(Received:
We 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 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