Synopsis

Ultrafast, Self-Propelled Particles

Physics 13, s110
New “Marangoni surfers” that whizz along at 10,000 body lengths per second offer new insight into active matter propelled by surface-tension gradients.
N. Jaensson/ETH Zurich

Particles that propel themselves along a fluid surface-tension gradient that they generate themselves are called Marangoni surfers. To understand the dynamics of this form of “active matter,” researchers have generally focused on how macroscopic Marangoni surfers induce surface-tension gradients by affecting either temperature or surfactant concentrations. Now, Kilian Dietrich of the Swiss Federal Institute of Technology (ETH) in Zurich and colleagues have investigated how both of these effects combine at the microscale, deriving a more comprehensive understanding of the factors that control the particles’ speeds [1]. In the process, they created micrometer-scale surfers that can race along at up to a few centimeters—or about 10,000 body lengths—per second.

The Marangoni surfers that the team created are thermally activated, meaning the surface-tension gradients that drive them are created by nonuniform fluid temperatures. Each particle is a silica sphere, one-half of which is coated with a thin layer of gold. When the researchers shone a laser onto the particle, the gold-covered side warmed more than the bare silica side. The higher temperature lowered the surface tension of the fluid on the gold side, propelling the particle in the opposite direction.

Though the temperature gradient is what propels the particle, Dietrich and colleagues found that surfactant-concentration gradients oppose its motion once the particle is underway. As it travels, the Marangoni surfer leaves a wake in which the surfactant concentration is lower—and surface tension is therefore higher. Ultimately, the competing effects of the two opposing gradients determine the speed of the particle. The researchers anticipate that improvements in propulsion control emerging from this understanding will allow new routes to manipulate active matter systems at fluid interfaces.

–Erika K. Carlson

Erika K. Carlson is a Corresponding Editor for Physics based in New York City.

References

  1. K. Dietrich et al., “Microscale Marangoni surfers,” Phys. Rev. Lett. 125, 098001 (2020).

Subject Areas

Fluid DynamicsSoft Matter

Related Articles

Soft Solid Flows Through a Pipe
Fluid Dynamics

Soft Solid Flows Through a Pipe

An ultrasoft material can move smoothly through a pipe, but the motion generates “furrows” on the material’s front surface. Read More »

Robotic Vacuum Cleaner for Microplastics
Fluid Dynamics

Robotic Vacuum Cleaner for Microplastics

Seong Jim Kim and Myoung-Woon Moon of the Korea Institute of Science and Technology have developed a device that can “vacuum” up tiny pieces of plastic floating on the surface of a body of water. Read More »

Bumpy Particles Take One Step to Become Glass
Materials Science

Bumpy Particles Take One Step to Become Glass

Roughing up the surfaces of particles in a colloidal system can smooth its transition into a glassy state. Read More »

More Articles