Acquisition of a Rheometer Enhanced with Scattering and Imaging for Complex Fluids Research and Education

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This award from the Instrumentation for Materials Research program allows Harvard University to acquire a rheometer enhanced with scattering and imaging for complex fluids research and education. One of the key features of all complex fluids is their response to stresses or strains; as with all "soft" materials, the larger scale structures that typify complex fluids make them more easily deformed and thus help define their most fundamental properties. Unfortunately, the relationship of microscopic properties to macroscopic response is still poorly understood for many important complex materials. Of particular interest here are the mechanical responses of foams and emulsions, colloidal suspensions, thin polymer films, and electrorheological suspensions. For example, concerning foams and emulsions, we will explore the influence on the bulk properties of the interfacial rheology of the surfactants used, the related interfacial elasticity, and the drainage of fluid from the interstitial space. In addition, researchers at Harvard University will investigate "jammed" states that dramatically influence the properties of weakly attractive colloidal suspensions. In order to examine these systems, they will develop instrumentation around a stress-controlled rheometer, which will be outfitted with two different optical probes: dynamic light scattering and a high-speed digital camera. This collaborative interdisciplinary research involves scientists from engineering, physics, and materials science.This award from the Instrumentation for Materials Research program allows Harvard University to acquire a rheometer enhanced with scattering and imaging for complex fluids research and education. We are all familiar with solids, liquids, and gases. However, many other common materials have properties that are intermediate to those of simple liquids or simple solids. For example, consider a foam such as used for shaving or washing dishes. These materials undergo a finite strain when exposed to a small stress (it is easy to do this experiment for yourself), which is characteristic of a solid, but the constituents of the foam (mostly water and air) are both fluids! Of course, this response is influenced greatly by the surfactants (basically large macromolecules) that reside at the interface between the gas and liquid. Materials whose macroscopic properties depend crucially on the different constituents are commonly referred to as complex fluids, which includes foams and emulsions, electrorheological suspensions (materials that respond to electric fields), polymer solutions, polymer films, etc. Because the macroscopic response is so dependent on the microscopic constituents, the proposed research will use modern optical and mechanical measurements to investigate further the world of complex fluids, and will elucidate more clearly the manner in which the microscopic and macroscopic properties are coupled.