Liquid directional transport: A new approach

New research published in Science challenges the conventional understanding of liquid spreading and demonstrates a groundbreaking method to control the direction of liquid movement. Traditionally, the spreading direction of liquids with different properties is believed to be determined solely by the surface design and cannot be tailored. However, this recent study introduces a novel approach using 3D macro ratchets with dual reentrant curvatures to make liquids with different surface tensions select their spreading directions on the same surface.

The fabrication of such intricate structures previously required a combination of and polishing treatments to eliminate microgroove-like surface defects resulting from the layer-by-layer printing process. This increased the complexity of manufacturing and hindered practical applications. However, the researchers, in a study published in the International Journal of Extreme Manufacturing, have simplified the surface topography and fabrication process while still maintaining the liquid directional steering function.

The study presents an innovative concept of transforming a perceived surface deficiency into a functional advantage. Rather than attempting to eliminate the rough surface defects of 3D printing through additional polishing treatments, the researchers leverage this apparent flaw to flexibly control the spreading phase map of liquids. By doing so, they achieve a surface design that is easy to fabricate or replicate without sacrificing the ability to direct the flow of liquids.

Zuankai Wang, a chair professor in the Department of Mechanical at The Hong Kong Polytechnic University and the corresponding author of the study, emphasizes the significance of this research, stating, “It provides a new design of surface that is easy to fabricate or replicate, without sacrificing the function of liquids directional steering.” Jing Sun, the first author of the paper and a postdoc at City University of Hong Kong, adds, “Essentially, it suggests new possibilities for making use of the often overlooked or useless part of materials to realize desired function.”

In nature, various biological surfaces possess the ability to transport liquids directionally, such as the back of desert beetles, the silk of spiders, and the beaks of birds. They achieve this by leveraging their unique surface morphologies or chemistries. Researchers have attempted to mimic this liquid movement through the control of asymmetric micro/nanostructures, wettability gradients, or external energy inputs. However, in all previous studies, liquids have always transported along directions that reduce surface energy. This leads to an intriguing question: Can liquids select their transport direction without altering the surface structure or requiring energy input?

In the Science article, researchers observed an unexpected liquid transport behavior on the Araucaria leaf. The leaf's surface consists of three-dimensional (3D) ratchets with transverse and longitudinal reentrant curvatures. Remarkably, liquids with low surface tensions and high surface tensions spread along opposite pathways on this leaf surface. This discovery challenges the conventional understanding and opens up new possibilities for controlling liquid transport phenomena.

In order to overcome the challenges posed by the microgroove-like surface defects resulting from the layer-by-layer 3D printing process, Sun and his team devised a novel approach. They designed a simplified dual-scale ratchet that could be fabricated in a single step using 3D printing. This ratchet featured an A-shaped island with a reentrant tip on a macroscopic scale, while microgrooves covered its surface on a microscopic scale. Surprisingly, their tests revealed that these simplified dual-scale ratchets could achieve liquid directional steering similar to that observed on the Araucaria leaf.

Eager to delve deeper into the effects of the second-tier microgroove structures, the researchers investigated the impact of different microgroove orientations. Sun explained, “We were curious about how and to what extent the orientation of microgrooves could influence the spreading dynamics of liquids.” Through experiments, they discovered that the orientation of microgrooves played a crucial role in controlling liquids with moderate wettability. In fact, liquids could even spread in opposite directions on ratchets with microgrooves oriented perpendicular and parallel to the ratchet-tilting direction.

Wang further elaborated on their findings, stating, “Microgrooves arranged perpendicular to the ratchet-tilting direction act as a delay valve, slowing down the spreading of liquids on the side surface of the ratchets. On the other hand, microgrooves parallel to the ratchet-tilting direction promote liquid spreading due to capillary wicking, making them more conducive to backward liquid spreading.” The team validated the role of microgrooves through theoretical analysis and simulation.

Intrigued by the possibilities that lie ahead, the researchers are now delving into the underlying mechanisms of liquid-solid interaction and exploring additional functions that these materials can exhibit. Wang emphasized their ongoing pursuit, stating, “What we currently understand is merely the tip of the iceberg. By employing advanced visualization tools, we can unravel the intricate microscale interactions between liquids and solid structures. Furthermore, we can potentially introduce other functions by incorporating different ingredients into the materials.”

This research not only offers insights into the fundamental understanding of liquid spreading behavior but also opens up a vast array of possibilities for leveraging surface defects to achieve desired functionalities. With further and advancements in visualization techniques, this field holds promise for the development of innovative materials and technologies.

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