Cunjing Lv

Sliding of water droplets on microstructured hydrophobic surfaces.

Sliding behaviors of liquid droplets on solid surfaces are among the fundamental results of wettability. To remedy the lack of quantitative correlation between sliding angle and roughness of the surface, which is known to be effective at enhancing wettability, we report in this paper the observation that the onset of water droplets sliding under gravity on inclined micropillar-structured hydrophobic surfaces always starts with detachment of the rear contact lines of the droplets from the pillar tops. We also establish an explicit analytical model, based on the observed mechanism, by which the sliding angle is fully determined by the fraction of water-solid interface area, droplet volume, and Youngs contact angle. This model gives predictions of sliding angles that agree well with experimental measurements.

Wetting of graphene oxide: a molecular dynamics study.

We characterize the wetting properties of graphene oxide by performing classical molecular dynamics simulations. With oxygen-containing functional groups on the basal plane, graphene becomes hydrophilic and the water contact angle decreases with their concentration, c. The concentration dependence displays a transition at c ≈ 11% as defined by the interacting range of hydrogen bonds with oxidized groups and water. Patterns of the oxidized region and the morphological corrugation of the sheet strongly influence the spreading of water droplets with their lateral spans defined by corresponding geometrical parameters and thus can be used to control their behavior on the surface. These results are discussed with respect to relevant applications in graphene oxide-derived functional materials and offer a fundamental understanding of their wetting and flow phenomena.

Condensation and jumping relay of droplets on lotus leaf

Dynamic behavior of micro water droplet condensed on a lotus leaf with two-tier roughness is studied. Under laboratory environment, the contact angle of the micro droplet on single micro papilla increases smoothly from 80° to 160° during the growth of condensed water. The best-known “self-cleaning” phenomenon will be lost. A striking observation is the out-of-plane jumping relay of condensed droplets triggered by falling droplets, as well as its sustained speed obtained in continuous jumping relays. The underlying mechanism can be used to enhance the automatic removal of dropwise condensation without the help from any external force. The surface tension energy dissipation is the main reason controlling the critical size of jumping droplet and its onset velocity of rebounding.

Substrate curvature gradient drives rapid droplet motion.

Making liquid droplets move spontaneously on solid surfaces is a key challenge in lab-on-chip and heat exchanger technologies. The best-known mechanism, a wettability gradient, does not generally move droplets rapidly enough and cannot drive droplets smaller than a critical size. Here we report how a curvature gradient is particularly effective at accelerating small droplets, and works for both hydrophilic and hydrophobic surfaces. Experiments for water droplets on tapered surfaces with curvature radii in the sub-millimeter range show a maximum speed of 0.28 m/s, two orders of magnitude higher than obtained by wettability gradient. We show that the force exerted on a droplet scales as the surface curvature gradient. Using molecular dynamics simulations, we observe nanoscale droplets moving spontaneously at over 100 m/s on tapered surfaces.

Freezing of sessile water droplets on surfaces with various roughness and wettability

This paper focus on the freezing delay time and the freezing time of sessile droplet on smooth, micro-structured and micro/nano-structured surfaces, and the whole freezing process are comparatively studied. The freezing delay time of the smooth surfaces with roughness smaller than the size of the critical ice nuclei is found to be much longer than superhydrophobic surfaces with hierarchical structures. Experimental data and theoretical analysis show that the surface roughness plays a very crucial role in nucleation. The freezing delay time could not be extended further on rough surface with more superhydrophobic for sessile droplet. In addition, decreased roughness can increase the free energy barrier for heterogeneous nucleation, result in significant freezing delay. On the contrary, the freezing time from the start of nucleation to the completion of freezing increases with the contact angle. In addition, surfaces with hierarchical roughness are found to have the longest freezing time.

Departure of Condensation Droplets on Superhydrophobic Surfaces

This article focuses on the departure of multidroplet coalescence on a superhydrophobic surface with nanoscale roughness. Out-of-plane jumping events triggered by multidroplet coalescence and a single fallen droplet are observed. Experimental data show that the departure of droplets due to the multidroplets coalescence and the jumping modes is dominant for the removal of condensed droplets from the substrate. The energy barrier is easier to overcome and the critical size of the self-propelled droplets could be further decreased in multidroplet coalescence jumping mode. A general theoretical model is developed which accounts quantitatively for determining the jumping velocity and the critical size of the multidroplet coalescence.

Sliding behavior of water droplet on superhydrophobic surface

We found experimentally that the advancing contact angles on micropillar-like superhydrophobic surfaces are hardly affected by the fraction of the water-solid interface area, while the receding contact angles and the sliding angles are strongly influenced by this geometrical parameter. Different from previous works, using particle image velocimetry (PIV) and particle tracking velocimetry (PTV) techniques, both rolling and slipping motions were observed for 5 μL water droplet during its sliding-down. We revealed that the rolling only occurs at the edge of the water droplet and the slip velocity is higher than the rolling velocity due to the low viscosity of water.

Dewetting Transitions of Dropwise Condensation on Nanotexture-Enhanced Superhydrophobic Surfaces.

Although realizing dewetting transitions of droplets spontaneously on solid textured surfaces is quite challenging, it has become a key research topic in many practical applications that require highly efficient removal of liquid. Despite intensive efforts over the past few decades, due to impalement of vapor pockets inducing strong pinning of the contact lines, how to realize the self-removal of small droplets trapped in the textures remains an urgent problem. We report an in situ spontaneous dewetting transition of condensed droplets occurring on pillared surfaces with two-tier roughness, from the valleys to the tops of the pillars, owing to the nanotexture-enhanced superhydrophobicity, as well as the topology of the micropillars. Three wetting transition modes are observed. It is found that a further decreased Laplace pressure on the top side of the individual droplets accounts for such a surprising transition and self-removal of condensed water. An explicit model is constructed, which quite effectively predicts the Laplace pressure of droplets trapped by the textures. Our model also reveals that the critical size of the droplet for transition scales as the spacing of the micropillars. These findings are expected to be crucial to a fundamental understanding, as well as a remarkable strategy to guide the fabrication, of optimum super-water-repellant surfaces.

Driving Droplet by Scale Effect on Microstructured Hydrophobic Surfaces

A new type of water droplet transportation mechanism on a microstructured hydrophobic surface is proposed and investigated experimentally and theoretically: a water droplet could be driven by scale effect under disturbance and vibration, which is different from the traditional contact angle-gradient-based method. A scale-gradient microstructured hydrophobic surface is fabricated in which the area fraction is kept constant, but the scales of the micropillars are monotonically changed. When additional water or horizontal vibration is applied, the original water droplet could move unidirectionally in the direction from the small scale to the large scale. A new model with line tension energy developed very recently could be used to explain these phenomena. When compared with the traditional contact angle-gradient smooth surface, it is also found that dynamic contact angle decreases with increasing the scale of the micropillars along the moving direction under disturbance. These new findings will deepen our understanding of the relationship between topology and dynamic wetting properties, and could be very helpful in designing liquid droplet transportation devices in microfluidic systems.

Drop impact upon superhydrophobic surfaces with regular and hierarchical roughness

Recent studies demonstrate that roughness and morphologies of the textures play essential roles on the dynamics of water drop impacting onto superhydrophobic substrates. Particularly, significant reduction of contact time has greatly attracted peoples attention. We experimentally investigate drop impact dynamics onto three types of superhydrophobic surfaces, consisting of regular micropillars, two-tier textures with nano/micro-scale roughness, and hierarchical textures with random roughness. It shows that the contact time is controlled by the Weber number and the roughness of the surface. Compared with drop impact on regular micropillared surfaces, the contact time can be finely reduced by increasing the Weber number on surfaces with two-tier textures, but can be remarkably reduced on surfaces with hierarchical textures resulting from the prompt splash and fragmentation of liquid lamellae. Our study may shed lights on textured materials fabrication, allowing a rapid drop detachment to realize broad appli...