Description
Solvent evaporation of a complex fluid, such as a colloidal dispersion, is a fundamental step to many industrial processes, including, for example, coating and printing. However, predictive control of the final structure requires an improved understanding of the physical mechanisms governing evaporation-driven flows, particle transport, and consolidation. Confined unidirectional drying offers a well-defined one-dimensional microfluidic geometry in which the coupled evaporation, flow, and transport can be quantitatively probed.
In confined drying geometries, recent theoretical models predict that evaporation-driven transport in colloidal dispersions is governed by distinct regimes, including a capillary-limited regime that is independent of the relative humidity (RH) [1,2]. However, these predictions have not been validated experimentally.
Drying experiments were performed on two aqueous dispersions (Ludox HS-40 and AS-40) of charged silica nanoparticles of different diameters (11 nm and 22 nm), confined in glass capillaries (0.1 × 1 × 100 mm³) and in self-built Hele-Shaw cells with similar confinement, in an RH-regulated environment. In this configuration, evaporation of water occurs only at one end of the capillary, leading to an evaporation-driven flow that concentrates the nanoparticles at the interface and results in the growth of a porous medium through which water transport is governed by capillary and Darcy-like flows. The evolution of the drying front was observed using multiple optical techniques, including Raman spectroscopy to measure the water content in the solid region, Mach–Zehnder interferometry to determine concentration profiles in the non-solid region, and bright-field and fluorescence microscopy to monitor evaporation flux and internal flows.
During drying, multiple phenomena can be observed, including the build-up of concentration gradients and a solid front, the evaporation rate and therefore the transport of water through this solid material, the invasion of air at the tip of the capillary, and crack formation in the solid material. Depending on the relative humidity, we observe both a flow-limited regime, in which the evaporation rate depends on RH, and a capillary-limited regime, in which evaporation is independent of RH. This regime is correlated with the size of the nanoparticles, which influences the pore size of the solid material and therefore the critical capillary pressure. Additionally, we show that cracks in the solid material do not significantly influence the transport of water through the material. All experiments lead to measurements of the permeability, the maximum packing fraction, and the concentration-dependent collective diffusion coefficient of the dispersion.
Overall, these results provide a direct experimental validation of evaporation-driven transport regimes in confined colloidal drying and establish confined unidirectional drying as a microfluidic platform for precise optical characterization of complex fluids.
[1] Vincent, O., Szenicer, A., & Stroock, A. D. (2016). Capillarity-driven flows at the continuum limit. Soft Matter, 12(31), 6656-6661.
[2] Pingulkar, H., & Salmon, J. B. (2023). Confined directional drying of a colloidal dispersion: kinetic modeling. Soft Matter, 19(12), 2176-2185.