At large scales, particulate suspensions flow like homogeneous viscous liquids, but at the particle scale, the role of the local heterogeneity brought by the particles cannot be neglected. The volume fraction also matters; in dense suspensions, particulate effects can be felt across distances much larger than the particle diameter. Therefore, whether a suspension should behave as a homogeneous or heterogeneous fluid is a matter of scale. Here, we consider the canonical situation of the pinchoff of suspension drops to study the behavior of suspensions at different scales. Initially, the filament of suspension thins down like a homogeneous liquid until reaching a critical thickness at which the thinning accelerates. Eventually, a region devoid of particles appears, and the breakup occurs similarly to a homogeneous viscous liquid. Although this problem have been studied for almost 20 y, the role of heterogeneity in the acceleration of the pinchoff is still not understood. We show that the onset of heterogeneity corresponds to the dislocation of the suspensions where local fluctuations in particle concentration increase. We derive scaling laws for the dynamics in the heterogeneous regime and develop a model to predict the coherence length at which the discrete nature of the particles appears, and we demonstrate that this length depends both on the particle size and on the volume fraction of the suspension. We extend this approach to polydisperse suspensions. Our work sheds light on the mesoscopic scale below which starts the heterogeneous regime and a continuum approach is not valid anymore.
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Droplet detachment and pinchoff of bidisperse particulate suspensions
When a droplet is generated, the ligament connecting the drop to the nozzle thins down and eventually pinches off. Adding solid particles to the liquid phase leads to a more complex dynamic, notably by increasing the shear viscosity. Moreover, it introduces an additional length scale to the system, the diameter of the particles, which eventually becomes comparable to the diameter of the ligament. In this paper, we experimentally investigate the thinning and pinchoff of drops of suspensions with two different sizes of particles. We characterize the thinning for different particle size ratios and different proportions of small particles. Long before the pinchoff, the thinning rate is that of an equivalent liquid whose viscosity is that of the suspension. Later, when the ligament thickness approaches the size of the large particles, the thinning accelerates and leads to an early pinchoff. We explain how the bidisperse particle size distribution lowers the viscosity by making the packing more efficient, which speeds up the thinning. This result can be used to predict the dynamics of droplet formation with bidisperse suspensions.
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 Award ID(s):
 1944844
 NSFPAR ID:
 10248464
 Date Published:
 Journal Name:
 Soft Matter
 ISSN:
 1744683X
 Format(s):
 Medium: X
 Sponsoring Org:
 National Science Foundation
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