Dynamic behavior of drops

Abstract

This dissertation consists of two investigations. In the first investigation we found that splash can be generated on superheated micro-patterned surface at a small Weber number, and there exists a correlation between the surface roughness and dynamic behaviour of drop upon its impingement. The second investigation arises from a trial test to study the directional motion of drops subject to a magnetic field, and we have accidentally discovered the anomalous and robust reverse rotation of a ferromagnetic nanoparticles seeded drop on both superheated surface above Leidenfrost point and superhydrophobic surface, relative to the rotational direction of the rotor containing two magnets. Subsequently, we replaced the drop with a solid ferromagnetic sphere, and conducted a systematic study on the reverse rotation phenomenon. It is found, that the sphere is always steady in a particular kinetic state, only determined by two parameters: 1. the distance from the magnets. 2. the rotation speed of the magnets. The sphere would always change its rotational direction upon a critical distance from the magnets for a certain rotation speed of the magnets. More precisely, the motion of the sphere can be classified into several distinct phases, i.e. the phase of direct rotation, the ‘flung out’ phase, transitional phase where the sphere is trapped on the orbit, and the phase of reverse rotation; for the sphere driven by two rotating magnets with the same polarity, it eventually stops at the center of the field when the magnetic strength is reduced to a certain value, but in the case of the opposite polarity, the diameter of the trajectory of rotation varies inversely with the magnetic field strength. These anomalous dynamic phenomena of ferromagnetic sphere in rotating magnetic fields were not reported before and were investigated for the first time.

Date of Award	2015
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology