Complex fluids are fluids which are usually comprised of more than one phase of which one is continuous and the others are dispersed in the form of meso-scale structures. Typical examples include polymer solutions and melts, colloidal suspensions, emulsions, foams etc. Many foodstuffs, pharmaceuticals, cosmetcis involve complex fluids during their processing or in their end product. Due to the presence of a dispersed phase, whose length scale is large compared to molecular scales, the structure and flow behaviour of complex fluids are dramatically different from simple (Newtonian) fluids, such as water and air.
We focus on a diverse class of problems spanning both the fundamental and technological aspects of complex fluid behaviour. A major emphasis been to elucidate the role of non-Newtonian rheology on flow field in model configurations, including the flow over a single stationary and rotating cylinder of circular, semi-circular, elliptic and square cross-sections and for two cylinders in a tandem arrangement, over a solid or a fluid sphere, etc. In particular, extensive results have been obtained on the detailed kinematics of the flow in terms of stream line patterns and isotherm contours to delineate the dead zones and local hot/cold regions. This information is crucial to achieve uniform product quality especially during the thermal processing of temperature sensitive materials (such as food-stuffs). Reliable values of the gross transport parameters including drag coefficient and Nusselt number encompassing wide ranges of the Reynolds number, Prandtl number and non-Newtonian flow parameters are also needed for process design calculations.
Complex fluids are also being investigated from the point of view of applications in nanotechnology. Nanomechanics and self-organized pattern formation in highly confined (< 50 nm) polymer films is being investigated with a view to harnessing the Van der Waal's and electric field induced instabilities for submicron patterning of soft materials for applications in functional surfaces (structural colors, control of adhesion, wetting and friction), MEMS/NEMS and sensors. New routes of micro/nano fabrication in polymers are being pursued based on electrospun photopatternable nanofibers of functional polymers, controlled dewetting of thin films and a combinations of a variety of bottom-up and top-down methods.
While traditional fluid mechanics focusses on the rheology of the fluid alone, many biological and microfluidic applications require the channel walls to be deformable. Our research has shown that the deformability of the channel walls can induce or suppress hydrodynamic instabilities which cause laminarturbulent transition in tubes and channels. While it is well known that laminar-turbulent transition in rigid tubes occurs at a Reynolds number near 2000, we find that in flow through soft, deformable tubes, the laminar flow becomes unstable at much lower Reynolds number. Interfacial instabilities that occur in multi-layer flows are shown to be suppressed by tuning the properties of the deformable wall, and this can be potentially used in polymer processing applications.
Another major thrust is to understand ageing and effect of deformation on a variety of soft glassy materials such as concentrated suspensions, glassy polymers and polymer nanocomposites. The common theme in all these systems is their jammed state wherein primary entity (particle or molecular segment) is physically arrested due to overall crowding of constituents. In such a state, the system explores only a small part of the phase space thereby leading to a glassy behavior. Recently we showed that how rheological characterization can give profound insight into the ageing of colloidal glasses compared to that of colloidal gels. We also investigated the effect of deformation on ageing suspensions and how influence of the same on relaxation dynamics can be used to predict long time rheological behavior of these materials.