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Former members:
Dr Emma Talib, Instability of oscillatory two-layer viscous flow,
PhD University of Manchester (2006).
Hugh Rice, Air fingers in rectangular channels of arbitrary aspect ratio,
MSc University of Manchester (2004).
Samina Ali, A theoretical and experimental study into the frozen wave instability,
MSc University of Manchester (2003).
Alexandra Heap, Airway reopening - the propagation of air fingers into elastic, fluid-filled tubes,
MSc University of Manchester (2002).

Current research can be separated into three main areas (for more detailed info on each project, please select the indvidual items in the left-handside table):

1. Dynamics of interfaces excited by horizontal oscillation:

Interfacial instabilities occur in many multi-layer flows connected to processes of industrial significance,
such as solvent extraction, coating and oil recovery. Viscosity can have subtle and surprising effects on the
stability of such interfaces. Under horizontal oscillation, stably stratified layers of immiscible liquids can become are become unstable to a spatially-periodic relief of the interface, which is often referred to as a "frozen wave", for suitable choices of frequencies and amplitudes of forcing. Intriguingly, increasing the viscosity of one of the layer can lead either to the enhancement or the suppression of the FW instability. We have recently combined experiments and a linear stability model to undestand and predict these dynamics. We are currently interested in the effect of viscosity on the growth of the waves beyond onset, since we observe qualitative changes in the dynamics toward localised states as we increase the viscosity ratio.

Fw wave instability in a horizontally vibrated granular slurry

An analog of the FW instability occurs in slurries, where one of the fluid layers is replaced by a collection of
grains. We propose to use our understanding of the FW in liquid layers to deepen the understanding of fluidized granular beds, since a close analogy can be made between the two phenomena. Other interesting granular systems have fluid analogs. Two initially well-mixed granular species can separate into bands of single species, when they are excited with horizontal oscillations (Mullin, T. Mixing and de-mixing Science 295, 1851 (2002)). If one of the grain types is replaced by a liquid, we observe that granular bands still form but now in the liquid. Are these phenomena related, i.e. to what extent does the collection of grains exhibit fluid-like behaviour?

2. Low Reynolds number interfacial fluid dynamics in rigid and flexible vessels: This research area comprises research on coating flows in rigid tubes of arbitrary cross-section. We are interested in understanding the nature of the transition from flows in three-dimensional geometries (square tube) governed by the Stokes equations to flows in large aspect ratio channels (Hele-Shaw cells) governed by Darcy's law. Intermediate aspect ratios are particularly relevant to microfluidic applications where channels typically have rectangular cross-section of aspect ratio of approximately 10! We find a scaling of the film volume with aspect ratio which enables us to predict coating flow for any cross-section based on the knowledge of the limiting cases. Further experiments and detailed three-dimensional simulations are underway to reveal the origin of the scaling law.

We are also interested in the interaction between fluid flows and elastic structures. For instance, the collapse and occlusion of the airways of the lungs can occur as the result of pulmonary disease. Once collapsed, the airway must be reopened quickly with minimal damage to the lining tissues. This reopening process happens through the propagation of an air finger into the fluid-lined, collapsed tube, and the mechanics of this process, involving both elasticity and fluid mechanics, are essential to understand.
We have studied the injection of air into a partially collapsed, fluid-filled elastic tube, as a benchtop model of lung airway reopening. Generally, the air propagates forming one single finger, which advances leaving a thin film of fluid behind. In the limit of strong collapse, however, our results indicate the presence of multiple reopening states, including the propagation of one asymmetric air finger, two fingers and a low pressure pointed finger.

We would like to uncover whether these new propagation states are caused by the change in the geometry of the cross-section of the tube (due to collapse) or by its elasticity. We are currently starting experiments on the propagation of air fingers in a rigid channel (no elasticity), where we simulate the collapsed geometry by altering the shape of the cross-section with different obstacles.

3. MHD convection in liquid metals.

Reviews of my research:
Recent work on the use of thermocapillary forces to suppress dripping from a ceiling received widespread attention with reviews in Nature Science Update, Science News (vol.159, No. 1,p. 7, 2001), Physics Today (February 2001) or in the AIP Physics News Update.

Experiments on the transition to turbulence in the Reynolds pipe were the focus of an article of Physics
Today, "New Experiments Set the Scale for the Onset of Turbulence in Pipe Flow'' by R. Fitzgerald in the February 2004 issue (Physics Today article on the Manchester Pipe Flow).