investigated. Although these contact angles on both surfaces were previously studied separately, the available data are of limited help to elucidate the effect of curvature orientation (concave or convex) on wettability due to the difference in surface structure. Here, we report on the three-phase contact region and wettability on the outer surface of CNT during the dipping and withdrawing experiment of CNT into an ionic liquid. Furthermore, the wettability on the inner surface was measured using a liquid within the same CNT. Our results show that the contact angle on the outer surface of the CNT is larger than that on the flat surface and that on the inner surface is smaller than that on the flat one. These findings suggest that the surface curvature orientation has a noticeable effect on the contact angle at the nanoscale because both inner and outer surfaces expose the same graphite wall structure and the contact line tension will be negligible in this situation. The presented results are rationalized using the free energy balance of liquid on curved surfaces.

Electrowetting Controls the Deposit Patterns of Evaporated Salt Water Nanodroplets

So-called "coffee-ring" stains are the deposits remaining after complete evaporation of droplets containing nonvolatile solutes. In this paper we use molecular dynamics to simulate the evaporation of salt water nanodroplets in the presence of an applied electric field. We demonstrate, for the first time, that electrowetted nanodroplets can produce various deposit patterns, which vary substantially from the original ringlike deposit that occurs when there is no electric field. If a direct current (dc) electric field with strength greater than 0.03 V/Å is imposed parallel to the surface, after the water evaporates the salt crystals form a deposit on the substrate in a ribbon pattern along the field direction. However, when an alternating current (ac) electric field is applied the salt deposit patterns can be either ringlike or clump, depending on the strength and frequency of the applied ac field. We find that an ac field of high strength and low frequency facilitates the regulation of the deposit patterns: the threshold electric field strength for the transition from ringlike to clump is approximately 0.006 V/Å. These findings have potential application in fabricating nanostructures and surface coatings with desired patterns.