Droplets on liquid surfaces: Dual equilibrium states and their energy barrier (original) (raw)

Hanging droplets from liquid interfaces

Journal of Fluid Mechanics

The impact of a heavier droplet falling into a deep pool of lighter liquid is investigated using three-dimensional numerical simulations. We demonstrate that the heavier droplets can hang from the surface of a lighter liquid using surface tension. The impact phenomenon and the evolution of the heavier droplet as a function of its size and release height are explored. A theoretical model is also formulated to understand the role of different forms of energy associated with the hanging droplet. We further solve the force balance equations for the hanging droplets analytically, and demonstrate that the results obtained from our simulations match very well the analytical solution. This research offers opportunities in many areas, including drug and gene delivery, encapsulation of biomolecules, microfluidics, soft robots, and remediation of oil spills.

Distributed evaporation of water-in-oil emulsion drops on solid surfaces

Physics of Fluids

We experimentally demonstrate capillary clustering of microdroplets when a microfluidic water-in-oil emulsion drop was placed on a solid substrate. Distributed evaporation rates occur within the cluster manifested by a size gradient of microdroplets. We show that the process depends upon the oil and substrate properties. We study the evaporative dynamics of the cluster and the individual microdroplets. We identify three different evaporation regimes of the microdroplets and model the process. Unlike bare droplets, the evaporation rate varies with time when the droplet size approaches the oil height. Our findings contribute to a better understanding of droplet evaporation in emulsions and on oil-infused surfaces. Also, it may have implications for soft materials, microdroplet chemistry, and biosystems involving droplets.

Numerical simulations and experiments on droplet coalescence dynamics over a liquid–air interface: mechanism and effect of droplet-size/surface-tension

SN Applied Sciences

Present study is on partial/complete coalescence dynamics of a droplet (surrounded by air) over a horizontal pool of the same liquid. Experimental and numerical studies are presented for both isopropanol and glycerol droplet of a constant diameter. Numerical study is presented in more detail for the isopropanol droplet to study the effect of diameter ($$D=0.035-6.7 mm$$ D = 0.035 - 6.7 m m ) and surface tension coefficient ($$\gamma =2-200 mN/m$$ γ = 2 - 200 m N / m ) on the coalescence dynamics. For partial coalescence of an isopropanol droplet and complete coalescence of a glycerol droplet, excellent agreement is demonstrated between our numerically and experimentally obtained interface dynamics; and a qualitative discussion on the mechanism of the partial and complete coalescence is presented. Three regimes of partial coalescence − viscous, inertio-capillary and gravity − proposed in the literature for a liquid-liquid system are presented here for the present liquid-air system wh...

Coalescence of particle-laden drops with a planar oil–water interface

Journal of Colloid and Interface Science, 2011

The coalescence mechanism of a particle-laden drop resting at an oil-water interface has been studied. Two mechanisms for drop coalescence are observed; (i) complete coalescence, in which the drop experiences total coalescence in one event, and (ii) partial coalescence, where a drop is observed to separate during coalescence, producing a smaller secondary drop that rebounds and comes to rest at the planar oil-water interface. For particle-laden drops of approximately 4mm in diameter, we show the critical condition for partial to complete coalescence to be dependent on the particle concentration, and the interparticle interaction energy. Colloidal silica spheres dispersed in 10(-4) M KNO(3) electrolyte solution are highly charged and remain dispersed in the drop. By increasing the solids concentration, we measure the transition from partial to complete coalescence at 20 wt.%. However, this critical condition can be reduced by increasing the interparticle interaction energy. In 1 M KNO(3) electrolyte solution, the particle surface charge is sufficiently screened such that particle clusters readily form in the water drop. With particle clustering, transition from partial to complete coalescence is measured at 8 wt.% solids.

Physicochemical properties of two-layer liquid drop

2010

The stability and geometry of two immiscible liquid drops resting on a nonwetting solid substrate (polypropyleneabbreviated as pp) are studied. The problem is a very simple situation of a non-miscible two-phase liquid system. The experimental observation of a micro drop of distilled water, tap water, salted water, acetone, benzene, cyclohexane, ethanol, n-heptane, nitrobenzene, glycerol or isopropylbenzene on top of a mercury drop resting on pp substrate is discussed, and the static situation of the drops is explored. It is found that there is a complete encapsulation of the mercury drop by only a drop of water (distilled, tap and salted) and glycerol , that depends on the volume of the droplets and the surface tension value of the upper drop. A drop of the rest of the mentioned liquids on Hg drop spreads, and so an unstable configuration, with no encapsulation, is obtained. _________________________________________________________________________________________________

Study of the formation of micro and nano-droplets containing immiscible solutions

Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2011

There is currently significant interest in the multiple resistance to treatment using drugs (MDR), developed by bacteria and malignant tumors. One of the alternatives to the existing medicines and treatment procedures in fighting MDR is strengthening the effects of cytostatics by improving their delivery methods. Such a method is represented by the generation, transport and use of micro-/nano-droplets which contain medicines. This approach can reduce the medicines consumption by generating micro-droplets which contain drugs incorporated in solvents substances; the micro-/nano-droplets can favour a faster delivery to the targets and a higher drug concentration in them.

Possible mechanism for multistage coalescence of a floating droplet on the air/liquid interface

Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2000

We study experimentally by means of atomic force microscopy (AFM) the jump-tocontact instability between two droplets in air, with radii ranging between 0.7 and 74 μm. This instability which occurs at the nanoscale is responsible for droplet coalescence. The AFM experiments were conducted in contact and frequency-modulation modes where the interaction force and the frequency shift are monitored while the two droplet interfaces approach each other. The critical distance d min at which the jump to contact takes place is determined by fitting the experimental curves by the theoretical expressions for the force and the frequency shift. The results point out the existence of two regimes. For submicrometer droplets, d min scales as (H R eq /γ) 1/3 where R eq is the equivalent droplet radius, H the Hamaker constant, and γ the surface tension of the liquid. For larger droplets, d min no longer depends on the droplet size and scales as (H/γ) 1/2. This second scaling is the one that controls droplet coalescence in most situations.

Mechanism of Non-Coalescence for Liquid Droplets at the Air-Liquid Interface

This work addresses the physical origin and the conditions of generation of non-coalescence-droplets (N-C-D). Our results showed that there is a potential link between the N-C-D and an ink-jet printing defect, i.e. the satellite dots. Ink-jet printing is more widely used in the manufacturing of bioactive surfaces. Eliminating print defects will make the ink-jet technology a more economic and attractive option for the manufacturing of the bioactive surfaces.