Modulating outcomes of oil drops bursting at a water-air interface

Abstract

Recent studies have shown that capillary waves generated by bursting of an oil drop at the water-air interface produces a daughter droplet inside the bath while part of it floats above it. Successive bursting events produce next generations of daughter droplets, gradually diminishing in size until the entire volume of oil rests atop the water-air interface. In this work, we demonstrate two different ways to modulate this process by modifying the constitution of the drop. Firstly, we introduce hydrophilic clay particles inside the parent oil drop and show that it arrests the cascade of daughter droplet generation preventing it from floating over the water-air interface. Secondly, we show that bursting behavior can be modified by a compound water-oil-air interface made of a film of oil with finite thickness and design a regime map which displays each of these outcomes. We underpin both of these demonstrations by theoretical arguments providing criteria to predict outcomes resulting therein. Lastly, all our scenarios have a direct relation to control of oil-water separation and stability of emulsified solutions in a wide variety of applications which include drug delivery, enhanced oil recovery, oil spills and food processing where a dispersed oil phase tries to separate from a continuous phase.

Figures (4)

• Figure 1: (a) Schematic of experimental setup showing a rising oil drop containing bentonite clay particles bursting at an air-water interface after being released upwards. (b) Scanning electron microscopy (SEM) image of bentonite clay particles with a median size of 20 $\mu$m. (c)-(i) Confocal laser scanning microscope (CLSM) image of an hexadecane (oil) drop in water with adsorbed clay particles at the oil-water interface forming a shell. Clay particles and bulk water are labeled using Rhodamine-B fluorescent red dye and Fluorescein green dye respectively with hexadecane not being dyed (therefore seen in black). (c) -(ii) Zoomed view of (c)-(i) showing the adsorption of particles at the oil-water interface. Yellow arrows show the shell of clay particles (dyed red) (d) Proof of self assembly of clay particles demonstrated by, (d)-(i) injecting a particle-laden oil drop into a bulk of water and, retracting the liquid (d)-(ii). Self assembled particles adsorbed at the oil-water interface form a shell around the oil droplet which crumples during retraction. Effect on cascade(Multimedia available online), (e)(i) without particlesKulkarni2024, (ii)-(iv) with particles showing early cessation with increasing particle concentration, $\varphi =$ 1, 3 and 6 % (f) Mechanism of cascade arrest, (i) Initial particle coverage (ii) Drop bursting leading to particle-laden oil film and daughter droplet generation with increased surface coverage below the interface (iii) Formation of a Pickering drop with final arrest with 90% drop surface coverage of particles.
• Figure 2: (a) Ratio of daughter droplet $R_{i}$ and parent drop radius, $R_p$ at each bursting event, $N_i$ in the absence of particles in three oils (hexadecane, silicone oil, pentane) Kulkarni2024 drop yielding the scaling, $R_{f}/R_p \sim N_i^{-2}$. (b) Decrease in number of bursting events required arrest cascade $N_f$ with increasing clay particle concentration ($\varphi$) in the oil drop exhibiting a scaling dependence of the form, $N_f \sim [\varphi^{-1/4}]$. Refer Fig. \ref{['Fig1']}(e) and (f) for symbols used here.
• Figure 3: (a) Schematic shows a rising water-in-oil compound drop generated using a co-axial nozzle. The inner water drop is dyed pink using rhodamine D while the water bath and the oil covering it are not. (b) Dependence of volume ratio, Vr of water-in-oil in a compound drop on the ratio of oil film thickness and water drop radius, ho/Rw. In the limit, $V_{r} << 1$ it reduces to $V_{r} \approx (h_{o}/R_w)^{1/3}$ and for $V_r >> 1$ it yields, $V_{r} \approx (3h_{o}/R_w)^{-1}$. Bursting process of a compound drop (Multimedia available online) (c) at low $V_{r} \approx 5\%$ (green arrows, initial encapsulated water drop) producing an encapsulated daughter water drop of higher $V_{r}$ (dotted arrows, movement direction) (d) at intermediate $0.04 < V_{r} < 40$ with suppression of daughter droplet production. (e) at high $V_{r} \approx 97\%$, leading to underwater oil film fragmentation (dotted arrows, hole expansion direction) and polydispersed daughter oil droplets. Note, waves after bursting approximately travel a distance of $1.5\pi R_p$ to cover the entire drop.
• Figure 4: Design map predicting encapsulation, spreading and film bursting at different dimensionless water drop radius, $R_w/h_p$ as a function of $V_r$ for $R_p = 2.2$ mm. Symbols are experimental data.