Manipulating droplet plays vital roles in fundamental research and practical applications from chemical reactions to bio-analysis. As an intriguing and active format, light control of droplets, typically induced by photochemical, photomechanical, light-induced Marangoni effects, or light-induced electric fields, enables remote and contactless control with remarkable spatial and temporal accuracy. However, current light control of droplets suffers from poor performances and limited reliability.  

Now, a research team led by Dr. DU Xuemin from the Shenzhen Institute of Advanced Technology, Chinese Academy of Sciences has reported a new smart material with high-efficiency and durable photo-induced charge regeneration capability, enabling light control of droplets with superior performances and reliability.  

This work was published in National Science Review on August 17, 2022.  

This smart material contains three core components: firstly, micro-size liquid metal particles owning to its superior photothermal and thermally conductive properties; secondly, polyvinylidene fluoride trifluoroethylene copolymer for its excellent ferroelectric and mechanical behaviors; thirdly, micro-pyramidal structures and low-surface-energy coatings of fluorinated SiO₂ nanoparticles for enhancing the superamphiphobicity.  

"Leveraging on the synergistic effect of these components, the photo-induced charged surfaces (PICS) possess a superior capability of real-time and in-situ photo-induced charge generation upon exposure to light illumination," said Dr. DU.   

This distinctive charge generation capability of the PICS is clearly revealed by scanning Kelvin probe microscopy (SKPM), which shows the real-time and in-situ generation/disappearance of the free surface charges upon exposure to ON/OFF light irradiation.  

This unique charge generation capability of the PICS exhibits no apparent degradation even in extreme environments including high relative humidity (~ 90%) for 72 hours and high temperature (70 ℃). The charge density of the PICS remains at stably high levels of 252 pC mm^-2 (peak to peak) even after 10000 ON/OFF irradiation cycles.  

"The outstanding efficiency, superior durability, and stability of the photo-induced charge regeneration in PICS is critical for light control of droplets," said Dr. DU. 

They demonstrated that the PICS imparts a new paradigm for controllable droplet motion, including high average velocity (~ 35.9 mm s^-1), unlimited distance, multimode motions (e.g., forward, backward, and rotation), and single-to-multiple droplet manipulation.  

They also extended light control of droplets to robotic and bio-applications, including transporting a solid cargo in a closed tube, crossing a tiny tunnel, avoiding obstacles, sensing the changing environment via naked-eye color shift, preparing hydrogel beads, transporting living cells, and reliable biosensing.  

"Our robust and biocompatible PICS not only provides insight into the development of new smart interface materials and microfluidics," said Dr. DU, "but also brings new possibilities for chemical and biomedical applications." 

Schematic illustrations of light control of droplet. (Image by Dr. DU)

Media Contact:
ZHANG Xiaomin
Email:xm.zhang@siat.ac.cn