W. N. Lacey Lectureship in Chemical Engineering

Fluid mechanics is often thought of as well developed so it might come as a surprise that flows in elementary configurations produce results with unexpected features.  I will try to make this case by describing two distinct problems that we have studied where seemingly modest variations in an elementary channel flow produce new effects.  First, we investigate some influences of fluid motion on surface-attached bacteria and biofilms.  In particular, we identify (a) upstream migration of surface-attached bacteria in a flow and (b) the formation of biofilm streamers, which are filaments of biofilm extended along the central region of a channel flow; these filaments are capable of causing catastrophic disruption and clogging of industrial, environmental and medical flow systems.  Second we consider flow in a T-junction, which is perhaps the most common element in many piping systems.  The flows are laminar but have high Reynolds numbers, typically Re = 100–1000.  It seems obvious that any particles in the fluid that enter the T-junction will leave following the one of the two flow channels.  Nevertheless, we report experiments that document that bubbles and other low density objects can be trapped at the bifurcation.  The trapping leads to the steady accumulation of bubbles that can form stable chain-like aggregates in the presence, for example, of surfactants, or give rise to a growth due to coalescence.  Our three-dimensional numerical simulations rationalize the mechanism behind this phenomenon.