Simultaneous impact of droplet pairs on solid surfaces

This study investigates the dynamics of the simultaneous impact of two droplets on a dry substrate. We develop a new micro-controlled droplet generator that releases two equally sized water droplets simultaneously on-demand, with no trailing droplets. The impact Weber number, based on impact droplet size and velocity, and the inter-droplet spacing relative to the impact droplet size are varied in the ranges of 54 to 155 and 1.32 to 2.25, respectively, leading to the strong interaction of the spreading lamellae that form a central uprising sheet, which eventually deposits or breaks into tiny droplets. We analyse the impact processes for both deposition and splashing of the uprising sheets. Simultaneous high-speed imaging from two orthogonal views of the droplet impacts quantifies the three-dimensional structure of the sheet morphology, including the temporal evolution of the rim-bounded ‘semilunar’ shape, surface waves, rim corrugations and finger formation, and deposition or splashing of the liquid sheet. The characteristics of the sheet surface waves and the rim instabilities are quantified. Novel scaling is developed for the maximum sheet height, sheet width and thickness, which considers the geometrical constraints and mass balance of the interacting lamellae to describe the temporal evolution of a ‘semilunar’ uprising sheet and is in good agreement with the measurements. The uprising sheet splashing generates larger droplets than those from splashing of single-droplet impacts, and it occurs due to the end-pinching of sheet fingers and at conditions that single-droplet impacts lead only to liquid deposition

Simultaneous impact of multiple droplets on a solid surface

This study develops a controlled generation of multiple droplets simultaneously to examine the physics of multiple droplet impacts on a solid surface. The developed droplet generator consists of a micro-controlled syringe pump connected with two needles. The control of the speed and steps of the motor of the syringe pump allows precise simultaneous ejection of a single droplet from each needle on demand. The droplet impacts on a solid substrate can lead to the interaction of the spreading lamellas between droplets to form an uprising liquid sheet. Simultaneous high-speed imaging from two views (front and side) of the sheet temporal evolution for three impact Weber numbers (We) of 55, 84, and 128 reveals the three-dimensional features, including the semicircular shape, instabilities, cusp formation and finger formation, of the sheet. The dependence of the height and periphery of the sheet on the impact Weber number is presented

A Study on the Relationship between Siltation and Flow Parameters of a Typical Alluvial River-Manas, A Tributary of Brahmaputra

Silt is defined as the granular material of size somewhere between sand and clay. Silt may occur as a soil or as sediment mixed with water as known as a suspended load in a body of water such as a river. It may also exist as soil deposited at the bottom of a water body. The natural process of the decay of organisms into the water can lead to the production of silt at the bottom of a river. Silt can be generated in the streams by mining, agriculture and other industries and sewage. Through these study an attempt has been made to record periodical observations of the river Manas, a tributary flowing into the Brahmaputra river, to record its different hydrodynamic properties viz. amount of silt carried, velocity during the period, discharge, cross-sectional properties etc. and to study its various aspects and then recommend some enhancing engineering solutions

On trapped surface formation in gravitational collapse II

Further to our consideration on trapped surfaces in gravitational collapse, where pressures were allowed to be negative while satisfying weak energy condition to avoid trapped surface formation, we discuss here several other attempts of similar nature in this direction. Certain astrophysical aspects are pointed out towards examining the physical realization of such a possibility in realistic gravitational collapse

Quantum oscillations in graphene in the presence of disorder and interactions

Quantum oscillations in graphene is discussed. The effect of interactions are addressed by Kohn's theorem regarding de Haas-van Alphen oscillations, which states that electron-electron interactions cannot affect the oscillation frequencies as long as disorder is neglected and the system is sufficiently screened, which should be valid for chemical potentials not very close to the Dirac point. We determine the positions of Landau levels in the presence of potential disorder from exact transfer matrix and finite size diagonalization calculations. The positions are shown to be unshifted even for moderate disorder; stronger disorder, can, however, lead to shifts, but this also appears minimal even for disorder width as large as one-half of the bare hopping matrix element on the graphene lattice. Shubnikov-de Haas oscillations of the conductivity are calculated analytically within a self-consistent Born approximation of impurity scattering. The oscillatory part of the conductivity follows the widely invoked Lifshitz-Kosevich form when certain mass and frequency parameters are properly interpreted.Comment: Appendix A was removed, as the content of it is already contained in Ref. 17. Thanks to M. A. H. Vozmedian