A new parallel plate shear cell for in situ real-space measurements of complex fluids under shear flow

Abstract

We developed and tested a parallel plate shear cell that can be mounted on top of an inverted microscope to perform confocal real-space measurements on complex fluids under shear. To follow structural changes in time, a plane of zero velocity is created by letting the plates move in opposite directions. The location of this plane is varied by changing the relative velocities of the plates. The gap width is variable between 20 and 200 microm with parallelism better than 1 microm. Such a small gap width enables us to examine the total sample thickness using high numerical aperture objective lenses. The achieved shear rates cover the range of 0.02-10(3) s(-1). This shear cell can apply an oscillatory shear with adjustable amplitude and frequency. The maximum travel of each plate equals 1 cm, so that strains up to 500 can be applied. For most complex fluids, an oscillatory shear with such a large amplitude can be regarded as a continuous shear. We measured the flow profile of a suspension of silica colloids in this shear cell. It was linear except for a small deviation caused by sedimentation. To demonstrate the excellent performance and capabilities of this new setup we examined shear induced crystallization and melting of concentrated suspensions of 1 microm diameter silica colloids.

Melting and crystallization of colloidal hard-sphere suspensions under shear

“…Previous measurements showed that in the parallel plate shear cell, the flow profile of a dilute colloidal suspension of these silica particles in ETPTA at a volume fraction of ϭ 0.22 was linear as expected (46). However, the flow profile of a partially crystallized suspension deviated from linearity.…”

“…The counterrotation principle creates a plane of zero velocity in which a collection of particles can be tracked for an extended time. In the present work, we also used a counter translating parallel plate shear cell with a small enough plate separation that we could observe individual particles throughout the gap with a confocal microscope (46). This enabled us to obtain local information about the complete system.…”

A multi-axis confocal rheoscope for studying shear flow of structured fluids

“…II-IV, we have demonstrated that our confocal rheoscope accurately measures the structural and stress responses of a complex fluid in a precisely controlled shear flow. Similar to other confocal rheometers 23,25,[29][30][31][32][33][35][36][37][38][39][40][41][42][43][44] this instrument allows us to investigate the interplay between a suspension's structure and its rheology. However, our device also allows for biaxial rheological measurements and the study of confined systems.…”

“…23,29,31,33,[39][40][41][42][43][44] Again, the simplest designs have one surface fixed in the laboratory frame, though independently moving both is possible and has the same advantages as in the analogous rotational design. 33,44 In these planar designs, with careful alignment of the boundaries, sample thicknesses of just a few particle diameters become possible, allowing exploration of thin film flow behaviors associated with confinement. 41,45,46 However, force measurement is a significant challenge in this flow geometry.…”

“…Using this technique, individual structures of interest can be followed in real time or, by scanning through the sample, the full three-dimensional (3D) structure can be mapped out, allowing for accurate reconstruction of flow profiles using particle velocimetry and detailed measurements of a material's dynamic microstructure. [29][30][31][32][33][34] Two types of experimental designs combining confocal microscopy with precise flow control have been reported. The major distinction between these two types is the geometry of the flow.…”