Optical images of Janus microspheres of different sizes and morphologies.
Optical images of Janus microspheres of different sizes and morphologies.

A new, low-cost, microfluidic route to synthesizing Janus microspheres has been developed by a team of Chinese and French researchers.

Do you use an e-reader? If so, you are already familiar with Janus beads. Originally used to describe spherical glass particles with one hydrophilic and one hydrophobic hemisphere, Janus beads now refer to any particle whose surface has two distinct physical properties. This unique structure has resulted in their widespread use across a range of sectors – in everything from catalysts and stabilizers to magnetic therapy and electronic ink. The synthesis of Janus particles requires the ability to selectively coat or functionalize one or both sides of a microsphere – and a team of researchers have just published a cost-effective and reliable route to do just this.

Electro-responsive Janus particles can be controlled by a changing electric field – this is the basis of electronic ink, and could potentially be used to produce flexible displays. Two-channel microfluidic techniques, involving the precise control of small volumes of fluids, had previously been used to prepare microscale Janus beads. But this work, published in Materials Letters [DOI: 10.1016/j.matlet.2014.12.032] has taken the approach to a new level. Led by the University of Science and Technology of China, the team have produced Janus carbon black (CB) / polytetrafluoroethylene (PTFE) microspheres with optical and electrical anisotropy. And they have done so using a single microfluidic channel, overcoming the need to match the rate of mixing with the rate of polymerization.

The simple, capillary-based microfluidic device used two syringe pumps to deliver the feed solution (consisting of CB, PTFE emulsion, distilled water) and continuous phase (silicon oil) at specific flow rates. When the two phases met at a T-junction, droplets formed, which were then collected and dried. The carbon black was found to coalesce into one hemisphere of the CB/PTFE beads to form the Janus structure. In addition, Xiang Li and his team could control both the size distribution of the Janus beads and the volume ratio of the CB hemisphere, to a high degree of accuracy, by varying operating parameters. By applying a voltage to the microspheres, their electrical properties could also be determined. Rotational motion was observed, along with translational motion at elevated voltages, demonstrating the anisotropy of the spheres and the reliability of their technique.

Materials Letters 142 (2015) 258–261 “Fabrication of electro- and color-responsive CB/PTFE Janus beads in a simple microfluidic device” DOI: 10.1016/j.matlet.2014.12.032