Drop impact on superhydrophobic surfaces has received significant attention because of the advantages of self-cleaning and anti-icing attained by minimum contact time with the surface. Drop hydrodynamics is generally assumed to be axisymmetric, and the contact time is still bounded below by a theoretical Rayleigh limit. In this study, we report an ellipsoidal drop impact on a superhydrophobic surface to demonstrate an efficient way to reduce the contact time and suppress the bounce magnitude by breaking the symmetry. The outcome of the bounce is characterized in terms of a geometric aspect ratio (AR) and Weber number of the drop by comparing the dynamics with a spherical drop. The experimental result shows that the bouncing of the ellipsoidal drop can reduce the contact time and maximum bounce height below the spherical one by at least 30% and 60%, respectively. The exceptional rim dynamics at high AR produces a liquid alignment along the principal direction, leading to the symmetry breaking in the mass and momentum distribution and the subsequent fast drop detachment, which is quantitatively rationalized by the numerical study. The distinct features of the ellipsoidal drop impact will provide an insight into shape-dependent dynamics and open up new opportunities for self-cleaning and anti-icing strategies.

In this paper, a systematic study of a fully developed electroosmotic flow of power-law fluids in a rectangular microchannel bounded by walls with different zeta potentials is described. Because the upper and lower layers of most microchannels are made of different materials, it is necessary to study the flow characteristics for cases in which the microchannels have different zeta potentials at each wall. The electrical potential and momentum equations were solved numerically using a finite element analysis. The velocity profiles and flow rates were studied parametrically by varying the fluid behavior index, channel aspect ratio, and electrochemical properties of the liquid and the bounding walls. The calculated volumetric flow rates in a rectangular microchannel were compared with those between two infinite parallel plates.

Manipulations of “complex” drops and their interactions with “complex” surfaces have attracted much attention in recent years. Here, we hypothesize that Janus drop impact on curved superhydrophobic surfaces can be highly affected by the curvature, Weber number (We), and viscosity difference between Janus components. The main objective is to predict the splitting of the low-viscosity component from the Janus drop. The study forms regime maps for the transition of non-splitting/splitting as a function of the parameters. It is shown that the We threshold, above which splitting occurs, can be tuned by altering the surface curvature and viscosity difference between low- and high-viscosity components. Asymmetric behavior observed in the two components is elucidated by establishing the relationship of the residence time and characterizing the dynamics by means of the horizontal momenta. The underlying mechanism for bouncing on asymmetric structures can provide a meaningful guidance on the designs of liquid purification or multi-material printing.

Enhancing drop deposition on solid surfaces has received significant attention in various fields. Breaking the circular symmetry in typical impact dynamics has opportunities for altering the mass and momentum distributions significantly and improving the deposition. Here, we study the impact dynamics of ellipsoidal drops on nonwetted solid surfaces to reduce the bounce magnitude as a function of the impact angle and ellipticity. Experimental and numerical studies reveal that the ellipsoidal drop with the impact angle shows a strong reduction in the maximum bounce height, compared with the spherical drops. The oblique drop impact exhibits a remarkable feature of the off-axis aligning process caused by asymmetric retraction dynamics. Axial momentum analyses help us to interpret the underlying principle behind the peculiar retraction dynamics and establish a transition map of the rebound and deposition for varying angles and ellipticities. We believe that a breakup of the symmetry in the dynamics can provide practical implications for the control of drop deposition in diverse applications, such as spraying, coating, and cooling.