45683-AC9
Using Colloidal Suspensions to Model Atomic Scale Lubrication Phenomena
As proposed, we have designed and built a new device enabling simultaneous rheometry and visualization of colloidal suspensions in a thin-film geometry. Our device is unique in its ability to realize a uniform shear flow of extremely high aspect ratio, confining fractions of a milliliter of sample between millimeter-sized parallel plates separated by as little as a few microns. The entire device is loaded onto a confocal microscope allowing direct observation of the suspension microstructure. Suspensions of micron-sized particles have been used to explore a wide range of phenomena, including nucleation, melting, plastic flow, and vitrification. In the present studies, we are exploring the effects of confinement on the phase behavior and rheology of these suspensions.
Applying oscillatory shear to a high volume fraction suspension, we have discovered intermittent transitions between an elastic crystalline phase and a viscous disordered fluid phase. This nonequilibrium behavior is highly unusual in a number of ways. Previous studies on shear melting of colloidal crystals have emphasized a smectic microstructure, consisting of particle layers sliding over one another, which emerges through a shear banding phenomenon. In our experiment, however, the smectic phase is not observed. As the applied strain amplitude is increased sufficiently far, the suspension goes directly from crystalline order to fluid-like disorder. Remarkably, the disordered phase isn't stable either and the suspension later makes a sudden transition back to its crystalline phase, which later destabilizes again. These intermittent phase transitions are mirrored by dramatic fluctuations in our stress measurements, as shown in Figure 1, where the large force spikes occurring when the suspension is disordered.
In the elastic regime, we use particle imaging velocimetry to extract oscillation amplitudes at the upper and lower boundaries of the crystal. The induced strain amplitude obtained from these measurements is a factor of 30 smaller than the applied strain amplitude defined by the relative motion of the upper and lower plates of the shear cell. Previous experiments with less confined suspensions have observed a far smaller difference between applied and induced strain, if any. This naturally raises questions about the general importance of slip in confined suspension rheology and, in particular, about its role in intermittent transitions between ordered and disordered phases at higher applied strain amplitudes. Our working hypothesis is that these transitions are part of a stick-slip phenomenon, in which competition between self-organization principles and instabilities allows the system to explore a range of induced strains. We are also considering possible connections with other complex fluid phenomena such as rheochaos and shear thickening.
We currently preparing to perform more in depth measurements and analysis of both linear and nonlinear aspects of the system's behavior as applied strain is varied, hoping to gain further insight into this phenomenon. To further probe the role of slip, we are designing experiments in which the boundary conditions are altered. Cornell's microlithography facilities give us the ability to replace the smooth surface morphology of our shearing plates with reproducible, patterned boundaries that grip the colloidal particles. The development of this process is nearly complete. We have already made numerous patterns and observed that they can significantly reduce slip. The consequences of patterned boundaries for colloidal phase and flow behavior is an important question in general and should find tribological applications as well.
Figure 1. Intermittent stress measurements
