New Observations of Particle Motions in Simple Geometries: Examples from Both Low and High Reynolds Numbers

The flow of particle-laden flows occurs widely, including bulk flows at
low or high Reynolds numbers. We give a few examples of our current work
in this area.
First 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 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 a growth due to coalescence. Our
three-dimensional numerical simulations rationalize the mechanism behind
this surprising phenomenon. Second, we consider low-Reynolds-number flows of colloidal solutions
in channels and porous systems involving chemical gradients. We document how salt
gradients, via a mechanism referred to as diffusiophoresis, can remove
particles from dead-end pores or deliver particles into such pores. The
transport can be size dependent and we explore the phenomenon using
experiments and modeling. Also, we show a final example to show how the effect can produce membraneless filtration.