Jump-to-contact instability: The nanoscale mechanism of droplet coalescence in air

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 mu m. 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 (HReq /gamma)(1/3) where R-eq is the equivalent droplet radius, H the Hamaker constant, and gamma the surface tension of the liquid. For larger droplets, d(min) no longer depends on the droplet size and scales as (H/gamma)(1/2). This second scaling is the one that controls droplet coalescence in most situations.