  Experimental measurements and numerical simulations are made on fluid flow
through cross-slot devices with a range of aspect (depth:width) ratios, 0.4 <
alpha < 3.87. For low Reynolds numbers Re, the flow is symmetric and a sharp
boundary exists between fluid streams entering the cross-slot from opposite
directions. Above an alpha-dependent critical value Re_c, the flow undergoes a
symmetry-breaking bifurcation (though remains steady and laminar) and a spiral
vortex structure develops about the central axis of the outflow channel. An
order parameter characterizing the instability grows according to a sixth-order
Landau potential, and shows a progression from second order to first order
transitions as alpha increases. A tricritical point occurs for alpha ~ 0.55.
The spiral vortex acts as a mixing region in the flow field and this phenomenon
can be used to drive enhanced mixing in microfluidic devices.
