Microfluidics enters a new field

Source: physicsworld.com | For years, the concept of “lab-on-a-chip” has fueled designs seeking to create miniaturized devices capable of performing a whole set of laboratory functions in the palm of your hand. While past efforts have struggled to effectively control liquids and materials on the micro-scale, a new device published in the Proceedings of the National Academy of Sciences offers a different approach to fluid manipulation. Following several years of collaboration, the team led by Govind Kaigala at IBM Research-Zurich and the group of Moran Bercovici at Technion-Israel Institute of Technology have now demonstrated that the key to dynamic control of fluid mechanics may be surprisingly electric.

The electric effect

Conventional techniques for guiding the microscale flow of fluids often rely on mechanical solutions, such as directly carving channels into polymers or glass. Although valves are sometimes added in order to dynamically control routing, the geometries of these systems are fixed, limiting them from attaining the true “lab-on-a-chip” goal of being able to perform multiple different experiments on one platform. More recent approaches have tried chemically modifying the chip’s surface to create a pattern of electric charge that dictates the path of a fluid; however, like the mechanical channels, the charge patterns are fixed and do not offer flexibility.

To control the motion of fluid in a way that is truly adjustable, the research team turned to electric fields. When liquid contacts a surface, it develops a layer of charge; applying an electric field to this layer moves the charges, dragging the liquid with them and creating a net flow. Taking advantage of this effect, the team designed a device that uses disk-shaped electrodes embedded in the bottom of a fluidic chamber to generate dipole-like flow patterns in the liquid when an electric field is applied. Placing multiple electrodes together in an array creates “virtual channels” that guide the fluid stream.

“Each electrode acts as a virtual micro conveyer belt whose directions and intensities can be controlled electronically,” says Federico Paratore, lead author of the work, “As a fun example, we showed that we can bend an otherwise straight streamline into a sine wave, and then change its phase with a click of a button—it’s like looking at an oscilloscope signal, but where the signal is made of liquid!”

Discovery of new flow patterns

Besides redirecting a streamline through different virtual channels, the researchers created new flow patterns that have not been previously observed. For instance, they used a ring-shaped electrode configuration to create an inner region of stagnation surrounded by an outer region of flow, which could be used as traps for cells and particles, or as “virtual vials” for performing chemical reactions. By changing the voltages on the electrodes, they could then invert the pattern to create an inner region of flow surrounded by an outer region of stagnation, which is useful for selective on-demand mixing. While more applications of these flow patterns have yet to be explored, the control and flexibility the team’s device offers suggest that the lab-on-a-chip dream may finally be within grasp.

About the author: Emily Toomey is a PhD student contributor to Physics World. Emily is studying superconducting nanoelectronics at the Massachusetts Institute of Technology.