Surface tension

About: Surface tension is a research topic. Over the lifetime, 25410 publications have been published within this topic receiving 695471 citations.

An explanation for the charge on water’s surface

Abstract: Measurements with different techniques point to a strong affinity of hydroxide ions for interfaces between water and hydrophobes, but some spectroscopic experiments do not detect excess hydroxide at the interface, while others do. Hydroxide ions are unusual in that they reduce the relative permittivity of an electrolyte solution more than other monovalent, monatomic ions. This implies that they suppress the collective dipole-moment fluctuations of nearby waters. We show that the absence of these fluctuations leads to a Hamaker-like force on the hydroxide ion that attracts it to regions where dipole-moment fluctuations are smaller than in bulk water, in other words, to regions of low relative permittivity. We show also that there is no contradiction between the picture of the basic, negatively charged interface and spectroscopic measurements. This is, in part, because the hydroxides are mostly below the outermost molecular layers. By combining a simple model for this fluctuation force with a modified Poisson–Boltzmann equation, we reproduce the dependence of the ζ-potential on pH, including the low isoelectric point, the approximate magnitude of the experimental surface charge density, and the Jones–Ray data for the dependence of surface tension on electrolyte concentration. We discuss also the apparent contradiction between molecular-dynamics simulations that deny and experiments that support a basic, negatively charged interface.

The mechanism of a splash on a dry solid surface

Abstract: From rain storms to ink jet printing, it is ubiquitous that a high-speed liquid droplet creates a splash when it impacts on a dry solid surface. Yet, the fluid mechanical mechanism causing this splash is unknown. About fifty years ago it was discovered that corona splashes are preceded by the ejection of a thin fluid sheet very near the vicinity of the contact point. Here we present a first-principles description of the mechanism for sheet formation, the initial stages of which occur before the droplet physically contacts the surface. We predict precisely when sheet formation occurs on a smooth surface as a function of experimental parameters, along with conditions on the roughness and other parameters for the validity of the predictions. The process of sheet formation provides a semi-quantitative framework for studying the subsequent events and the influence of liquid viscosity, gas pressure and surface roughness. The conclusions derived from this framework are in quantitative agreement with previous measurements of the splash threshold as a function of impact parameters (the size and velocity of the droplet) and in qualitative agreement with the dependence on physical properties (liquid viscosity, surface tension, ambient gas pressure, etc.) Our analysis predicts an as yet unobserved series of events within micrometres of the impact point and microseconds of the splash.