Droplet on a liquid substrate: Wetting, dewetting, dynamics, instabilities

doi:10.1016/j.cocis.2020.101398

Current Opinion in Colloid & Interface Science

Volume 51, February 2021, 101398

https://doi.org/10.1016/j.cocis.2020.101398 Get rights and content

Abstract

Droplets on a liquid substrate (‘liquid lenses’) play an important role in various branches of engineering, including microfluidics, chemical engineering, environment protection, etc. In the present paper, we discuss basic phenomena characteristic for liquid lenses. We recall classical results on the shape of an equilibrium droplet and the kinds of droplet wetting. We overview briefly the main theoretical approaches used for the analysis of droplet dynamics, discuss the phenomena accompanying a droplet impact, physical effects used for droplet manipulations, and the factors that determine the interaction between droplets. We describe the main types of droplet instabilities leading to oscillations, self-propulsion, and disintegration of droplets. Some promising directions of further research are listed.

Introduction

The dynamics of liquid droplets on a solid substrate was studied extensively in the past decades [1, 2, 3]. In the case of a solid substrate, the description of the contact line dynamics is rather complex, and it needs an extension of the analysis beyond the standard hydrodynamic approach. The motion of a viscous droplet on a dry solid surface with a finite contact angle contradicts the nonslip condition, which is a cornerstone of the viscous fluid dynamics. The dynamic phenomena, among them the difference between the dynamic and static contact angles and the existence of the dynamic contact angle hysteresis, attracted the great interest of scientists for a long time, and the exploration of those phenomena led to a significant progress in interfacial science.

The dynamics of liquid droplets floating on the surface of another liquid (‘liquid lenses’) attracted, until recently, much less attention. A widespread opinion is that liquid lenses play an important role in engineering, but from the point of view of science, in a contradistinction to the dynamics of droplets on a solid substrate, that problem is not especially interesting: basically, it was solved by Neumann [4] and Langmuir [5]. The goal of this review is to attract the attention of scientists to that physical object and the relevant scientific problems. In the present paper, we give an overview of basic phenomena characteristic for liquid lenses, including their wetting, dewetting, dynamics, and instabilities.

Section snippets

Liquid lens vs. liquid film

First, let us recall the classical results on the statics of the liquid lens and the kinds of droplet wetting.

Theoretical approaches

Let us discuss briefly the main theoretical approaches used for studying the droplet dynamics.

The standard mathematical model for the description of the droplet's motion is the Navier–Stokes equations (or the Stokes equations in the case of a low Reynolds number) supplemented by appropriate boundary conditions at the interfaces and the triple line. A direct solution of that problem is rather difficult technically, because in addition to the calculation of flow fields, it is necessary to track

Droplet instabilities

With the enhancement of the intensity of kinetic processes (heat and mass transfer, evaporation, dissolution, etc.) or an external action (e.g., vibration), a plethora of phenomena characteristic for out-of-equilibrium systems is observed in floating droplets. These phenomena are generally similar to those observed in liquid layers or in finite aspect ratio systems, but there is a significant peculiarity: the very planar shape of the droplet, i.e., the region where the pattern formation takes

Outlook

In conclusion, let us mention some promising directions of research that were not included into the present review.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Droplet on a liquid substrate: Wetting, dewetting, dynamics, instabilities

Abstract

Introduction

Section snippets

Liquid lens vs. liquid film

Theoretical approaches

Droplet instabilities

Outlook

Declaration of competing interest

Colloids Surf, A

Soft Matter

J Chem Phys

Langmuir

Phys Rev Lett

Langmuir

Phys Fluids

Phys Fluids

Phys Rev Appl

Proc Natl Acad Sci USA

Wetting: statics and dynamics

Rev Mod Phys

Capillarity and wetting phenomena: drops, bubbles, pearls, waves

Wetting and spreading dynamics

Vorlesungen über die Theorie der Capillarität

Oil lenses on water and the nature of monomolecular expanded films

J Chem Phys

Conceptual aspects of line tensions

J Chem Phys

Shape of interfaces, drops, and bubbles

Experimental and numerical investigation of the equilibrium geometry of liquid lenses

Langmuir

Can water float on oil?

Langmuir

Impact of droplets on immiscible liquid films

Soft Matter

Spreading and penetration of a micro-sized water droplet impacting onto oil layers

Phys Fluids

Intermolecular and surface forces

Spreading of nonvolatile liquids in a continuum picture

Langmuir

Experimental observation of critical wetting

Phys Rev Lett

Wetting transition and line tension of oil on water

Wetting of surfactant solutions by alkanes

Chem Phys Chem

Equilibrium thickness of large liquid lenses spreading over another liquid surface

Langmuir

Nucleation and growth of droplets at a liquid-gas interface

Phys Rev E

Creating nanoscale emulsion using condensation

Nat Commun

A theory for the estimation of surface and interfacial energies. I. derivation and application to interfacial tension

J Phys Chem

Another look at the interfacial interaction parameter

J Colloid Interface Sci

Four-phase merging in sessile compound drops

J Fluid Mech

Compound sessile drops

Soft Matter

Liquid/liquid dewetting

Langmuir

Impact of energy dissipation on interface shapes and on rates for dewetting from liquid substrates

Sci Rep

Dewetting at a polymer-polymer interface: film thickness dependence

Langmuir

Structure formation in thin liquid-liquid films

The competition between the liquid-liquid dewetting and the liquid-solid dewetting

J Chem Phys

Alternative pathways of dewetting for a thin liquid two-layer film

Phys Rev E

Nonlinear stability analysis of a two-layer thin liquid film: dewetting and autophobic behavior

J Colloid Interface Sci

Low Reynolds number motion of bubbles, drops and rigid spheres through fluid-fluid interfaces

J Fluid Mech

Direct numerical simulation of free-surface and interfacial flow

Annu Rev Fluid Mech

Dynamics of a drop at a fluid interface under shear

Phys Rev E

Long-scale evolution of thin liquid films

Rev Mod Phys