Very-low-Re chaotic motions of viscoelastic fluid and its unique applications in microfluidic devices: A review

This paper presents a comprehensive review on the peculiar phenomena of elasticity-induced instabilities, transition to turbulence and elastic turbulence in very low Reynolds number (Re) viscoelastic fluids flows, as well as their particular applications of those viscoelasticity-induced phenomena in microfluidic devices. It is well-known that the addition of polymers or surfactants additives into normal solvents (like water) can make the solutions show remarkable viscoelastic properties, which are varying with the additives concentration and temperature. The elasticity of the solution stems from the flexible molecular chains, which can be stretched or reassemble themselves in fluid motion, and hereby induce the flow instabilities due to the initial perturbation. These instabilities are common in the flow devices with curvilinear streamlines, such as Taylor–Couette geometry, cone-and-plate geometry and plate-and-plate geometry, and all occur at relatively low-Re, hence called elastic instabilities. In microscopic flow, Re is naturally quite low and the flow is definitely laminar for Newtonian fluids. Nevertheless, flow instabilities, even chaotic flow patterns happen for viscoelastic fluids, provided the fluid elasticity is strong enough. For a practical purpose, triggering elastic instabilities or turbulence in the microchannel devices will be favorable for mixing enhancement, reaction acceleration and potential heat transfer enhancement, which are usually limited by the laminar flow nature greatly. Using viscoelastic fluid coupling to curvatures of geometry, the low-Re irregular flow behaves counter-intuitively compared with normal fluids and can functionally work in particularly designed microfluidic devices. With wide applications of microfluidics in the fields of chemical science, medical engineering and biotechnology, etc., and combination with microfabrication technology, the instabilities’ occurrence and applications will be a promising way for functional microchip design with multipurpose. Till now, though huge developments in microfluidics using viscoelastic fluid flow have been achieved, challenges and research interests still remain for the future.