Self-generated Motion of Droplets Induced by Korteweg Force

Abstract

Phase-separating droplets exhibit self-generated motion depending on the composition of the constituent solution of a biocompatible material, aqueous two phase system. For higher concentrations than the equilibrium concentration, a droplet moved unidirectionally even in a homogeneous concentration field. For lower concentrations, the droplets hardly moved. The translational motion we observed is an effect of convection caused by Korteweg force.Chemical methods for controlling droplet motion provide the droplets with stimulus-responsive functions such as performing self-propelled motion 16 or changing in the mode of motion depending on the materials in the surrounding environment. Therefore, chemical methods are considered to be the most preferable in the development of functional fluids for accomplishing particular tasks in microscopic spaces beyond the control of external forces. However, conventional chemical methods are performed by releasing the high interfacial energy inherent in the surface of a substrate 1,46,7b or droplet 2,3,7a,7c due to chemical reactions and by converting that gain into kinetic energy. Thus, in the former case, the droplets can move only on a specific substrate. In the latter case, the droplets inevitably consist of materials with differing chemical properties from the surrounding media to produce high interfacial energy. For use of self-propelled droplets in vivo, the range of application of these systems is limited. To overcome these limitations, we employed biocompatible material, aqueous two phase systems (ATPS), to make droplets move without surfactants and pretreatment of a substrate.In our system, the interface in the initial stage does not have the driving force to propel droplets because the ATPS have quite low interfacial energy. The driving force is generated as follows. When an aqueous phase that differs from the equilibrium composition comes into contact with another aqueous phase, the composition in the vicinity of the interface changes depending on the difference between the initial and the equilibrium free energies, giving rise to a concentration gradient at the liquid liquid interface. As a result, an interfacial energy arises at the liquidliquid interface creating a composition-dependent force called Korteweg force.812 This force generates convective flow around the droplet, inducing motion at the center of the mass of the droplet. The fact that this method makes droplets move not by consuming energy inherent in the interface but by using interfacial energy generated by variations in the composition of the phase is a large point of difference from conventional methods. In this paper we experimentally investigate the effect of composition of the phase on droplet motion.Experiments were performed using ATPS of poly(ethylene glycol) (PEG) + Na 2 SO 4 . PEG 8000 (weight-average molecular weight: M w = 8000) and Na 2 SO 4 (high grade) was obtained from Wako Pure Chemical Ind., and both were used without further purification. Aqueous solution containing 13% w/w Na 2 SO 4...

Nonequilibrium Interfacial Tension in Simple and Complex Fluids

“…Notwithstanding this transient, nonequilibrium character, the effective interfacial tension between miscible fluids plays a key role in a wide range of research fields of both academic and practical interest [19], from geosciences (e.g., in mantle convection, magma fragmentation, and the dynamics of Earth's core [20,21]) to hydrology [22], oil recovery [10], filtration and flow in porous media (e.g., in a chromatography column [23]), fluid removal [24], aquifer and soil remediation [25,26], and the modeling of hydrodynamic instabilities, e.g., in Rayleigh-Taylor [27] and Hele-Shaw [28,29] flows. Very recently, convection induced by Korteweg stresses has been proposed as a new mechanism for self-propulsion of droplets [30] and vesicles [31], demonstrating artificial chemotaxis and opening new scenarios in active matter and drug delivery systems. We may thus state quite generally that effective interfacial tension plays an important role in all miscible multifluid systems and at all length scales.…”

Spatiotemporal control of cargo delivery performed by programmable self-propelled Janus droplets

“…Emulsion droplets, in contrast, have the potential to be composed of bio-compatible liquids. It was also shown that emulsion droplets can self-propel by Marangoni stresses 10 that are maintained by chemical reactions 11 , micellar solubilization 12,13 , or liquid-liquid phase separations [14][15][16][17][18][19] . However, the realization of self-propelled Janus droplets has been proven difficult [20][21][22][23][24] in spite of the achievements obtained with emulsion droplets.…”

Self-propelled droplets

“…Other studies were concerned with binary liquid mixtures featuring a two-phase region in their phase diagram. In [4,5] the binary system acetone-hexadecane and in [6,7] mixtures of aqueous solutions of poly-(ethylene glycol) (PEG) and sodium sulfate (Na 2 SO 4 ) were studied. However, the droplets driven by phase separation dynamics show only short active periods and their estimated cruising range is typically below 20 times their diameter.…”