The surface properties of materials are often critical because they dictate how they react with their external environment. An important property currently under intense investigation is how surfaces interact with liquids like water.
In a wide range of industrial applications, superhydrophobic surfaces (SHPOS) play an important role. They predominantly consist of rough solid protrusions that function to entrap air to minimize the liquid to solid interface. Ideally, there should be a relatively small spacing between protrusions to ensure greater stability of the superhydrophobic state. This condition, however, increases the lateral adhesion force that reduces the mobility of drops of liquid.
The behavior of droplets of liquid on rough surfaces occur in one of two states: either Cassie, where the liquid partially floats on a layer of gas or air; and Wenzel, where the liquid droplets are in full contact with the surface, trapping them there.
Therefore, for the superhydrophobic condition, we need to achieve optimization of the stability of the Cassie state while at the same time minimising the lateral adhesion force. This dichotomy presents a considerable challenge for the SHPOS with high performance.
Nature demonstrates this effect well. The Salvinia molesta plant is a floating water weed that has oval-shaped leaves which are covered in many waxy hair-like structures on the upper surface of the leaf. The leaves display a persistent Cassie state when submerged, due to the hydrophobic eggbeater-shaped trichomes (hairs) which bear hydrophilic pins on the top.
At the hydrophilic/hydrophobic boundary, the water/air contact line is pinned in the vertical direction. The structure exhibits strong adhesion due to the presence of the hydrophilic patches. At the same time, the pinning effect is found to diminish the mobility of the contact line in the horizontal plane.
In related studies, the Nepenthes pitcher plant has stimulated research into slippery liquid-infused porous surfaces (SLIPS), with a molecularly smooth lubricant fixed to the top of the microstructure which facilitates fast liquid drop transportation. SLIPS have been shown to be promising substrates where a low lateral adhesion force for liquid drops is required.
However, drops on a liquid infused slippery surface demonstrate both a smaller contact angle and shedding velocity compared with the SHPOS. Therefore, to obtain a structure with the stability of the Cassie state combined with the minimization of the lateral adhesion force, requires a combination of SLIPS and SHPOS.
Difficulties arise with the introduction of a stable air cushion between protrusions with slippery surface due to the low surface tension of the common liquids like water. Responding to this challenge, the materials surface science research team headed by Professor Xu Deng at the University of Electronic Science and Technology of China (UESTC) collaborated with Professor Periklis Papadopoulos from the University of Ioannina, to propose a Salvinia-like slippery surface (SSS). In effect, this is a new slippery rough surface that permits high mobility for liquid droplets.
The Salvinia-type slippery surface consists of a lubricant-infused, cross-linked polydimethyl siloxane (PDMS) layer on the top of the structures with hydrophobic side walls. An additional energy barrier is created by the lubricant, which works against quasi-static and dynamic impalement by the pillar structures. The oil layer at the top of the structure also functions as a lubricant and this aids in the reduction of the adhesion and significantly improves the liquid drop mobility.
In this combination, the drops on the SSS demonstrate the stable Cassie state and avoids the strong pinning force on the hydrophilic patches of the Salvinia plant leaf. Researchers compared a control surface with the same structure without lubricant with the SSS and found that the SSS demonstrates increased stability against forces from both pressure and impact. It exhibited enhanced lateral mobility of the water drops and reduced hydrodynamic drag.
As a consequence of its ease of fabrication and its enhanced performance properties, the SSS engineered liquid-repelling surface offers multiple applications in the transporting of viscous fluids, industrial pipelines, and microfluidic apparatus. Enhancing the mobility of liquid droplets passing over rough surfaces opens up a multitude of other potential applications. It could assist with improving the efficiency of water harvesting and transportation in arid regions. It may offer valuable advantages in improving condensation heat transfer for power plant heat exchangers and could be applied to prevent dangerous frosting and ice formation on aircraft wings.
Further information: Xiaomei Li et al, Salvinia-like slippery surface with stable and mobile water/air contact line, National Science Review (2020). DOI: 10.1093/nsr/nwaa153
Keywords: superhydrophobic surfaces, liquid to solid interface,
mobility, lateral adhesion force, liquid droplets, hydrophilic/hydrophobic
boundary, adhesion, pinned, polydimethyl siloxane, lubricant, liquid-repelling surface