Square liquid drops

Tuning round liquid drops into different shapes.

AS we are taught in physics, under most conditions, a drop of liquid on a surface has a round shape as it tends to minimise its surface energy. Two researchers, Rafael Schulman and Kari Dalnoki-Veress of McMaster University, Canada, argued that this need not be the case. Through a series of experiments, the duo has shown that drops do behave very differently and can assume oval or even square shapes depending on the tension between two thin stretchable films, and these, in fact, become highly tunable. In their paper, published in “Physical Review Letters”, they state that adjusting the tension allowed the contact angle and droplet morphology to be controlled. By exploiting these elastic boundaries, droplets can be made elliptical, with an adjustable aspect ratio, and can even be transformed into a nearly square shape.

They have also suggested that such stretched and squeezed drops could act as a tiny lens with adjustable optical properties and also for liquid patterning in displays. Dalnoki-Veress said that the results were “counter-intuitive” and “fun”.

The duo placed a 30 to 300 micrometre diameter drop of either glycerol or polyethylene glycol on an elastic film anchored to a silicon substrate. In a perpendicular direction, they stretched a second elastic film and lowered it over the drop.

When the top film’s tension was equal in all directions, the round drop flattened into a pancake. When the film’s tension was greater along one direction, the drop assumed an oval shape. When the entire set-up was suspended and both the top and bottom films were stretched in perpendicular directions, the shape became square.

When the focal properties of the suspended sandwiched drops were measured using a laser beam, it was found that the flatter drops had longer focal lengths. The focal spot shape also varied, with square drops producing a cross-shaped pattern and oval drops creating a line.