The droplets show rotational and translational behaviors under an applied voltage, enabling the visualization of electric field distribution within microelectrodes

Existing sensor probes for microelectrical devices can measure only their average electric properties, providing no information on their spatial distribution. Liquid crystal droplets (LCDs) – microscopic droplets of soft matter that respond to electric field – are promising in this regard. Accordingly, researchers from Japan recently visualized the electric field and electrostatic energy distribution of microstructured electrodes by recording the motion of LCDs under an applied voltage, making for high detection accuracy and spatial resolution.

Microelectromechanical systems (MEMS) involve the use and development of micron-sized electrical devices such as microelectrodes, sensors, and actuators that are integrated into computer and smartphone chips. Fabricating such integrated MEMS devices is usually a challenging task as these devices often deviate from their original design owing to the defects introduced during their fabrication and operation. This, in turn, limits their performance. Therefore, it is crucial to identify and rectify these defects.

One way to identify and rectify these defects is by measuring the spatial distribution of electric properties of these devices. However, standard sensor probes do not offer the required spatial resolution, and can only determine the spatially averaged-out electric properties. Due to this, it is possible to detect only the presence of defects, not their location. Fortunately, liquid crystal droplets (LCDs)–micron-sized droplets of soft matter with molecular orientational order–offer hope on this front. LCDs respond strongly to external stimuli such as an electric field, and can thus act as a high-resolution probe.

Capitalizing on this promise, Dr. Shinji Bono and Prof. Satoshi Konishi from Ritsumeikan University, Japan, have now utilized LCDs for visualizing the electric properties of microstructured electrodes via a technique called particle imaging electrometry. Their findings were published in Volume 13 of the journal Scientific Reports on 16 March 2023.

Dr. Bono explains the research methodology. “The LCDs were dispersed on microelectrodes arranged in a comb-like structure atop a glass slab. Their molecular orientations, determined using polarized optical microscopy, were randomly distributed when the electric field was absent. Then, a voltage was applied across the electrodes.” Because of this, the LCDs between the electrodes and in front of the electrode ends underwent rotation, their molecular orientations lining up perpendicular and parallel to the electrodes, respectively. This alignment, revealed by COMSOL simulations performed by the researchers, corresponded to the direction of the electric field, and occurred faster with increasing voltage. The relaxation frequency of rotation was found to vary as the square of the applied voltage.

Further, at high voltages, the LCDs showed translation (linear motion) towards the electrodes, especially their endpoints, the regions with maximum electrostatic energy density. Based on this behavior, the researchers could produce an array of LCDs via periodic modulation of the energy density in a micro-capacitive MEMS device. The LCD array, in turn, served as a periodic modulator of the refractive index, a number characterizing the light bending ability of a material.

These results thus demonstrate that the electric properties of microelectrodes and microelectric devices can be visualized simply by observing the rotational and translational behavior of LCDs under an electric field. Moreover, the technique provides a high spatial resolution (10 μm) as well as high detection accuracy (5 μV/μm). In light of these features, Prof. Konishi has high hopes for its applications. “It will help improve the design and fabrication of integrated microelectrical devices by providing information on the defect location, which so far has remained unavailable. In turn, more sophisticated MEMS technology may become available soon,” he concludes.

And we are just as eager to find out !

Title of original paper: Rotation and transportation of liquid crystal droplets for visualizing electric properties of microstructured electrodes
Journal: Scientific Reports
DOI: 10.1038/s41598-023-31026-8

About Shinji Bono from Ritsumeikan University, Japan

Dr. Shinji Bono is a Lecturer at the Department of Mechanical Engineering at the College of Science and Engineering at Ritsumeikan University since 2020. He completed his Ph.D. from Kyoto University in 2016 and worked at Waseda University from 2016 to 2020. He is also a member of The Institute of Electrical Engineers of Japan. Dr. Bono received The Japanese Liquid Crystal Society’s Best Paper Award in 2020. Over the last eight years, he has published a score of research articles, which have been cited around 70 times. His research interests include liquid crystals, soft matter physics, and microelectromechanical systems.

About Professor Satoshi Konishi from Ritsumeikan University, Japan

Satoshi Konishi received a BS degree in 1991 in Electronics Engineering, an MS degree in 1993, and a PhD in 1996 in Electrical Engineering from the University of Tokyo in Japan. He is currently a Professor at Ritsumeikan University, Japan, where he joined as faculty in 1996. He is also a Visiting Professor with Shiga University of Medical Science, Japan since 2007. His research interests concern microelectromechanical systems (MEMS), covering broad ranges from fundamental to applied fields. His current research focuses on biomedical MEMS, especially multiscale interfaces in biomedical engineering.

Funding information

This study was supported by JPSJ Grant-in-Aid for Early-Career Scientists (Grant Number 20K14433) and Ritsumeikan University’s Global Innovation Research Organization.