Growing numbers of applications including proteomics, cosmetics, and thin film coatings use novel viscoelastic materials that derive their rheological properties from micro scale structure created by the inclusion of long chain molecules, nano particles and dispersed fluids. Characterization of these novel materials is often difficult due to the small volumes initially formulated. Therefore, micro rheology and thin film rheology techniques have been employed to characterize these materials. Micro rheology commonly refers to analyzing the motion of micro probe particles to measure fluid response at small length scales. This method examines a small area around the probe particle, ignoring a fluid s micro structure. We demonstrate a MEMs parallel plate rheometer for micro rheology that confines viscoelastic materials to length scales on the order of a micrometer, but probes the entire material response to dynamic oscillatory shear. The MEMs Parallel Plate rheometer uses a 1 mm square nano positioner stage to apply a controlled sinusoidal strain, with displacement up to 15 micrometers. The instrument monitors two displacements on the MEMS platform, which measure the stress and strain in the sample. The device is suitable therefore for strain- or stress-controlled operation. Through physical modeling of the system, both storage and loss moduli can be extracted for a wide range of frequencies, from 0.5 to 500 Hz. The confinement of the fluid is set by adjusting the gap between the stage and a transparent cover plate that allows optical observation. By decreasing this gap, the increasing effects of confinement can be observed. Because the strain is applied to the entire fluid body, this device examines the effects of confinement on the entire micro structure. Furthermore, this device uses less than 1 nL of material, which is beneficial for these types of novel experimental materials.
Citation: Complex Fluids Group Highlight
Pub Type: Websites
Rheology, MEMS, Complex Fluids, Viscoelastic Fluids and Gels