Here, we show that water droplets exhibit non-coalescence in air when deposited on oil-infused surfaces. This surprising phenomenon is due to the oil meniscus that surrounds each water droplet; when the menisci of two or more neighboring droplets overlap, the droplets spontaneously zip together to minimize the system's free energy and an oil film is squeezed upward from the substrate to form a barrier between the colliding droplets (Figure 1). While droplet coalescence will eventually occur due to film drainage, the time required for film rupture is orders of magnitude larger compared to water droplets submerged in an oil bath, such that oils of moderate viscosity can sustain non-coalescing droplets for over 24 hours. The prolonged non-coalescence of water droplets on an oil-infused surface is due to the inherent asymmetry of droplet collision, where a large-area oil film is squeezed between the droplets from the bottom-up. This is in sharp contrast to water droplets submerged in oil, which exhibit a point-contact during collision resulting in a small-area oil film with a faster drainage time. These findings should enable the construction of air-stable droplet networks useful for drug screening, bio-sensing, and synthetic biology.Figure 1:  Water droplets on oil-infused surfaces exhibit non-coalescence.

Non-coalescence of water droplets on oil-infused surfaces

247th National Meeting (2014)

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