Not all bubbles are round. In fact, a new laser-based technique has been developed to make square bubbles, donut bubbles, and V-shaped bubbles. The researchers claim in the January Physical Review E that many other shapes can be made on-demand. These vapor bubbles and the liquid jets they create could have practical uses in moving and bending nanostructures, as well as for manipulating biological cells.
The spherical shape of a soap bubble minimizes surface tension, whereas vapor bubbles–like those in boiling water–are often round because they are expanding outwards equally in all directions, from a tiny starting point. For decades researchers have been controlling the onset of vapor bubbles by focusing lasers inside a liquid. The laser rapidly heats a small region of the liquid above the boiling point, so that a bubble literally explodes out of the liquid. As the bubble cools, the vapor recondenses and the bubble collapses on itself in less than a millisecond.
Claus-Dieter Ohl of Nanyang Technological University in Singapore and his colleagues were curious to see what shapes they could make with these short-lived vapor bubbles. They followed the path of earlier work going back to the 1970s, in which researchers placed a holographic plate in the beam line of a laser, to create multiple focal points. However, Ohl and his collaborators used a newer device called a spatial light modulator, a two-dimensional reflective array that is essentially a computer-controlled holographic plate. Each pixel in the array separately controls the phase of the light ray bouncing off of it. When a lens concentrates all the rays, they interfere at the focal plane to create high intensity regions of any desired shape.
Using this modulator, the group shot a pulsed laser into a thin chamber holding yellow printer ink, which readily absorbed the green laser wavelength of 532 nanometers. A high speed camera captured the microscopic bubbles that formed after each pulse. In their first example, the researchers focused the laser onto five points at the center and corners of a diamond. As the five resulting vapor bubbles expanded, they coalesced into one square-shaped bubble before collapsing 20 microseconds later. “A lot of shapes are possible, but corners tend to be rounded as the bubbles expand,” Ohl says. When the focal region was extended into a short line segment, the bubble that developed was an ellipsoid. In addition, two of these lines touching, at an angle to each other, formed a puffy V-shaped bubble, and a “focal ring” made a donut-shaped bubble.
In conjunction with all this bubble blowing, the team performed simulations that reproduced the recorded evolution of bubble shapes. The simulations highlighted possible applications in microfluidics for these new bubble geometries. For example, the V-shaped bubbles appear to generate high-speed (10 meters per second or more) jets of fluid when the bubbles collapse. These strong jets can be used to move and bend carbon nanotubes and nanowires, as the team recently demonstrated using jets that form between two collapsing bubbles . Other bubbles might be useful in biology. Specifically, Ohl’s group is looking into trapping and compressing red blood cells in the “hole” of donut-shaped bubbles. This squeezing could test for malaria infection, which tends to stiffen red blood cells.
“This paper illustrates an exciting way to break spherical symmetry in bubble dynamics,” says Andrew Szeri, a mechanical engineer at the University of California, Berkeley. Engineer Pei Zhong of Duke University in Durham, North Carolina, is also impressed. “This work is innovative and has potential in a broad range of applications, especially in microfluidics,” he says. “The technique also provides a powerful tool for exploring bubble dynamics in complex geometry.”
- P. A. Quinto-Su et al., “Manipulation and Microrheology of Carbon Nanotubes with Laser-Induced Cavitation Bubbles,” Phys. Rev. Lett. 104, 014501 (2010).