xuemin du, shenzhen institutes of advanced technology, science advances

Photocontrol of a water droplet moving upward. (Courtesy: X Du)

A super-slippery material that regenerates its surface charge when illuminated could pave the way for next-generation interfacial materials and microfluidics. The new material is a combination of a copolymer, tiny liquid metal particles and lubricant-trapping microstructures, and its developers say it could find applications in lab-on-a-chip devices, biological diagnostics and chemical analysis.

Slippery lubricant-infused porous surfaces (SLIPS) show much promise for devices that are self-cleaning, anti-icing and able to resist “fouling” by microorganisms that might otherwise accumulate on structures such as boat hulls or microfluidic chips. Such lubricants do have their downside, however. For one, they act as a physical screen for the material beneath them, thereby masking any desirable properties (such as surface charge) it might have. Such screening is not good for applications in which droplets and liquids need to be manipulated and transported across the slippery surface in a controlled way.

Robust charge regeneration capability

Researchers led by Xuemin Du of the Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences, have now developed a slippery material that does not suffer from these screening effects. The new light-induced charged slippery surface (LICS), as it is called, consists of three core components: micro-sized Ga-In liquid metal particles for efficiently converting absorbed light into local heat; poly(vinylidene fluoride-co-trifluoroethylene) copolymer for its excellent ferroelectric behaviour; and microstructures coated with a layer of hydrophobized SiO2nanoparticles for trapping the lubricant.

In a series of experiments detailed in Science Advances, the team used light to control the movement of droplets placed on the new LICS, moving them at speeds as high as around 18.8 mm/s and over distances as long as around 100 mm. These droplets, which can be either microscopic or macroscopic (their volumes ranged from 10-3 to 1.5 x 103 µL) can also climb up flat or curved surfaces thanks to the charge on the LCIS – something that is not possible for current SLIPS.

“The LICS can rapidly reach as high as 1280 pico-Coulombs per square mm in 0.5 s when exposed to light illumination,” Du explains. “Its robust charge regeneration capability shows no apparent decay even after being exposed to 10 000 cycles of impulse near-infrared irradiation, or even immersed in silicone oil for six months.”

According to the team, the LICS could be used to create steerable droplet-based robots and for performing chemical reactions. It could also be integrated into a pump-free microfluidic chip, allowing for reliable biological diagnosis and analysis in a closed design.

The researchers now plan to further optimize their control of the droplets. “We will also be expanding the biochemical applications of these intelligent polymers and LICS microfluidic chips,” Du tells Physics World.

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