Interfacial dynamics in complex fluids

Abstract

Complex fluids refer to those with internal microstructures whose evolution affects the macroscopic dynamics of the material, especially the rheology [7]. Examples include polymer solutions and melts, liquid crystals, gels and micellar solutions. Such materials often have great practical utilities since the microstructure can be manipulated via processing flow to produce outstanding mechanical, optical or thermal properties. A good example is main-chain liquid-crystalline polymers (LCPs). Their molecular backbone is rodlike, with a degree of rigidity, such that the polymer assumes an anisotropic orientational order due to spontaneous alignment of the molecules. This order, further enhanced by extensional flows, leads to exceedingly high strength and modulus in the Kevlar fiber, a commercially successful product of du Pont. An important way of utilizing complex fluids is through composites. By blending two immiscible components together, one may derive novel or enhanced properties from the composite, and this is often a more economical route to new materials than synthesis. Moreover, the properties of composites may be tuned to suit a particular application by varying the composition, concentration and, most importantly, the interfacial morphology. Take polymer blends for example [11]. Under optimal processing conditions, the dispersed phase is stretched from drops into a fibrillar morphology. Upon solidification, the long fibers act as in situ reinforcement and impart great strength to the composite. The effect is particularly strong if the fibrillar phase is liquid crystalline [2]. The dispersed phase may also be solid as in colloidal dispersions, or gas as in thermoplastic foams. From a scientific viewpoint, the essential physics in all such composites is the coupling between interfacial dynamics and complex rheology of the components. Despite their practical importance, our current knowledge of two-phase complex fluids is very limited. The main difficulty is that these materials have a myriad of internal boundaries, which move, deform, break up and reconnect during processing. This leads to a seemingly intractable mathematical problem, and also hampers experimental observation and measurement. A secondary difficulty is that the rheology of each component alone is highly complex, with the internal microstructure coupled with the flow field. Thus, these materials feature dynamic coupling of three disparate length scales: molecular conformation inside each component, mesoscopic interfacial morphology and macroscopic hydrodynamics. An understanding of the interfacial dynamics in complex fluids is a major fundamental challenge as well as a significant practical need. The problem involves several traditional disciplines: mathematical modeling, numerical computation, soft-matter physics, fluid mechanics, material science and engineering. An objective of the workshop is to explore new research directions in the context of multi-disciplinary interactions.