Recent surge in the development of superhydrophobic/superoleophobic surfaces has been motivated by surfaces like fish scales that have hierarchical structures, which are believed to promote water or oil repellency. In this work, we show that the under-water oil repellency of fish scales is entirely due to the mucus layer formation as part of its defense mechanism, which produces unprecedented contact angle close to 180[deg]. We have identified the distinct chemical signatures that are responsible for such large contact angle, thereby making fish scale behave highly superoleophobic inside the water medium. In absence of the mucus layer, it is found that the contact angle decreases quite dramatically to around 150[deg], making it less oleophobic, the degree of such oleophobicity can then be contributed to its inherent hierarchical structures. Hence, through this systematic study, for the first time we have conclusively shown the role of the fish's mucus layer to generate superoleophobicity and negate the common notion that hierarchical structure is the only reason for such intrinsic behavior of the fish scales.

In this paper we analyze the inviscid regime (for which viscosity is unimportant and the flow occurs due to the balance between the capillary and the inertial effects) that invariably precedes the classical century-old Washburn regime during capillary filling. We demonstrate that a new nondimensional number, namely, the product of the Ohnesorge number and the ratio between the filling length ($\ell${}) and the radius of the capillary (R), dictates the occurrence of this regime and the other well-known regimes in a capillary filling problem. We also identify that this inviscid regime occurs for the time that is of the order of the capillary time scale and, as has been quantified before [Quere, Eur. Phys. Lett. 39, 533 (1997); Joly, J. Chem. Phys. 135, 214705 (2011)], is characterized by the filling length being linearly proportional to the filling time. We establish the universality of this regime by pinpointing the existence of this regime (showing appropriate dependencies of the capillary radii and density) from existing experimental and Molecular Dynamics Simulation results that signify disparate ranges of length and time scales.

This study presents a theoretical and an experimental study of the effects of an applied external magnetic field on the maximum spreading of a ferrofluid droplet impacting on a solid substrate. Although many studies have explored the theoretical modeling of the droplet impact scenario, a theoretical model representing the impact of ferrofluid droplets of different magnetic characteristics, strongly affected by the magnetic field, is yet to be addressed. In this study, we developed a theoretical model based on the principle of the conservation of energy to predict the maximal deformation of both diamagnetic and paramagnetic ferrofluid droplets upon impact under the influence of the magnetic field. The physics behind the variation of maximum drop spread, as a function of Weber number (We), Reynolds number (Re), and magnetic Bond number (Bo_m) for 5–45, 150–400, and 150–3000, respectively, was studied. By validating the theoretical model with the experimental observations, we demonstrated that the proposed theoretical model could successfully predict experimental observations. Through theoretical analysis and extensive experimental investigations, a rational understanding was formulated which allowed us to comment on the effect of all the governing dimensionless numbers (We, Re, and Bo_m) on the maximum spreading of a ferrofluid droplet upon impact.

We present the evidence of universality in conical tip formation during the freezing of arbitrary-shaped sessile droplets. The focus is to demonstrate the relationship between this universality and the liquid drop shape. We observe that, in the case of asymmetric drops, this universal shape is achieved when the tip reconfigures by changing its location, which subsequently alters the frozen drop shape. The proposed “two-triangle” model quantifies the change in the tip configuration as a function of the asymmetry of the drop that shows a good agreement with the experimental evidence. Finally, based on the experimental and theoretical exercise, we propose the scaling dependence between the variations in the tip configuration and the asymmetry of the drop.