MEMS PARALLEL PLATE RHEOMETER FOR OSCILLATORY SHEAR MICRO RHEOLOGY MEASUREMENTS
Gordon Christopher1, Nicholas Dagalakis2, Steven Hudson1, and Kalman Migler1
1Complex Fluids Group, Polymers Division, MSEL, NIST
2Systems Integration Group, Intelligent Systems Division, MEL, NIST
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. These applications also often involve flow through confined geometries which deform the micro structure, altering the materials’ rheology and limiting their effectiveness. 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. A number of methods exist to study thin films, but they rely on complex modeling of probe geometry, contact area and material properties to extract anything more than the elastic modulus of a film
We propose a MEMs parallel plate rheometer for micro rheology that will confine viscoelastic materials to length scales on the O(1) um, but probe the entire material response to dynamic oscillatory shear. The MEMs Parallel Plate rheometer uses a 1mm square nano positioner stage to apply a controlled sinusoidal strain. Through physical modeling of the system, both storage and loss moduli can be extracted for a wide range of frequencies. 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 10 nL of material, which is beneficial for these types of novel experimental materials.