Altering Emulsion Stability with Heterogeneous Surface Wettability

Abstract

Emulsions-liquid droplets dispersed in another immiscible liquid-are widely used in a broad spectrum of applications, including food, personal care, agrochemical, and pharmaceutical products. Emulsions are also commonly present in natural crude oil, hampering the production and quality of petroleum fuels. The stability of emulsions plays a crucial role in their applications, but controlling the stability without external driving forces has been proven to be difficult. Here we show how heterogeneous surface wettability can alter the stability and dynamics of oil-in-water emulsions, generated by a co-flow microfluidic device. We designed a useful methodology that can modify a micro-capillary of desired heterogeneous wettability (e.g., alternating hydrophilic and hydrophobic regions) without changing the hydraulic diameter. We subsequently investigated the effects of flow rates and heterogeneous wettability on the emulsion morphology and motion. The experimental data revealed a universal critical timescale of advective emulsions, above which the microfluidic emulsions remain stable and intact, whereas below they become adhesive or inverse. A simple theoretical model based on a force balance can be used to explain this critical transition of emulsion dynamics, depending on the droplet size and the Capillary number-the ratio of viscous to surface effects. These results give insight into how to control the stability and dynamics of emulsions in microfluidics with flow velocity and different wettability.

Figure 1

(a) Schematic diagram of the experimental setup of oil-in-water emulsions generated by a single co-flow microfluidic capillary, which has heterogeneous surface wettability. The continuous phase is 2% PVA aqueous solution, and the droplet is paraffin oil. (b) The glass micro-capillary, initially hydrophilic, is chemically modified to have a segment of a hydrophobic wall using a mono-layer of Octyltriethoxysilane (OTES) coating. (c) Outline of the experimental procedures to make a micro-capillary with a desired pattern of heterogeneous wettability.

Figure 2

(a) Droplet length, D, as a function of the flow rate of inner (oil) phase, q_in, for different flow ratios between the inner (oil) to outer (water) flow rate in an unmodified, hydrophilic capillary: ϕ = 1/3 (▲), ϕ = 1 (), ϕ = 3 (), ϕ = 7 (). (b) Effect of flow ratio on the dimensionless droplet length D, normalized with the capillary diameter, d. The dashed line is the best fit of the power-law relation: D/d ~ ϕ^10/9.

Figure 3

(Supporting movies) Phase diagram of (oil-in-water) emulsion dynamics after passing a hydrophobized section (marked in red along the micro-capillary), revealing the effects of droplet size and velocity, in terms of the Capillary number Ca = μU/σ, on the changing behaviors of the droplets. The upper (shaded) regime above the dashed line indicates (i) passing oil-in-water droplets without changing their size or speed (×) for different flow rates. In contrast, at lower speeds (below the dashed line) critical changes are observed, including (ii) adhesion of oil droplets on the hydrophobic wall with an increasing advancing contact angle (◦, ▫, ◊, , ■), (iii) inversion of oil droplets to become water-in-oil emulsions (▼), and (iv) break-up of the oil droplets with unstable lubrication films (Δ, ∇, ●). See Supplementary information for the details.

Figure 4

(a) Convective time scale, Δt = D/U, of the oil-in-water emulsions (of speed U and length D) for different flow rates. The dashed line indicates the average critical convective time, Δt^*, of the oil droplets. When Δt < Δt^* the oil droplets are intact, passing without changing their dynamics, whereas for Δt > Δt^*, the droplets alter their morphologies with unstable lubrication film after traveling the hydrophobized section. (b) The critical capillary number for the oil droplets to cause the adhesion on the hydrophobic surface in the capillary. The solid line indicates a power-law fit of Ca^* ~ (D/λ)^β, where λ is the length of the hydrophobized session, ≈10 d.