Dripping From Faucets and Ceilings

Understanding dripping better can bring about such things as higher-quality inkjet printing and more uniform deposition of DNA onto gene chips. Solving the fundamental Navier-Stokes fluid equations (involving such variables as fluid velocity and pressure) for a single drop from a faucet and then observing dripping with a fast camera to determine the equations' essential features, Purdue researchers (Osman Basaran, 765-494-4061, obasaran@ecn.purdue.edu) have implemented a streamlined model that enables computers to simulate sequences of hundreds of drops. (Previous Navier-Stokes-based approaches have only been able to consider the genesis of a single drop.)

Among the team's observations: as flow rate increases, a phenomenon known as "period doubling" can occur, in which drops make a transition from falling at a single time interval (such as every 5 seconds) to two characteristic intervals (such as 4 s followed by 2 s). The authors also predict the possibility of a (yet-to-be-observed) hysteresis effect, in which the previous history of the flow can influence the subsequent dripping pattern. (Ambravaneswaran et al., Phys. Rev Lett., 18 Dec.)

On the heels of the new dripping model comes an experiment showing how to prevent drips from a ceiling. Spreading a layer of silicone oil on the underside of a flat surface suspended above a gas layer, University of Texas researchers (Harry Swinney, 512-471-4619, swinney@chaos.ph.utexas.edu) found that making the bottom of the gas layer about 10 degrees warmer than the top of the liquid layer could prevent dripping from occurring for weeks at a time. Normally, the smallest disturbance to such a liquid layer creates unstable variations in thickness along the layer, which leads to dripping. But the Texas researchers showed that heating the gas layer can warm up the thicker regions of the liquid layer. Since surface tension decreases with increasing temperature, oil gets pulled along the gas-liquid interface from the warmer regions of lower surface tension to the colder regions of higher surface tension. Thus, heating from below can restore and stabilize the flat boundary between the liquid and the gas.

This work can provide insights for designing more uniform coatings on materials. (Burgess et al., upcoming paper in Phys. Rev. Lett.)