A singularity is in general a point where a parameter fails to be well-behaved in some particular way. Singularities in science are ubiquitous and often provide a method to probe basic theories and gain fundamental insights into a particular problem.
The cross-slot microchannel is a common flow geometry that is typically used to create a planar extensional flow field. The symmetry of the geometry results in the occurrence of an isolated singularity point at the precise center of the flow field. Elongation occurs as fluid accelerates away from the stagnation point along the axis of the outlet channels.The image here shows the nature of the flow field in a cross-microchannel with a Newtonian fluid at U = 0.01 m/s, Re << 1. The streaks are generated by stacking a sequence of images of fluorescent particles flowing in channel. The image is symmetric about two axes, which coincide at the stagnation point at the center. Flow enters through the top and bottom channels and exits through the left-and right-hand channels.