Avalanche dynamics: dripping water drops.

A hot shower in a cold bathroom can generate a thick mist of water droplets. Those droplets that collect on the bathroom window or mirror initially form a thin film on the surface. But over time, these droplets grow larger, coalesce, and begin to drip downward, engulfing other drops along the way.

Now, researchers have taken a closer look at this commonplace but rarely studied phenomenon of water-droplet "avalanches." The experiment was "the first of its kind," says physicist Franco M. Nori of the University of Michigan in Ann Arbor.

Nori and Michigan colleagues Britton Plourde and Michael Bretz describer their results in a paper scheduled for publication in the Oct. 25 PHYSICIAL REVIEW LETTERS.

In recent years, researchers have studied avalanches and other collective effects in a variety of systems ranging from sandpiles (SN: 7/15/89, p.40) to regions of a material magnetized in different directions (SN: 3/31/90, p.207). But in these systems, the avalanches involved well-defined individual units, such as sand grains or magnetic domains.

Nori and his co-workers were interested in the dynamicsof avalanches in which the units could grow in size. For water droplets continuously sprayed on a slanted surface, an avalanche occurs when individual droplets reach a critical mass, at which point they begin to run down the surface, capturing other droplets stationed along their paths.

It took the researchers nearly two years to design and construct an apparatus (see diagram) for measuring the size and duration of water droplet avalanches and the time between successive avalanches. In the end, they clearly demonstrated that their system shows the same kind of loading-unloading cycles that typify sanpile avalanches. Although water droplets are continuously deposited on a slanted surface, water runs off the surface at irregular intervals rather than continuously.

This effect proved particularly strong when the researchers used low spray rates and water chilled to near freezing. The lower temperature makes the water more viscous, which proves more cohesion between droplets, Nori says. This allows the droplets to gather into larger clusters, setting the stage for occasional huge avalanches that practically clear the surface.