Researchers use ultrasound to control the orientation of the tiny particles
(Nanowerk News) Acoustic waves may be able to control how particles sort themselves. While researchers have been able to separate particles based on their shape — say, bacteria from other cells — for years, the ability to control their movement remained a largely unsolved problem, until now. Using ultrasound and nozzle technology, Penn State researchers have separated, controlled, and expelled various particles based on their shape and various properties.
They publish their results in a journal Small (“Ultrasound Manipulation and Extrusion of Active Nanorods”).
“We engineered a microduct nozzle and applied ultrasonic energy to the system,” said associated author Igor Aronson, Penn State’s Dorothy Foehr Huck and J. Lloyd Huck Chair Professor of Biomedical Engineering and professor of chemistry and mathematics. “Nozels play two roles. This concentrates the fluid flow, which other researchers have done. But other than that, the walls of the nozzle reflect the acoustic waves of the ultrasound energy.”
Aronson and his collaborators are working with tiny materials called nanorods, which are some of the best-studied synthetic self-propelled particles, according to Aronson. Because they are similar in size and have similar swimming speeds to bacteria, said Aronson, many of the conclusions drawn from observing nanorods can be applied to the movement of bacteria. For this reason, they are often used as a proof of concept for future separation assignments.
In this case, the nanorods are half platinum and half gold. The researchers placed the nanorods in a nozzle, shaped like a miniature syringe, and then added hydrogen peroxide. Hydrogen peroxide decomposes – or burns – on the platinum half of each nanorod, forcing them to swim mimicking the behavior of bacteria.
The researchers applied ultrasound to the nozzle, generating acoustic waves that, together with the fluid flow, are able to separate the nanorod particles, stick them together, or expel them from the nozzle.
“The concept of separation hinges on the fact that nanorods and spherical particles respond differently to acoustic radiation and generate fluid flow,” said Aronson. “By controlling the shape of the nozzle and the frequency and amplitude of the acoustic radiation, we can force particles of different shapes and material properties to behave differently. This is especially true for active particles like nanorods: They can swim independently, and the control is very challenging.”
