Workshop Microrheology and Rheological Phenomena in Microfluidics location:Karlsruhe, Germany date:4-5 October 2006
The term „Microrheometry” is primarily associated with either the tribological study of sliding friction in thin films or the monitoring of a local rheological response using nanometer-sized particles. The nanorheological response of molecularly-thin films has been studied extensively in the past using AFM techniques as well as variants of the Surface Force Apparatus (SFA). Numerous particle-based microrheometric techniques have also been developed over the past decade: variants include the analysis of thermally-excited (or ‘passively driven’) micron-sized particles through either diffusing wave spectroscopy or the microscopic tracking of single particles or pairs of particles. Actively-driven techniques that directly measure the forces on microscopic beads have also been developed using optical traps and magnetic
Whilst all the techniques above probe the microrheological response on the submicron scale, many industrial processing operations, as well as the emerging field of microfluidics, lead to flows on an intermediate or ‘meso-scale’ range (roughly spanning the range of 1m - 100m) that cannot be readily probed with either conventional bulk rheometry or nanoscale measurements of the apparent viscosity or surface friction. There are few established experimental techniques that are capable of quantitatively measuring the true viscometric material functions of a bulk sample of a complex fluids under homogeneous deformation conditions on the meso to micro-scales.
We therefore focus in this paper on the development of a new technique, the Flexure-based Microgap Rheometer (FMR), that is capable to measure the viscometric properties of small bulk fluid samples (<10l) in adjustable gaps that cover meso-scale dimensions of 200m down to micro-scale dimensions of 1m. We demonstrate the capability of the FMR by characterizing the complex and gap dependent micro”bulk”rheometrical properties of minute amounts of typical microstructured food and cosmetic products and correlate the microgap-dependent rheological response to the static and transient microstructure of these complex systems.