The flexure-based microgap rheometer (FMR)

Abstract

SynopsisWe describe the design and construction of a new microrheometer designed to facilitate the viscometric study of complex fluids with very small sample volumes (1-10 µl) and gaps of micrometer dimensions. The Flexure-based Microgap Rheometer (FMR) is a shear-rate-controlled device capable of measuring the shear stress in a plane Couette configuration with directly-controlled gaps between 1 µm and 200 µm. White light interferometry and a three-point nanopositioning stage using piezo-stepping motors are used to control the parallelism of the upper and lower shearing surfaces which are constructed from glass optical flats. A compound flexure system is used to hold the fluid sample testing unit between a drive spring connected to an 'inchworm' motor and an independent sensor spring.Displacements in the sensing flexure are detected using an inductive proximity sensor. Ready optical access to the transparent shearing surfaces enables monitoring of the structural evolution in the gap with a long working-distance video-microscope. This configuration then allows us to determine the microgap-dependent flow behavior of complex fluids over 5 decades of shear rate. We demonstrate the capability of the FMR by characterizing the complex stress and gap dependent flow behavior of a typical microstructured food product (mayonnaise) over the range of gaps from 8 to 100 µm and stresses from 10 to 1500 Pa. We correlate the gap-dependent rheological response to the microstructure of the emulsion and changes induced in the material by prolonged shearing.

Lateral capillary forces of cylindrical fluid menisci: a comprehensive quasi-static study

“…By fixing the dimensions of the confining pads and the meniscus fluid and height beforehand, the setup allowed for repeatable experiments. Our setup may be roughly likened to an upscaled version of the flexure-based microgap rheometer developed by Clasen et al to study microgap-dependent flow behavior of complex fluids [48]. The experimental results obtained for circular pads were then supported by both a finite-element (section 3) and an analytical model (section 4) predicting restoring forces for small perturbation of cylindrical fluid menisci, with good accuracy (section 5).…”

Rheology of microgels in single particle confinement

“…The study of the conformational and dynamical behaviors of such fluids in confinements is thus of great interest from an industrial as well as a fundamental point of view. When the characteristic length scale of the flow geometry approaches the microstructural length scale of the fluid, boundary effects such as wall-slip (Barnes 1995;Black and Graham 1999), cohesive failure (Reimers and Dealy 1998), and adhesive failure (Migler et al 1993) occur on the same length scale as the overall dynamics of the sample (Clasen and McKinley 2004;Clasen et al 2006;Baik et al 2011) and can no longer be neglected. Also, the rheological behavior of thin fluid films of complex materials can be different from the corresponding bulk behavior (Clasen and McKinley 2004;Clasen et al 2006).…”

Ex vivo rheology of spider silk

“…The imposed shear rate (␥), defined as the ratio of the actuated plate velocity (v) and the inter-plate gap (h), ␥=v/h, can be varied over the range 2ϫ10 -4 <␥<4ϫ10 2 ·s -1 . Further details of the instrumentation are provided elsewhere (Clasen et al, 2006;Clasen and McKinley, 2004).…”

“…These large deformations influence the development of non-equilibrium texture morphologies in the spun silk (Vollrath and Knight, 2001) that lead to observational phenomena such as super-contraction (Vollrath et al, 1996) and shape-memory effects (Emile et al, 2006). In order to address these issues, two new micro-rheometric instruments have been constructed: a flexure-based micro-rheometer (Clasen et al, 2006;Clasen and McKinley, 2004;Gudlavalleti et al, 2005) for steady and oscillatory shearing measurements and a capillary break-up micro-rheometer for extensional rheometry (Bazilevsky et al, 1990;McKinley and Tripathi, 2000).…”