Interfacial waves in two liquid layers driven by horizontal oscillation

When a closed vessel containing two stably stratified, immiscible liquids is oscillated in the horizontal direction, the flat interface between the two liquids is known to undergo a symmetry breaking bifurcation to two-dimensional (2-D) 'frozen wave' driven by interfacial shear, similar to the Kelvin-Helmholtz instability. In this thesis we present an experimental study on the dynamics of this interfacial wave as a function of the vibrational Froude number (W, square root of the ratio of vibrational to gravitational forces). The onset of the 'frozen wave' is followed by a nonlinear growth of the wave to large amplitudes, which precedes a secondary instability to three-dimensional (3-D) waves.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Steep capillary-gravity waves in oscillatory shear-driven flows

We study steep capillary-gravity waves that form at the interface between two stably stratified layers of immiscible liquids in a horizontally oscillating vessel. The oscillatory nature of the external forcing prevents the waves from overturning, and thus enables the development of steep waves at large forcing. They arise through a supercritical pitchfork bifurcation, characterized by the square root dependence of the height of the wave on the excess vibrational Froude number (W, square root of the ratio of vibrational to gravitational forces). At a critical value Wc, a transition to a linear variation in W is observed. It is accompanied by sharp qualitative changes in the harmonic content of the wave shape, so that trochoidal waves characterize the weakly nonlinear regime, but ‘finger’-like waves form for W Wc. In this strongly nonlinear regime, the wavelength is a function of the product of amplitude and frequency of forcing, whereas for W <Wc, the wavelength exhibits an explicit dependence on the frequency of forcing that is due to the effect of viscosity. Most significantly, the radius of curvature of the wave crests decreases monotonically with W to reach the capillary length for W =Wc, i.e. the lengthscale for which surface tension forces balance gravitational forces. For W <Wc, gravitational restoring forces dominate, but for W Wc, the wave development is increasingly defined by localized surface tension effects

Efficient manipulation of microparticles in bubble streaming flows

Oscillating microbubbles of radius 20–100 μm driven by ultrasound initiate a steady streaming flow around the bubbles. In such flows, microparticles of even smaller sizes (radius 1–5 μm) exhibit size-dependent behaviors: particles of different sizes follow different characteristic trajectories despite density-matching. Adjusting the relative strengths of the streaming flow and a superimposed Poiseuille flow allows for a simple tuning of particle behavior, separating the trajectories of particles with a size resolution on the order of 1 μm. Selective trapping, accumulation, and release of particles can be achieved. We show here how to design bubble microfluidic devices that use these concepts to filter, enrich, and preconcentrate particles of selected sizes, either by concentrating them in discrete clusters (localized both stream- and spanwise) or by forcing them into narrow, continuous trajectory bundles of strong spanwise localization