A Sliding Plate Microrheometer for Monitoring Structure Evolution in Self-Assembling Peptide Solutions and Other Complex Fluids

The conformational and dynamical behaviour of viscoelastic polymer melts and solutions close to solid boundaries is of great interest for the polymer processing industry. In particular, when the gap approaches microscopic scales, boundary effects such as wall-slip [1], cohesive [2] and adhesive failure [3] occur on the same scale as the overall deformation of the bulk sample and their impact on the measured rheological properties can no longer be neglected. These effects are especially important when structural elements (such as those encountered in self-assembling biopolymer solutions, microgels and emulsions) dominate the viscoelastic properties of the fluid system or when the characteristic microscopic length scales such as the inter-chain separation or the mesh spacing of a gel approach the characteristic dimension of the probe volume. Previous microrheometer designs such as the Surface Force Apparatus (SFA) [4], imbedded probe particles [5] or AFM techniques [6] focus mainly on the studies of thin films in the nanometer range. On the other hand many industrial processing operations as well as the emerging field of microfluidics lead to flows on an intermediate or ‘meso-scale’ range that cannot be readily probed with either bulk rheometry or nanoscale measurements of apparent viscosity or surface friction. With the exception of the work of Granick and co-workers [7] there are few established experimental techniques that are capable of measuring on the meso to micro-scales. The aim of this paper is therefore to introduce a new design of a Flexure-based Microgap Rheometer (FMR) that allows the determination of the viscometric properties of small fluid samples (<10μl) in adjustable gaps that cover meso-scale dimensions of 200 μm down to micro-scale dimensions of 1 μm.