a Department of Chemical and Biomolecular Engineering, The
b Hatsopoulos Microfluids Laboratory,
Department of Mechanical Engineering,
c Department of Chemical Engineering,
The
We
explore the interplay of fluid inertia and fluid elasticity in planar entry
flows by studying the flow of weakly elastic solutions through microfabricated planar contraction geometries. The
small characteristic length scales make it possible to achieve a wide range of Weissenberg numbers (0.4 < Wi < 42) and Reynolds numbers (0.03 < Re <
12), allowing access to a large region of Wi-Re space
that is typically unattainable in conventional macroscale
entry flow experiments. Experiments are carried out using a series
of dilute solutions (0.78 <c/c*< 1.09) of a high molecular weight
polyethylene oxide, in which the solvent viscosity is varied in order to
achieve a range of elasticity numbers, 2.8 < El = Wi/Re
< 68. Fluorescent streak imaging and micro-Particle Image
Velocimetry (µPIV) are used to characterize the
kinematics, which are classified into a number of flow regimes including
Newtonian-like flow at low Wi,
steady viscoelastic flow, unsteady diverging flow and
vortex growth regimes. Progressive changes in the centreline
velocity profile are used to identify each of the flow regimes and to map the
resulting stability boundaries in Wi-Re space.
The same flow transitions can also be detected through measurements of the
enhanced pressure drop across the contraction/expansion which arise from fluid viscoelasticity.
The results of this work have significant design implications for lab-on-a-chip
devices, which commonly contain complex geometric features and transport
complex fluids, such as those containing DNA or proteins. The results
also illustrate the potential for using microfabricated
devices as rheometric tools for
measuring the extensional properties of weakly elastic fluids.