Abstract
Understanding the correlation between structure and rheology in colloidal suspensions is important as these suspensions are crucial in industrial applications. Moreover, colloids exhibit a wide range of structures under confinement that could considerably alter the viscosity. Here, we use a combination of experiments and simulations to elucidate how confinement induced structures alter the relative contributions of hydrodynamic and repulsive forces to produce up to a ten fold change in the viscosity. We use a custom built confocal rheoscope to image the particle configurations of a colloidal suspension while simultaneously measuring the viscosity. We find a non-monotonic trend to the viscosity under confinement that is strongly correlated with the microstructure. As the gap decreases below 15 particle diameters, the viscosity first decreases from its bulk value, shows fluctuations with the gap, and then sharply increases for gaps below three particle diameters. Further, we compare our experimental results to two simulations techniques that enables us to determine the relative contributions of hydrodynamic and short range repulsive stresses. The first method uses the lubrication approximation to find the hydrodynamic stress and includes a short range repulsive force between the particles and the second is a Stokesian dynamics simulation that calculates the full hydrodynamic stress in the suspension. We find that the decrease in the viscosity at moderate confinements has a significant contribution from both the hydrodynamic and repulsive forces whereas the increase in viscosity at gaps less than three particle diameters arises primarily from short range repulsive forces. These results provide new insights to the unique rheological behavior of confined suspensions and further enable us to tune the viscosity by changing properties such as the gap, polydispersity, and the volume fraction.
Highlights
Since simulations of colloidal and granular systems have both implicated hydrodynamic lubrication forces as the origin for the rapid viscosity increase as the gap is decreased to several particle diameters [26,40], we focus on the large Pe regime where Brownian forces are negligible, the system is no longer thinning, and the viscosity depends weakly on the shear amplitude [10,41]
We use our imaging capability to test the hypothesis that changes in the suspension microstructure are correlated to the observed variations in viscosity in each of these regimes. To address whether these structural changes act through short-range repulsive forces or hydrodynamics, both of which are present in the lubricationrepulsion dynamics simulations, we compare our results to the Stokesian dynamics simulations, which calculate only the hydrodynamic stress contributions and do not include additional short-ranged repulsive interactions
We find that the hydrodynamic interactions from lubrication-repulsion dynamics show quantitative agreement with the Stokesian dynamics simulations at large gaps but show larger decreases under further confinement, even though the Stokesian dynamics simulations show layering under confinement
Summary
This murkiness arises in part due to the difficulty of conducting studies that combine measurements of structure and rheology with simulations in order to distinguish how different structures alter the relative contributions of hydrodynamic and short-ranged interaction forces Such studies in granular suspensions suggest that a rich interplay will occur in colloidal systems. Attempts to simultaneously image the particle arrangements under confined flows studied suspension transport through capillaries and showed the flow rate changes with the particle density and ordering [28,29] In such measurements, it is difficult to determine a structure-dependent viscosity since the total flow rate results from an average over a range of shear rates. This comparison enables us to determine how microstructure alters the balance of short-ranged and hydrodynamic forces to determine the measured viscosity trends
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