Drop mobility on superhydrophobic microstructured surfaces with wettability contrasts

Manipulation of drop motion has attracted considerable attention recently as it is pertinent to industrial/biological applications such as microfluidics. Wettability gradients/contrasts applied to microtextured, superhydrophobic surfaces are probable candidates for engineering drop motion by virtue of their wettability controllability and low contact angle hysteresis. In the present work, we present a systematic study of drop mobility induced via wettability contrasts. A millimetre-sized water drop, placed on the boundary between two surfaces with distinct, uniform arrays of pillars, immediately moved toward the surface more densely populated with asperities, which was relatively more hydrophilic. The velocity of the motion was found to increase proportionally with the difference in pillar densities on each surface, in circumstances where the rear side surface had sufficiently small contact angle hysteresis. To elucidate the underlying mechanism of drop motion, we implemented a surface energy analysis for each motion event. Motion was initiated by the excess surface free energy due to drop deformation and directed in favour of energy minimisation. Lastly, we propose a theory to predict the direction of the drop which at the same time acts as the criterion for the motion to ensue

Experimental investigation of drag coefficient of free-falling deformable liquid gallium droplet

In this article, the effect of shape and deformation on the drag coefficient of a free-falling liquid gallium droplet in water in a terminal state is investigated experimentally. The temperature of the dispersed and continuous liquid was varied in order to examine the effect on the liquid–metal droplets. The falling droplets were imaged using a high-speed camera, and a simple model was developed to predict drag coefficient over a Reynolds number range of 103 < Re < 104. The drag coefficients of the deformed liquid gallium droplets were found to be larger than that associated with a solid sphere and the associated Weber number was below 4.5. It was found that the shape of all droplets in our experiment were oblate spheroid. A correlation has been established to predict the aspect ratio of a liquid gallium droplet moving in quiescent water. The deformation is highly dependent on interfacial surface tension and inertial force, while the viscosity ratio and pressure distribution have negligible effect

Droplet Motion on Contrasting Striated Surfaces

Liquid droplets move readily under the influence of surface tension gradients on their substrates. Substrates decorated with parallel microgrooves, or striations, presenting the advantage of homogeneous chemical properties yet varying the topological characteristics on either side of a straight-line boundary, are considered in this study. The basic type of geometry consists of hydrophobic micro-striations/rails perpendicular to the boundary, with the systematic variation of the width to spacing ratio, thus changing the solid–liquid contact fraction and inducing a well-defined wettability contrast across the boundary. Droplets in the Cassie–Baxter state, straddling the boundary, move along the wettability contrast in order to reduce the overall surface free energy. The results show the importance of the average solid fraction and contrasting fraction in a wide range of given geometries across the boundary on droplet motion. A unified criterion for contrasting striated surfaces, which describes the displacement and the velocity of the droplets, is suggested, providing guidelines for droplet manipulation on micro-striated/railed surfaces. The authors would like to acknowledge the support of the European Space Agency through ESA Contract No. 4000129506/20/NL/PG and the support received from the Engineering and Physical Sciences Research Council (EPSRC) through Grant No. EP/P005705/1. The authors also acknowledge the EC-RISE-ThermaSMART project, which received funding from the European Union's Horizon 2020 research and innovation program under Marie Skłodowska-Curie Grant Agreement No. 778104