Drying dynamics of a charged colloidal dispersion in a confined drop

Abstract

We performed a thorough investigation of the drying dynamics of a charged colloidal dispersion drop in a confined geometry. We developed an original methodology based on Raman micro-spectroscopy to measure spatially-resolved colloids concentration profiles during the drying of the drop. These measurements lead to estimates of the collective diffusion coefficient of the dispersion over a wide range of concentration. The collective diffusion coefficient is one order of magnitude higher than the Stokes-Einstein estimate showing the importance of the electrostatic interactions for the relaxation of concentration gradients. At the same time, we also performed fluorescence imaging of tracers embedded within the dispersion during the drying of the drop, which reveals two distinct regimes. At early stages, concentration gradients along the drop lead to buoyancy-induced flows. Strikingly, these flows do not influence the colloidal concentration gradients that generate them, as the mass transport remains dominated by diffusion. At longer time scales, the tracers trajectories reveal the formation of a gel which dries quasi homogeneously. For such a gel, we show using linear poro-elastic modeling, that the drying dynamics is still described by the same transport equations as for the liquid dispersion. However, the collective diffusion coefficient follows a modified generalized Stokes-Einstein relation, as also demonstrated in the context of unidirectional consolidation by Style et al. [Crust formation in drying colloidal suspensions, Style et al., Proc. R. Soc. A 467, 174 (2011)].

Drying-induced back flow of colloidal suspensions confined in thin unidirectional drying cells

“…Because of the thin geometry, it has been assumed that suspension ow is almost ideal. However, recent studies have revealed that vertical ows are induced spontaneously, even in a thin liquid layer owing to drying-induced increase in the solute concentration [18][19][20][21] and in the concentration of suspended particles. 22 These examples suggest that ow is complex, even in a drying thin liquid layer between parallel plates.…”

Buoyancy-driven dispersion in confined drying of liquid binary mixtures

“…[11][12][13]. We will assume an initial concentration Φ i = 0.001 and radius a = 5 nm for the silica nanoparticles, leading to a diffusivity D 4.37 × 10 −11 m 2 /s according to the Stokes-Einstein relation for a temperature of 25 • C (we do not consider here enhanced values due to colloidal interactions occurring for such systems at high concentrations [4,16]). For such a very dilute silica dispersion, the solutal expansion coefficient at the reference concentration Φ i is well approximated by β s ρ s /ρ w − 1 1.2 where ρ s 2200 kg/m 3 is the density of silica, and ρ w 1000 kg/m 3 that of water.…”

“…When these gradients are orthogonal to gravity, they inevitably generate buoyancydriven flows, that then alters the drying process when coupled to the overall mass transport. Solutal buoyancy-driven free convection is generally relevant at relatively large scales, but many recent experiments reported such flows in confined microfluidic geometries (10-100 µm): drying of confined [1][2][3][4] and sessile droplets [5][6][7], or any other microfluidic configuration generating concentration gradients [8,9]. At small scales, buoyancy-driven flows are expected to play a minor role on mass transport, owing to the high viscous dissipation and fast solute diffusion [10].…”

Integrating optical coherence tomography with gravimetric and video analysis (OCT-Gravimetry-Video method) for studying the drying process of polystyrene latex system

“…Gravimetric analysis and videography, which record global water loss and film appearance respectively, are most common methods 7 , 17 . Nuclear magnetic resonance profiling (NMR) 11 , Raman microspectroscopy 18 and infrared microscopy 19 can non-destructively measure the distribution of water concentration inside the latex, but lack the resolution to see particles. With microns or sub-micron resolutions, environmental scanning electron microscopy (ESEM), ellipsometry, atomic force microscopy (AFM) were used to detect particles’ packing process, during which particles gathered with their Brownian motions restricted 9 , 20 – 22 .…”