Formation and stability of liquid and molten beads on a solid surface

We present an experimental study of the formation and stability of small-scale beads deposited onto a solid surface by sweeping a droplet stream over it. We are concerned particularly with beads formed from molten droplets deposited on a cold substrate, as in the work of Gao & Sonin (1994), but some results of isothermal deposition are also shown. We show that a molten bead forms with parallel contact lines which have been arrested by freezing while the bead itself is still largely in a liquid state, and that the still-molten material is stable when the contact angle is less than ½π and unstable when it exceeds ½π, consistent with Davis's (1980) theory. In addition, we present a relatively simple inviscid theory for small-scale (small Bond number) beads which allows us to compute the wavelength associated with the maximum growth rate of the instability, and show that it agrees with the dominant wavelength in the experiments.