  We examine experimentally the deformation of flexible, microscale helical
ribbons with nanoscale thickness subject to viscous flow in a microfluidic
channel. Two aspects of flexible microhelices are quantified: the overall shape
of the helix and the viscous frictional properties. The frictional coefficients
determined by our experiments are consistent with calculated values in the
context of resistive force theory. Deformation of helices by viscous flow is
well-described by non-linear finite extensibility. Under distributed loading,
the pitch distribution is non-uniform and from this, we identify both linear
and non-linear behavior along the contour length of a single helix. Moreover,
flexible helices are found to display reversible global to local helical
transitions at high flow rate.
