In ac electrowetting, hydrodynamic flows occur within a droplet. Two distinct flow patterns were observed, depending on the frequency of the applied electrical signal. The flow at low-frequency range was explained in terms of shape oscillation and a steady streaming process in conjunction with contact line oscillation. The origin of the flow at high-frequency range has not yet been explained. We suggest that the high-frequency flow originated mainly from the electrothermal effect, in which electrical charge is generated due to the gradient of electrical conductivity and permittivity, which is induced by the Joule heating of fluid medium. To support our argument, we analyzed the flow field numerically while considering the electrical body force generated by the electrothermal effect. We visualized the flow pattern and measured the flow velocity inside the droplet. The numerical results show qualitative agreement with experimental results with respect to electric field and frequency dependence of flow velocity. The effects of induced-charge electro-osmosis, natural convection, and the Marangoni flow are discussed.

Non-axisymmetric drops can significantly alter impact dynamics via rebound suppression when compared to axisymmetric drops. In this study, we focus on ellipsoidal drop impact on a non-wetting surface and investigate the effects of the geometric aspect ratio (AR) and the Weber number (We) on the dynamics and outcomes of impacts, both experimentally and numerically. Non-axisymmetric spreading features are characterized by scrutinizing the maximal extensions along the x-axis (D mx ) and y-axis (D my ) with respect to AR and We. The ratio of the maximal extensions depends strongly on AR, following our scaling relation D mx /D my ∼ AR 1/2 . Experimental and numerical studies show that increasing AR induces a high degree of axis switching during retraction, thereby resulting in the prevention of drop rebound, where axis switching denotes alternate expansion and contraction along the principal axes. We determine the transition between rebound and deposition (rebound suppression) over the AR and We domains and discuss the transition based on a non-axial distribution of the kinetic energy. The understanding of ellipsoidal drop impacts will potentially provide applications to surface patterning, cleaning, and cooling.

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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.

Non-axisymmetric drops impacting on a solid surface can alter impact dynamics significantly, thereby resulting in rebound suppression. Here, we present a method to control the bounce height of drops impacting on heated surfaces with ellipsoidal shaping. Experimental and numerical studies are used to investigate the effects of the geometrical aspect ratio (AR) of the drop on bouncing dynamics, which shows that maximum bounce heights of ellipsoidal drops can be reduced below spherical cases to nearly 40%. Control of bounce height can be explained in terms of a non-axial kinetic energy distribution during retraction. Interestingly, the non-axisymmetric hydrodynamics allows us to reduce contact time below this theoretical limit, which is explored both experimentally and numerically as a function of AR. This work may provide an understanding of bouncing dynamics on non-wetting surfaces for applications in surface cooling and cleaning.