Professor Xue Longjian’s team (NISE-Lab) from the School of Power and Mechanical Engineering (WHU) has made newadvances in the research of intelligent bionic anisotropic materials, with related findings published in Advanced Science.

The first author of the paper is Li Qian, a 2017 doctoral student from the School of Power and Mechanical Engineering (WHU), and the corresponding author is Xue Longjian. The research was funded by the National Key R&D Program of China and the National Natural Science Foundation of China.

Surfaces with micro- and nanoscale features are critical for many plants and insects to survive in nature. For instance, anisotropic surfaces enable butterflies to get rid of water droplets along the outward direction of wings, water striders to walk on water, plants to trap pollen and insects. By mimicking natural surfaces, diverse engineered materials with anisotropic wetting properties have made a contribution to advanced applications in intelligent microfluidic, printing industry, self‐cleaning coating, biomedical applications, and so on.

However, the dynamic changes of surface properties usually need to be carried out ex situ, limiting their potential applications. In addition, although many smart surfaces can be used to transport droplets, the ability to analyze the property of droplets and to sort them is still missing. It is universally known that lotus leaves exhibit isotropic super-hydrophobicity due to their unique papilla structures, and butterfly wings exhibit anisotropic wettability due to their microstructure which causes the difference between the sliding angles forming the forward and the reverse direction of their wings.

Inspired by the wetting features of lotus leaves and butterfly wings, Professor Xue Longjian’s team (NISE-Lab) designed a smart surface, which is referred to as TMAS. As is shown in Figure 1, TMAS possesses the ability to reversibly and in situ manipulate water droplets, and to distinguish acidic/basic liquids, and their strengths by mechanical strain.

Figure 1: the smart surface TMAS inspired by (A) the isotropic-wetting lotus leave and (B) the anisotropic-wetting butterfly wings

TMAS is prepared by growing triangular micropillar array on a pre‐stretched thin poly(dimethylsiloxane) (PDMS) film. Different triangular micropillar arrays, such as a “head-to-tail” arrangement and a “shoulder-by-shoulder” arrangement (Figure 2), can be presented by adjusting the direction and the strength of the strain applied to the PDMS film. Due to the anisotropic shape of the triangular, direction A and direction E are defined.

When the corresponding period in the y direction (Py) increased from 40 µm to 51 µm, the sliding angles in both direction A (SAA）and direction E (SAE) slowly increased, and TMAS presented the isotropic wettable behaviors like lotus leaves, but there is no statistic difference in SAs in direction A and E.. When Py decreased from 40 µm to 24 µm, both SAA and SAE sharply increased, and water droplets could not slide in direction A, even with the surface turned upside down, showing a SAA of −180°.

The surface presented the anisotropic wettability like butterfly wings (ΔSA = 258.3 ± 2.5°). But for the design of micropillar arrays in other shapes, such as circular, squared and hexagonal micropillar arrays, there is no such kind of reversible switches from the state of isotropic wettability to the state of anisotropic wettability.

Figure 2: (A) the “head-to-tail” TMAS, (B) the “shoulder-to-shoulder” TMAS, (C) the definition of sliding direction and sliding angle, (D) the changing regulation of SAA and SAE as the period of the array (Py) changes in the direction of y, (E) the changing regulation of the anisotropy degree (ΔSA) of designs of micropillar arrays in other shapes.

It is for the differences of the three‐phase contact line (TCL) in direction A and direction E that TMAS can get the anisotropic results. When Py=24 μm, the energy barrier at the continuous TCLE is much larger than that at the discrete TCLA. When a water droplet slides in direction A, due to the strong pinning effect of TCLE, the TCL of a droplet in the receding angle is pinned in place, making it impossible for the droplet to slide. When the water droplet changes to slide in direction E, the receding angle is TCLA which has a weak pinning effect, so the droplet will get serious deformation under the effect of the gravity and will slide down when it comes to a certain angle. In the whole process from being unable to slide down in direction A to it being easy to slide down in direction E, the droplet is always in the Cassie mode. Therefore, a situ shift between the states of super‐sticky and easy‐sliding of water droplets is achieved.

Based on the features of the situ shift, TMAS also showed powerful ability to transport water droplets. In the transportation process, TMAS could pick up a droplet from various tilting angles and be ready to release the droplet by just rotating TMAS in direction E. Depending on the stretching state, TMAS can transfer droplets with various volumes (from 2 to 12 µL). Taking advantage of a strong adhesion in direction A, TMAS can not only transport droplets to a horizontal surface, but also deposit a droplet onto titled, vertical, and even upside-down surfaces, showing a large degree of freedom and a large tolerance rate.

Based on the fact that mechanical stress can adjust the morphology of the TCL in the advancing and receding angles in real-time, TMAS also possesses the excellent ability to distinguish acids and bases and their pH strengths. When one acidic droplet and one alkaline droplet are separately placed on two TMAS with different periods and are slowly stretched, the acidic droplet falls first; only a further stretching can release the alkaline droplets, easily identifying acid and base solutions. As for two kinds of acid solutions or base solutions with different pH values, TMAS can also identify the strength of acid/base. When TMAS is slowly stretched, the strong acid droplet and the weak base droplet will slide down first, sliding earlier than the weak acid droplet and the strong base droplet.

The intelligent bionic surface, TMAS, which combines the features of the wettability of butterfly wings and lotus leaves, not only realizes the situ shift between the states of super‐sticky and easy‐sliding of water droplets, but also possesses the exciting ability to pick up and release a droplet from various tilting angles, to distinguish acids and bases, and to tell pH values, offering a new design principle for new chemical sensors.

Link tothepaper:https://onlinelibrary.wiley.com/doi/full/10.1002/advs.202001650

Website of NISE-Lab: http://niselab.whu.edu.cn/

DOI: 10.1002/advs.202001650

Written by: Chen Min

Rewritten by: Li Tiantian

Edited by: Qin Zichang, Sylvia and Hu Sijia