Evaporation-Driven Water-in-Water Droplet Formation

Abstract

We present new observations of aqueous two-phase system (ATPS) thermodynamic and interfacial phenomena that occur inside sessile droplets due to water evaporation. Sessile droplets that contain polymeric solutions, which are initially in equilibrium in a single phase, are observed at their three-phase liquid−solid−air contact line. As evaporation of a sessile droplet proceeds, we find that submicron secondary water-in-water (W/W) droplets emerge spontaneously at the edges of the mother sessile droplet due to the resulting phase separation from water evaporation. To understand this phenomenon, we first study the secondary W/W droplet formation process on different substrate materials, namely, glass, polycarbonate (PC), thermoplastic elastomer (TPE), poly(dimethylsiloxane)-coated glass slide (PDMS substrate), and Teflon-coated glass slide (Teflon substrate), and show that secondary W/W droplet formation arises only in lower-contactangle substrates near the three-phase contact line. Next, we characterize the size of the emergent secondary W/W droplets as a function of time. We observe that W/W drops are formed, coalesced, aligned, and trapped along the contact line of the mother droplet. We demonstrate that this W/W multiple emulsion system can be used to encapsulate magnetic particles and blood cells, and achieve size-based separation. Finally, we show the applicability of this system for protein sensing. This is the first experimental observation of evaporation-induced secondary W/W droplet generation in a sessile droplet. We anticipate that the phenomena described here may be applicable to some biological assay applications, for example, biomarker detection, protein sensing, and point-of-care diagnostic testing.

How liquid-liquid phase separation induces active spreading

“…Discussion and conclusions.-In contrast to binary or ternary droplets on partially wetted surfaces with pinned contact lines [20,21,36,37,[55][56][57][58][59][60], here we report unexpected spreading of phase-separating binary-mixture droplets on fully wetted surfaces with free contact lines. Interestingly, we find that Cox-Voinov law is not applicable anymore for such droplets, because phase separation accelerates the speed of moving contact line.…”

Architecture-Driven Fast Droplet Transport without Mass Loss

“…Droplet motion control has always been a crucial subject of great concern to researchers due to the potential applications in the fields of energy, medicine, and biotechnology. − The inspiration for reaching this purpose comes from nature, where parts of many creatures, such as spider silk, rice leaves, the back of a desert beetle, the beak of a shorebird, , and the peristome surface of Nepenthes , , possess the unique capability of holding and transporting water droplets. The key to achieving spontaneous fast droplet transport is to increase the driving force and suppress the moving resistance.…”

Evaporation-Induced Diffusion Acceleration in Liquid-Filled Porous Materials

“…Surface evaporation from a microfluidic system causes two main functions: fluid transport and material exchange. 32 For example, evaporation would limit the radial permeability of liquid in porous media, 33 control the wetting behavior of liquid on the surface, 34 − 36 and cause the “coffee ring” effect in ink-jet printing. 37 − 39 Evaporation-induced water flow within a porous carbon sheet can drive ions through the narrow porous gap and generate voltage, 40 − 42 which may be used for physiological information monitoring.…”

“…Surface evaporation from a microfluidic system causes two main functions: fluid transport and material exchange . For example, evaporation would limit the radial permeability of liquid in porous media, control the wetting behavior of liquid on the surface, − and cause the “coffee ring” effect in ink-jet printing. − Evaporation-induced water flow within a porous carbon sheet can drive ions through the narrow porous gap and generate voltage, − which may be used for physiological information monitoring. , In energy and environmental applications, evaporation accelerates salt transport in salt marshes; for a hydrogel-based evaporator, water evaporates through the surface of the gel, which promotes the separation of Na + , Cl – and pure water, thus realizing seawater purification. ,− In biomedical engineering, surface evaporation from foam dressing affects the rate of wound healing due to changes in moisture permeability and internal nutrient transfer . During the storage and use of flexible electronic materials, evaporation affects the distribution of the electrolyte concentration in hydrogels, thus changing the conductivity of the material and its functions. , Further, evaporation was found to reduce the speed of wicking flow in paper-based diagnostics. − Given that the diffusion time scale is much larger than that of wicking flow, the change of wicking flow by evaporation should influence the diffusion process .…”