Droplet motion over a surface with wettability gradient has been simulated using molecular dynamics (MD) simulation to highlight the underlying physics. GROMACS and Visual Molecular Dynamics (VMD) were used for simulation and intermittent visualization of the droplet configuration respectively. The simulations mimic experiments in a comprehensive manner wherein micro-sized droplets are propelled by surface wettability gradient against a number of retarding forces. The liquid-wall Lennard-Jones interaction parameter and the substrate temperature were varied to explore their effects on the three-phase contact line friction coefficient. The contact line friction was observed to be a strong function of temperature at atomistic scales, confirming the experimentally observed inverse functionality between the coefficient of contact line friction and increase in temperatures. These MD simulation results were successfully compared with the results from a model for self-propelled droplet motion on gradient surfaces.

Equilibrium and dynamic electrowetting behavior of ultrathin liquid films of surfactant (SDS) laden water over silicon substrate (with native oxide) is investigated. A nonobtrusive optical method, namely, image analyzing interferometry, is used to measure the meniscus profile, adsorbed film thickness, and the curvature of the capillary meniscus. Significant advancement of the contact line of the liquid meniscus, as a result of the application of electric field, is observed even at relatively lower values of applied voltages. The results clearly demonstrate the balance of intermolecular and surface forces with additional contribution from Maxwell stress at the interline. The singular nature of Maxwell stress is exploited in this analysis to model the equilibrium meniscus profile using the augmented Young-Laplace equation, leading to the in situ evaluation of the dispersion constant. The electrowetting dynamics has been explored by measuring the velocity of the advancing interline. The interplay of different forces at the interface is modeled using a control volume approach, leading to an expression for the interline velocity. The model-predicted interline velocities are successfully compared with the experimentally measured velocities. Beyond a critical voltage, contact line instability resulting in emission of droplets from the curved meniscus has been observed.

Droplet motion on a surface with chemical energy induced wettability gradient has been simulated using molecular dynamics (MD) simulation to highlight the underlying physics of molecular movement near the solid-liquid interface including the contact line friction. The simulations mimic experiments in a comprehensive manner wherein microsized droplets are propelled by the surface wettability gradient against forces opposed to motion. The liquid-wall Lennard-Jones interaction parameter and the substrate temperature are varied to explore their effects on the three-phase contact line friction coefficient. The contact line friction is observed to be a strong function of temperature at atomistic scales, confirming their experimentally observed inverse functionality. Additionally, the MD simulation results are successfully compared with those from an analytical model for self-propelled droplet motion on gradient surfaces.

No abstract

No abstract

No abstract

It is observed that the presence of negatively charged, suspended nanoparticles significantly changes the electric-field-induced spreading and contact line dynamics of partially wetting liquid films. Image-analyzing interferometry is used to accurately measure the meniscus profile, including the spatial change in the meniscus curvature. The nanoparticle-containing meniscus exhibits enhanced spreading with an increase in the particle size and weight fraction. The instantaneous contact line velocities are measured using video microscopy and a frame-by-frame analysis of the extracted images. The effects of electric field polarity reversal on the flow toward the contact line are explored as well. The movement of the meniscus is analyzed taking into account the capillary forces and Maxwell-stress-induced flows. An analytical model based on the Young-Laplace equation is used to analyze the electric-field-induced contact line motion, and the model-predicted velocities are compared to the experiments.

However, surfaces with a high contact angle (CA > 150°) can also exhibit "sticky" behavior characterized by high CAH; such surfaces are termed parahydrophobic. [7] A rose petal is a prime example exhibiting such parahydrophobic behavior; this socalled petal effect [8] has attracted significant research attention [7] in an attempt to understand this somewhat puzzling wetting state.Research over the last two decades has led to noteworthy progress in the synthesis of functional surfaces inspired by nature. [9] Self-cleaning surfaces inspired by the lotus leaf [10] have potential applications in enhancing the efficiency of solar cells, reducing surface drag, enhancing fluidic transport, and preventing water corrosion of batteries and fuel cells. [5] Surfaces exhibiting the petal effect have been proposed for separation processes [11] and collection of water via directional liquid transport. [12][13][14] It has also been recently demonstrated that parahydrophobic surfaces are ideal for facilitating bubble nucleation and departure during boiling. [15,16] Models of the wetting state are typically used to design surface topologies that provide the desired wetting characteristics. [17] Unlike the lotus leaf, for which the wetting state has been confirmed and is well accepted, [4] a recent review of the current understanding of parahydrophobic surfaces reveals a lack of experimental evidence to confirm the postulated wetting states on the rose petal. [7] As the surface morphology of natural surfaces is typically complex, and the features themselves delicate, it is difficult to obtain an accurate characterization of the microscopic wetting state. The existing explanation of the petal effect is based on a partially wetting "Cassie-impregnating" state. [7] The Cassieimpregnating wetting state is adapted to the dual-scale surface features of the rose petal, [18] namely, microscale papillae (bumps) with nanoscale striae (folds) [18] on top of each micropapilla. [19] Water droplets on the petal are thought to penetrate the gap between micro-papillae but not wet the nanoscale features. Although it was postulated that high apparent CA and high adhesion exhibited by surfaces could be explained by a mixed wetting state, [20] there has been no direct experimental evidence or visualization of these hypothesized wetting states on a rose petal. One recent study visualized the wetting state on a rose petal using top-down optical microscopy, [21] and postulated the trapping of air at the surface; however, it is difficult to view the liquid-air and liquid-solid interfaces underneath the droplet using this technique. Progress has been made on visualizing the liquid-air and liquid-solid interfaces between and below condensing droplets [22,23] and moving droplets [24] on microstructured surfaces using scanning electron microscopy (SEM).The rose petal features surface structures that offer unique wetting properties. A water droplet placed on a rose petal forms a high contact angle but exhibits significant contact angle hysteresis, such that re...