Self-propelling Microdroplets Generated and Sustained by Liquid-liquid Phase Separation in Confined Spaces

Flow transport in confined spaces is ubiquitous in technological processes, ranging from separation and purification of pharmaceutical ingredients by microporous membranes and drug delivery in biomedical treatment to chemical and biomass conversion in catalyst-packed reactors and carbon dioxide sequestration. In this work, we suggest a distinct pathway for enhanced liquid transport in a confined space via self-propelling microdroplets. These microdroplets can form spontaneously from localized liquid-liquid phase separation as a ternary mixture is diluted by a diffusing poor solvent. High speed images reveal how the microdroplets grow, break up and propel rapidly along the solid surface, with a maximal velocity up to ~160 um/s, in response to a sharp concentration gradient resulting from phase separation. The microdroplet self-propulsion induces a replenishing flow between the walls of the confined space towards the location of phase separation, which in turn drives the mixture out of equilibrium and leads to a repeating cascade of events. Our findings on the complex and rich phenomena of self-propelling droplets suggest an effective approach to enhanced flow motion of multicomponent liquid mixtures within confined spaces for time effective separation and smart transport processes.Comment: This is the authors' submitted version of the manuscrip

Surface Nanodroplets: Formation, Dissolution, and Applications

Droplets at solid-liquid interfaces play essential roles in a broad range of fields, such as compartmentalized chemical reactions and conversions, high-throughput analysis and sensing, and super-resolution near-field imaging. Our recent work has focused on understanding and controlling the nanodroplet formation on solid surfaces in ternary liquid mixtures. These surface nanodroplets resemble tiny liquid lenses with a typical height of <1 μm and a volume of subfemtoliters. The solvent exchange is based on the process of displacing a droplet liquid solution by a poor solvent to create a transient oversaturation for droplet formation. A quantitative understanding of growth dynamics of surface nanodroplets in ternary liquid mixtures not only provides insight into the liquid-liquid phase separation induced by solvent addition in general but also has made it possible to control the droplet size well. This review article will summarize our findings in the last ∼5 years from the research with our collaborators. The first part will explain the fundamental aspects that are key to the formation and stability of surface nanodroplets. In the second part, we will highlight the applications of nanodroplets in chemical analysis and functional surface fabrication and finally point out future directions in droplet-based applications

Enhanced displacement of phase separating liquid mixtures in 2D confined spaces

Displacing liquids in a confined space is important for technological processes ranging from porous membrane separation to CO2 sequestration. The liquid to be displaced usually consists of multiple components with different solubilities in the displacing liquid. Phase separation and chemical composition gradients in the liquids can influence the displacement rate. In this work, we investigate the effects of liquid composition on the displacement process of ternary liquid mixtures in a quasi-2D microchannel where liquid−liquid phase separation occurs concurrently. We focused on model ternary mixtures containing 1-octanol (a model oil), ethanol (a good solvent), and water (a poor solvent). These mixtures are displaced with water or with an ethanol aqueous solution. As a comparison, for some experiments, water was displaced by ternary mixtures. The bright-field and fluorescence imaging measurements reveal distinct phase separation behaviors. The spatial distribution of subphases arising from phase separation and the displacement rates of the solution are impacted by the initial ternary solution composition. The boundary between the solution and displacing liquid changes from a defined interface to a diffusive interface as the initial 1-octanol composition in the solution is reduced. The displacement rate also varies non-linearly with the initial 1-octanol composition. The slowest displacement rate arises at intermediate 1-octanol concentration, where a stable three-zone configuration forms at the boundary. At very low 1-octanol concentration, the displacement rate is fast, associated with droplet formation and motion driven by the chemical concentration gradients formed during phase separation. The excess energy provided from phase separation may contribute to the enhanced displacement at intermediate to high 1-octanol concentrations but not at low 1-octanol concentration with enhancement from induced flow in confinement. The knowledge gained from this study highlights the importance of manipulating phase separation to enhance mass transport in confinement for a wide range of separation processes

Microfluidic device coupled with total internal reflection microscopy for in situ observation of precipitation

In situ observation of precipitation or phase separation induced by solvent addition is important in studying its dynamics. Combined with optical and fluorescence microscopy, microfluidic devices have been leveraged in studying the phase separation in various materials including biominerals, nanoparticles, and inorganic crystals. However, strong scattering from the subphases in the mixture is problematic for in situ study of phase separation with high temporal and spatial resolution. In this work, we present a quasi-2D microfluidic device combined with total internal reflection microscopy as an approach for in situ observation of phase separation. The quasi-2D microfluidic device comprises of a shallow main channel and a deep side channel. Mixing between a solution in the main channel (solution A) and another solution (solution B) in the side channel is predominantly driven by diffusion due to high fluid resistance from the shallow height of the main channel, which is confirmed using fluorescence microscopy. Moreover, relying on diffusive mixing, we can control the composition of the mixture in the main channel by tuning the composition of solution B. We demonstrate the application of our method for in situ observation of asphaltene precipitation and $$\beta$$-alanine crystallization