Small is Big - Bio-inspired and microfluidics-enabled structures for manipulating liquids

Sep 20, 2017

Professor Liqiu Wang (the second from right) demonstrates the manipulation of nano- droplets with his PhD students, Xin Tang, Pingan Zhu and Ye Tian (from the left).


Nature has always been our inspiration source of innovations. Chinese Kung Fu developed effective moves from hunting skills of powerful beasts like snakes, eagles, and tigers; airplanes mimic the skillful flight of birds; legged robots imitate legged animals such as dogs and spiders. Nowadays, state-of-the-art technology enables us to unveil mysteries of the microscopic world and thus invent at microscale with precision. Prof Liqiu Wang and his PhD students, from the Department of Mechanical Engineering, have been using the precision of microfluidics in manipulating tiny amount of liquids and engineering nano-/micro- structures to mimic evolutionarily-optimized microstructures in insects that interact with liquids, and thus developed a series of techniques for manipulating liquids precisely: water collecting, liquids repelling, and droplets manoeuvring. The breakthroughs have yielded three articles published in the prestigious journal Nature Communications in 2017 (Tian Y., Zhu P.A., Tang X., Zhou C.M., Wang J.M., Kong T.T., Xu M. and Wang L.Q. 2017, Large-scale water collection of bioinspired cavity-microfibers, Nature Communications (in press); Zhu P.A., Kong T.T., Tang X. and Wang L.Q. 2017, Well-defined porous membranes for robust omniphobic surfaces via microfluidic emulsion templating, Nature Communications 8, 15823; Tang X., Zhu P.A., Tian Y., Zhou X.C., Kong T.T. and Wang L.Q. 2017, Mechano-regulated surface for manipulating liquid droplets, Nature Communications 8, 14831).

Unique structural and topological features of spider-silks and their web enable them being a super water collector witnessed by a large number of water droplets handing on them in the early morning. With the microfluidic technology, Prof. Wang’s team has precisely fabricated robust microfibers with spindle cavity-knots and different topological fiber-networks in mimicking these features. These microfibers are endowed with unique surface roughness, mechanical strength, and long-term durability, thus enabling a super performance in collecting water. The maximum water volume collected on a single knot is almost 495 times the knot volume; the water collection is even more efficient and scalable with their networks. These light-weighted yet tough, low-cost microfibers offer promising opportunities for large-scale water collection in water-deficient areas.

On a sunny summer day, beaches are full of joys: kids build sand castles; adults swim or surf waves. However, people have to suffer the discomfort of getting clothes wet. Inspired by springtail cuticle, Prof Wang’s team has fabricated liquid-repellent surfaces that can eliminate this distressing situation. The fabrication technique is based on microfluidic-droplets templates, similar to the method for making shaped cookies using baking molds. The functional surfaces repel both water and oils attributed only to springtail-cuticle-mimicked microstructures. The work offers deep insights of liquid-repelling structures and benefits our daily life significantly with applications of these super-liquids-repelling surfaces in various fields including clothes, cookers, and building walls where repelling liquid is relevant.

Some semiaquatic insects can readily walk on water and climb up menisci slope due to the dense hair mat and retractable claws of complementary wettability on their tarsi. Inspired by this, Prof. Wang’s team created a mechano-regulated surface whose adhesive force to liquid droplets can be simply switched through mechanical regulation. The mechano-regulated surface functions as a “magic hand” that can capture and release multiple tiny droplets precisely in a loss-free manner, and works for both water and oil droplets down to nano-litre scale. These surfaces are relevant and crucial in various high-precision fields such as medical diagnosis and drug discovery where the precise transferring of tiny liquid is a must.

Learning from nature paves the way for creating microstructures with unique features to interact with liquids on-demand. Small yet powerful, these microstructures can manipulate liquids of volume much larger than their dimensions effectively and precisely. With these techniques developed in HKU, drinking water can be gathered directly from the air in deserts, clothes are never been wetted on rainy days, and liquids can be conveniently handled like solids.