are the liquid density and surface tension).This law is also observed to hold on partially wettable surfaces, provided that liquidsof low viscosity (such as water) are used. The law is interpreted as resulting fromthe eﬀective acceleration experienced by the drop during its impact. Viscous dropsare also analysed, allowing us to propose a criterion for predicting if the spreading islimited by capillarity, or by viscosity.

Maximal deformation of an impacting drop

— The chemistry of transition metal colloids (J. S. Bradley) It was not intended to describe the cluster beam technique together with the generation and the properties of naked clusters. The present book is remarkable for various reasons. The editor has succeeded in bringing the contribution of various authors in a homogeneous form which is important for the reader. Günter Schmid considered the interdisciplinary character of cluster science, summarizing contributions from chemists, physicists and material scientists. The synthesis, physical and chemical properties, and first applications, as well as the structure of these nano-sized particles are treated. The editor recommends the book to scientists, working in research as well as in practice, who wish to gain a fundamental insight into one or more areas of \"the world of small metal particles\". I wish to extend these recommendations especially to young scientists with respect to the \"Perspectives\" of this field (cf. the last small chapter 7 [G. Schmid]); a plenty of prospecting aspects represents here such a lot of innovation that the future of cluster and colloid science can be viewed with great optimism. I wish this valuable and interesting book, inspite of the price, a broad spread.

Introduction to Modern Colloid Science

Electrowetting — From statics to dynamics

Inkjet printing is a noncontact and additive patterning method, which is able to distribute a broad variety of functional materials directly onto a desired substrate with high efficiency. Bypassing many sophisticated processing approaches, inkjet printing affords a facile and low-cost processing platform, and thereby enhances the device cost-effectiveness. This review highlights recent advances in optical devices fabricated by inkjet printing, ranging from imaging to solar cells, photodetectors, anticounterfeiting, sensors, and optical waveguide systems, with detailed descriptions of the appropriate regulation of implementation process including inkjet printing apparatus, ink formulation, and the undertaking substrate, which results in device optimization. Finally, a conclusion and an outlook on future perspectives are proposed.

Emerging progress of inkjet technology in printing optical materials

We experimentally investigate the effect of an electric field applied between a Leidenfrost droplet and the heated substrate on which it is levitating. We quantify the electro-Leidenfrost effect by imaging the interference fringes between the liquid-vapour and vapour-substrate interfaces. The increase of the voltage induces a decrease of the vapour layer thickness. Above a certain critical voltage the Leidenfrost effect is suppressed and the drop starts boiling. Our study characterizes this way to control and/or to avoid the Leidenfrost effect that is undesirable in many domains such as metallurgy or nuclear reactor safety.

Effect of an electric field on a Leidenfrost droplet

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 ( ) and the Weber number ( ) on the dynamics and outcomes of impacts, both experimentally and numerically. Non-axisymmetric spreading features are characterized by scrutinizing the maximal extensions along the -axis ( ) and -axis ( ) with respect to and . The ratio of the maximal extensions depends strongly on , following our scaling relation . Experimental and numerical studies show that increasing 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 and 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.

Ellipsoidal drop impact on a solid surface for rebound suppression

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.

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Bouncing of an ellipsoidal drop on a superhydrophobic surface

We report that electrically driven shape deformation of a drop strongly affects the impacting behavior. The experiment shows that when a nonaxisymmetric drop impacts on hydrophobic substrates, it alternately spreads and recoils along two principal axes, subsequently suppressing its rebound. The rebound suppressions of the drops are investigated as a function of the Weber number, and consequently the critical We causing rebound is considerably higher than that of an axisymmetric drop. We rationalize the impacting behaviors originated from the breakup of axisymmetry using the three-dimensional numerical simulation. The simulations of nonaxisymmetric drops completely predict the axis-switching and reveal the consequent kinetic energy transfer between the horizontal principal axes rather than the vertical axis.

Suppressing drop rebound by electrically driven shape distortion

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.

Control of a bouncing magnitude on a heated substrate via ellipsoidal drop shape

Controlling bouncing drops on solid surfaces has gained significant attention because of the benefit of low residence time in anti-icing and self-cleaning strategies. Given that the drop shape at the moment of impact is classically assumed to be spherical, the residence time on a flat surface is bounded by a theoretical Rayleigh limit. In this study, we investigated the impact dynamics of oblate and prolate ellipsoidal drops to demonstrate the concept of modifying the residence time by shaping like raindrops. Experimental and numerical studies show that the initial shape plays a vital role in an increase or reduction in bounce speed of the drop, which is explained by scaling the maximum spreading time. The hydrodynamic features of ellipsoidal drops are analyzed by quantifying the temporal variations in diameters, heights, velocity fields, momenta, and energy dissipation. We believe that the ellipsoidal drop impact can provide an efficient pathway for controlling the residence time in practical applications.

Sungchan Yun

Papers

Ellipsoidal drop impact on a solid surface for rebound suppression

Bouncing of an ellipsoidal drop on a superhydrophobic surface

Suppressing drop rebound by electrically driven shape distortion

Control of a bouncing magnitude on a heated substrate via ellipsoidal drop shape

Spreading Dynamics and the Residence Time of Ellipsoidal Drops on a Solid Surface