Researchers in Hong Kong have discovered a novel approach to regulating liquid spreading dynamics on surfaces. Traditionally, it was believed that the spreading direction of liquids was determined solely by the surface design and could not be customized. However, this new research challenges that notion by demonstrating that liquids with different surface tensions can be made to select their spreading directions on the same surface.
Macro Ratchets
The key to achieving this customization lies in the use of 3D macro ratchets with dual reentrant curvatures. These sophisticated structures can be fabricated using 3D printing, but the layer-by-layer printing process introduces microgroove-like surface defects. Previous approaches required additional polishing treatments to eliminate these defects, increasing complexity and limiting practical applications.
In this study, the researchers took a different approach. Instead of eliminating the surface defects, they harnessed them to regulate the spreading phase map of liquids. By designing simplified dual-scale ratchets with microgrooves, they were able to achieve liquid directional steering similar to that found in natural phenomena.
Further experiments revealed that the orientation of microgrooves plays a crucial role in regulating liquids with moderate wettability.
“It provides a new design of surface that is easy to fabricate or replicate, without sacrificing the function of liquids directional steering,” said Zuankai Wang, researcher in the Department of Mechanic Engineering at The Hong Kong Polytechnic University.
“Microgrooves arranged perpendicular to the ratchet-tilting direction serve as delay valve to slow down the spreading of liquids on the side surface of ratchets, whereas microgrooves parallel to the ratchet-tilting direction would promote the spreading of liquids due to capillary wicking, thus the latter is more beneficial for the backward spreading of liquids.”
Liquid Spreading
The findings of this study challenge conventional thinking and open up new possibilities for the design and fabrication of surfaces that can precisely control liquid spreading. The researchers are now delving deeper into the mechanisms of liquid-solid interactions and exploring additional functionalities that can be achieved by incorporating different ingredients into the materials.
This research demonstrates how seemingly undesirable surface defects can be leveraged to create functional surfaces, turning waste into treasure. By embracing and understanding the intricacies of microscale structures, engineers can unlock new applications in various fields, including oil-water separation, water harvesting, heat management, microfluidic, advanced manufacturing and biomimetics.
“What we know is just the tip of the iceberg, more advanced visualization tools may be employed to reveal how the liquid and solid structures interact in the microscale, or we can even introduce other functions by adding different ingredients to the materials,” said Wang.
The possibilities are vast, and further investigations will undoubtedly reveal more about the potential of these innovative approaches to liquid manipulation.
The full research paper, titled “Selective liquid directional steering enabled by dual-scale reentrant ratchets” can be found in the International Journal of Extreme Manufacturing, at this link.
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