Stages That Lead to Drop Depinning and Onset of Motion.

In this paper, we consider drops that are subjected to a gradually increasing lateral force and follow the stages of the motion of the drops. We show that the first time a drop slides as a whole is when the receding edge of the drop is pulled by the advancing edge (the advancing edge drags the receding edge). The generality of this phenomenon includes sessile and pendant drops and spans over various chemically and topographically different cases. Because this observation is true for both pendant and sessile cases, we exclude hydrostatic pressure as its reason. Instead, we explain it in terms of the wetting adaptation and interfacial modulus, that is, the difference in the energies of the solid interface at the advancing and receding edges. At the receding edge, a slight motion exposes to the air a recently wetted solid surface whose molecules had reoriented to the liquid and will take time to reorient back to the air. This results in a high surface energy at the solid-air interface which pulls on the triple line, that is, inhibits the motion of the receding edge. On the other hand, at the advancing edge, a slight advancement does not change the nature of the solid interfacial molecules outside the drop, and the advancing side's sliding can continue. Moreover, the solid molecules under the drop at the advancing edge take time to reorient, and hence, their configuration is not yet adapted for the liquid and therefore not adapted for retention of the advancing edge. Therefore, in sliding-drop experiments, the advancing edge moves before the receding one, typically a few times before the receding edge moves. For the same reason, the last motion of the receding edge usually happens as a result of the advancing edge pulling on it.

Comment on "Comparison of the Lateral Retention Forces on Sessile, Pendant, and Inverted Sessile Drops".

Tadmor et al.'s 2009 PRL article shows experiments of pendant drops with ∼30% higher retention forces than their sessile analogues. A recent article (de la Madrid, R. et al. Langmuir 2019, 35, 2871) seemingly explains this result theoretically using a drastically different experimental system that shows a ∼3% higher force that exceeds the scatter in three out of four data points. The differences between the two experimental systems might have allowed the two theories to coexist, but Tadmor's theory, which can explain both, allows an understanding of the solid-liquid interaction, which the newer theory lacks.

The Influence of Gravity on Contact Angle and Circumference of Sessile and Pendant Drops has a Crucial Historic Aspect.

Normally, pendant drops adapt contact angles that are closer to 90° than their sessile analogues. This is due to the drop's weight that pulls the pendant drop and straightens its contact angles. In this paper, we show a case in which the opposite happens: sessile drops that adapt contact angles that are closer to 90° than their pendant analogues. To achieve these peculiar states, one needs to increase the effective gravity on the drops and then relax it again to 1 g. Apparently, this and other phenomena depend not only on the direction of the gravitational force but also on the drop's history. We show that the drop's contact angle (and resultant area) is affected by two types of histories: short-term history and long-term history. For example, if we gradually increase the effective gravity on the drop, decrease it back to 1 g, and then repeat this cycle again and again, we see that the first cycle is drastically different, whereas other cycles approach a plateau in their behavior. In addition to drop's history, we explain these observations in terms of volume conservation, drop contact area, and pinning effect. This study may be generalized for other body forces such as electrical and magnetic or accelerating systems.

Droplets Sliding down a Vertical Surface under Increasing Horizontal Forces.

We have investigated the retention forces of liquid drops on rotating, vertical surfaces. We considered two scenarios: in one, a horizontal, centrifugal force pushes the drop toward the surface (?pushed drop? case), and in the other, a horizontal, centrifugal force pulls the drop away from the surface (?pulled drop? case). Both drops slide down as the centrifugal force increases, although one expects that the pushed drop should remain stuck to the surface. Even more surprising, when the centrifugal force is low, the pushed drop moves faster than the pulled drop, but when the centrifugal force is high, the pushed drop moves much slower than the pulled drop. We explain these results in terms of interfacial modulus between the drop and the surface.

Why Drops Bounce on Smooth Surfaces.

It is shown that introducing gravity in the energy minimization of drops on surfaces results in different expressions when minimized with respect to volume or with respect to contact angle. This phenomenon correlates with the probability of drops to bounce on smooth surfaces on which they otherwise form a very small contact angle or wet them completely. Theoretically, none of the two minima is stable: the drop should oscillate from one minimum to the other as long as no other force or friction will dissipate the energy. Experimentally, smooth surfaces indeed show drops that bounce on them. In some cases, they bounce after touching the solid surface, and in some cases they bounce from a nanometric air, or vacuum film. The bouncing energy can be stored in the interfaces: liquid-air, liquid-solid, and solid-air. The lack of a single energy minimum prevents a simple convergence of the drop's shape on the solid surface, and supports its bouncing back to the air. Therefore, the lack of a simple minimum described here supports drop bouncing on hydrophilic surfaces such as that reported by Kolinski et al. Our calculation shows that the smaller the surface tension, the bigger the difference between the contact angles calculated based on the two minima. This agrees with experimental finding where reducing the surface tension, for example, by adding surfactants, increases the probability for bouncing of the drops on smooth surfaces.

Solid-Liquid Work of Adhesion.

We establish a tool for direct measurements of the work needed to separate a liquid from a solid. This method mimics a pendant drop that is subjected to a gravitational force that is slowly increasing until the solid-liquid contact area starts to shrink spontaneously. The work of separation is then calculated in analogy to Tate's law. The values obtained for the work of separation are independent of drop size and are in agreement with Dupré's theory, showing that they are equal to the work of adhesion.

A Novel Technique Enables Quantifying the Molecular Interaction of Solvents with Biological Tissues.

The pharmaceutical industry uses various solvents to increase drug penetrability to tissues. The solvent's choice affects the efficacy of a drug. In this paper, we provide an unprecedented means of relating a solvent to a tissue quantitatively. We show that the solvents induce reorientation of the tissue surface molecules in a way that favors interaction and, therefore, penetrability of a solvent to a tissue. We provide, for the first time, a number for this tendency through a new physical property termed Interfacial Modulus (Gs). Gs, which so far was only predicted theoretically, is inversely proportional to such interactions. As model systems, we use HeLa and HaCaT tissue cultures with water and with an aqueous DMSO solution. The measurements are done using Centrifugal Adhesion Balance (CAB) when set to effective zero gravity. As expected, the addition of DMSO to water reduces Gs. This reduction in Gs is usually higher for HaCaT than for HeLa cells, which agrees with the common usage of DMSO in dermal medicine. We also varied the rigidities of the tissues. The tissue rigidity is not expected to relate to Gs, and indeed our results didn't show a correlation between these two physical properties.