How Fast do Microdroplets Generated During Liquid–Liquid Phase Separation Move in a Confined 2D Space?

Abstract

How liquid transport occurs in confined spaces is relevant to many industrial and lab-scale processes, ranging from enhanced oil recovery to drug delivery systems. In this work, we investigate propelling microdroplets that form from liquid–liquid phase separation in a quasi-2D chamber, focusing on the direction and speed of microdroplets in response to local composition gradients. The confined ternary solution in our experiments comprises a model oil (the main one being octanol), a good solvent (ethanol), and a poor solvent (water). It is displaced by water. Depending on the initial solution composition, water-rich or oil-rich microdroplets, ∼1/4–1/3 of the height of a narrow and wide microchamber form and move spontaneously as the ternary solution mixes with and is displaced by water diffusing from a deep side channel. Microdroplet movement is followed in situ using high-speed bright-field imaging or fluorescence imaging when the solution is doped with a dye. Local ethanol composition gradients are estimated from the variation of fluorescence intensity in the local continuous liquid surrounding of the mobile microdroplets. From phase separation of the ternary solution with high oil concentration, mobile oil-rich microdroplets form in a water-rich zone, accompanying the formation of water-rich microdroplets in an oil-rich zone. The fast movement of oil-rich microdroplets induces directional flow transport that mobilizes water-rich microdroplets close to the water-rich zone. Regardless of the initial composition of the solution, displacement of oil-rich microdroplets extends linearly with time. The average microdroplet speed increases with the initial oil concentration in the ternary solution. The fastest speed of oil-rich microdroplets observed in our experiments is ∼150 μm/s along the surface of a hydrophobic wall. The presence of a sharp ethanol composition gradient is thought to be the primary driving force for the fast movement of oil-rich microdroplets in confinement. Our results demonstrate the potential of enhancing liquid transport in confinement through composition gradients arising from phase separation.

Review of microscale dynamics of dilution-induced asphaltene precipitation under controlled mixing conditions

“…In the study of the liquid-liquid separation, diffusive-dominated mixing causes the branch-like the structure of droplets, as shown in Figure [85], as shown in Figure 7(a). Arends et al [88,89] found a jetting phenomenon of the droplets, and the speed of the jetting was related to the composition of the solution. Meng et al [87] successfully applied the proposed…”

Review of Microscale Dynamics of Dilution-Induced Asphaltene Precipitation under Controlled Mixing Conditions

“…In the study of the liquid–liquid separation, diffusive-dominated mixing causes the branch-like the structure of droplets, as shown in the figure, as shown in Figure a. Arends et al , found a jetting phenomenon of the droplets, and the speed of the jetting was related to the composition of the solution. Meng et al successfully applied the proposed quasi-2D chamber to the study of liquid–solid separation systems such as oiling-out crystallization, as shown in Figure b.…”

Flows of a nonequilibrated aqueous two-phase system in a microchannel

“…These flows are not only set by the interaction between the different species and the gradient in composition, but also can be influenced strongly by the presence of solid surfaces and the wettability of each phase to the surface. For example, nucleated droplets may experience unidirectional propulsion due to Marangoni flow 27,29,30 set by a gradient in composition. Colloidal particles present in a phase separating mixture may experience phoretic motion, as the composition near the particle surface changes.…”