Abstract

We give a few examples of unusual flow phenomena that occur in relatively simple geometries. First, in an introductory, short survey we sketch or briefly illustrate examples with (i) micro patterned substrates, (ii) a variant of the classical Saffman-Taylor viscous fingering instability, and (iii) an unusual flow effect that occurs for motile but surface-attached bacteria. Second, in the main part of the talk we describe the seemingly ordinary flow of bubbles moving through a T-junction or bifurcation. A T -junction is perhaps the most common element in many piping systems so it should be expected that the flow structures there are well understood. In our experiments the flows are laminar but have high Reynolds numbers, typically Re=100-1000. For a two-phase flow in this geometry it seems obvious that any particles in the fluid that enter the T-junction will leave following the one of the two main 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 bubble growth due to coalescence. Our three-dimensional numerical simulations of the corresponding single-phase flow rationalise the mechanism behind this phenomenon and highlight unrecognized richness of the three-dimensional flow at the T-junction.