Drying of Complex Suspensions
Entry by Leon Furchtgott, APP 225 Fall 2010.
Drying of Complex Suspensions" Lei Xu, Alexis Berges, Peter J. Lu, Andre R. Studart, Andrew B. Schofield, Hidekazu Oki,6 Simon Davies, and David A. Weitz, Phys. Rev. Lett. 104, 128303 (2010).
The authors investigate the 3D structure and drying dynamics of complex mixtures of emulsion droplets and colloidal particles, using confocal microscopy. Air invades and rapidly collapses large emulsion droplets, forcing their contents into the surrounding porous particle pack at a rate proportional to the square of the droplet radius. By contrast, small droplets do not collapse, but remain intact and are merely deformed.
Drying of suspensions of colloids is important in a lot of areas, from the coffee-ring effect to technologically important phenomena such as paint and cosmetics drying. This paper seeks to understand a slightly simplified version of these phenomena: the drying of a mixture of an emulsion and colloidal particles. Most emulsions scatter light significantly making them difficult to image; this paper gets around this challenge and resolves the 3D structure of the mixture using confocal microscopy.
The authors suspend colloidal spheres of PPMA with radius 1 micron in DHN. separately, they create an emulsion of an aqueous phase, comprising equal volumes of water and glycerol, and PGPR-90 surfactant in nonpolar DHN; a homogenizer creates polydisperse droplets from microns to tens of microns. They combine the particle suspension and emulsion to create a particle-droplet mixture. The particular components ensure that the refractive indices of all the components are matched to limit scattering. The authors can then image the entire 3D structure with confocal fluorescence microscopy with single-particle resolution (Figure 1).
The authors observe that the particles first jam into a solidified pack, throughout which emulsion drops are dispersed; a front of air then passes through the entire system. When this drying front reaches large emulsion droplets, the droplets unexpectedly collapse and their internal contents are forced into the pore space between the surrounding colloids, driven by an imbalance of pressures at the droplets' interfaces with air and with the solvent (Figure 2).
By contrast, small droplets are deformed by the drying front, yet remain intact without bursting (Fig 4 a-c). The authors make a simple model based on coupling the Laplace pressure to Darcy's law to estimate the threshold radius for evacuation and the rate of large-droplet evacuation. (Figure 3).
The authors observe two qualitatively different behaviors: droplets that evacuate and collapse, creating large voids; and droplets that remain intact during drying, yielding void-free particle packs. They use this dichotomy to produce hierarchical materials with several different controllable length scales, by varying droplet and particle sizes (Fig 4 d-f).