Lattice Boltzmann Study of Near-Wall Multi-Phase and Multi-Component Flows

Abstract

Recent developments of lab-on-a-chip devices call for better understanding of smallscale multi-phase and multi-component (MPMC) flows for the optimal design,fabrication and operation of these devices. In this thesis, the lattice Boltzmann method(LBM) was used to investigate a range of MPMC flows near various substratesmainly at small scales, with the focuses on the b Lotus Effectb , mobility in diffuseinterface modeling (DIM), substrate control for droplet manipulation and bubbleentrapment during droplet impact.First, a 2D droplet moving in a channel made of one smooth and one grooved wallwas studied. It was found that the wettability and the topography of the grooveaffected the flow much more under small scales than under macroscopic scales. Withthe grooved surface being sufficiently hydrophobic, the droplet was lifted andcompletely attached to the other wall, resulting in significantly reduced drag. Forhydrophilic grooved surfaces, the effects of the two factors were found coupled witheach other and a variety of interesting phenomena resulting from them were captured.Some of the simulations are expected to be helpful in elucidating the b Lotus Effectb .Next, the mobility in DIM was found to be closely related to the slip velocity of thethree-phase lines, and it was discovered that it may even determine the routes throughwhich a near-wall MPMC system evolves. Such mobility-dependent bifurcations werestudied in detail through droplet dewetting, and also illustrated by droplet motions ona heterogeneous surface. Thirdly, droplets on surfaces with given wettabilitydistributions and temporal variations were investigated in order to devise fast dropletmanipulation methods. Several kinds of droplet behaviors were found under differentsubstrate controls. When proper hydrophobic confinement and wettability switch wereapplied, rapid transport of droplets toward a desired direction was achieved. Keyfactors for such droplet transport were explored and their relations were identified.Finally, droplet impacts onto homogeneous surfaces were investigated. Several typesof bubble entrapment during such processes were discovered and analyzed, andconditions for entrapment prevention were preliminarily estimated.In conclusion, investigations of several kinds of near-wall MPMC flow problems andsome simulation issues on DIM have been carried out by using LBM. The resultssuggest that LBM is a fairly useful tool in the modeling and simulation of MPMCflows, especially those found in digital microfluidics involving complex physics andsurface chemistry. They may also provide better understanding of MPMC flows overcomplicated surfaces in nature such as lotus leaves, and for some industrialapplications involving droplets.