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Topological fluid diodes control liquid flow in only one direction in a tube by adjusting the surface microstructure. The research was published in Science Advances, a magazine subsidiary of Science. "Topology" means to study the topography and landform of a particular place. Wang Diankai and his colleagues use the term "topology" to express the directional movement of fluids by controlling the micro-morphology of the material surface. The wettability of the surface is changed by adjusting the unique structure formed by the microstructure of the surface. The design of topological fluid diodes solves a phenomenon that was discovered by Professor Manoj k. Chaudhury and Professor Ankur Chaudhury as early as 2005, but which cannot be explained, that oil on the hydrophobic surface breaks through the initially slowly expanding barrier.

Design Inspiration
Without the directional movement of liquids, many animals and plants on the planet would be extinct. For example, desert beetles collect water from the hydrophilic areas behind them, and then use the fluid channels formed by the hydrophilic and hydrophobic areas to transport the collected water spontaneously and directionally to the mouth. Another example is cactus, which collects water vapor through thorns in the desert. The collected water is transmitted to cactus spontaneously and directionally along the outside of thorns. Of course, such examples are not confined to deserts. Lips like pitcher grass and lizard skin have similar functions.

U-shaped Island Array & Re-entrant Structure
In the manufacture of Topological Fluid Diodes, the research team constructed a complex surface structure with a special array. The overall structure of the surface is a U-shaped island arrays. Each U-shaped island arrays consists of a U-shaped groove with a re-entrant structure at the top of the groove. The concave angle structure is not designed to look good, but to change the wettability of the surface.

The liquid diode consist of U-shaped island arrays spatially confined in periodically patterned fences. The width of the U-shaped islands is designed to decrease gradually from the opening end to the other end; thus, two diverging side-channels are naturally formed with- in fences. The inner side of the cavity in the U-shaped island is specially designed with a reentrant structure to constrict the backflow of the liquid. The width and length of the cavity, the total width and length of the island, and the spacing between individual islands can be varied. All surfaces are fabricated on a silicon wafer using standard microelectromechanical system (MEMS) process).

According to an earlier study by Professor Anish Tuteja of the University of Michigan, this concave structure can turn a hydrophilic surface into a hydrophobic surface without any chemical modification. When the droplet is on the surface, it does not spread out in disorder as is common in life, but in a single direction. Although there is a small degree of wetting in the opposite direction, the wetting is quickly cut off by a fluid diode.

Not only water, but also other liquids with different surface tensions, densities and wettability, such as ethanol and glycol, have been tried. It is found that these liquids have similar phenomena on fluid diodes. This proves that the fluid diode has universal applicability.

Research Beackground
The design of a fluid diode solves a phenomenon that has been difficult to explain in physics for more than a decade. As early as 2005, Professors Manoj K. Chaudhury and Ankaur Chaudhury found that on a hydrophobic surface linearly aligned with water droplets, oil expands very slowly in its initial state. But as the oil gradually accumulates, joins and covers the water droplets, the oil expands rapidly. Although some studies have attempted to explain this phenomenon since then, no one has been able to give an answer to how oil can break through and overcome the barrier of slow expansion at the initial stage, so it has become an open mystery.

Corner Flow
Until the study of directional flow of liquid in a fluid diode, the author found that a precursor liquid film played a key role - the follow-up liquid was more willing to follow the "predecessor" footprint, and the vanguard forces pulled the large forces forward. This can be attributed to a phenomenon called corner flow. Drink coffee with astronauts -- to be exact, smoke coffee - for example. Under the condition of weightlessness in space, the flow of liquid is free and disordered. However, due to the angular flow effect, liquids tend to move along the cup wall.

Hydraulic Jump
In the topological fluid diode, a part of the liquid will flow preferentially along the side wall of the fence, from both sides into the U-shaped groove of the fluid diode, forming a precursor liquid film, but it will not exceed the height of the concave structure. Subsequent liquids are blocked by concave angular structures and accumulated in U-shaped grooves. When the blocked liquid accumulates to a certain amount, it will break through the shackles of concave angle structure and converge with the precursor. Then, a "hydraulic jump" will occur, which will cross the U-shaped island barrier and flow forward. So as a whole, the flow of liquid in the topological fluid diode is not a continuous event, but a discrete event.

The forward direction of the fluid diode is always in the on state, so the reason of its reverse blocking state can also be found from the surface structure. When the liquid tries to flow backwards in the fluid diode, most of the liquid blocked by the concave angle structure will wet the concave angle structure from above, and the concave angle structure blockades the precursor liquid film below, forming a re-entrant pinning. Thus, the subsequent liquid can not merge with the precursor liquid film, and can not move forward smoothly.

Although the liquid will break through the concave angle structure when the pressure is high to a certain extent, because the forward conduction state of the fluid diode is very good, the liquid is willing to run forward, so the reverse pressure is difficult to increase to the extent of breaking through the concave angle structure, which contributes to the one-way flow of the liquid on the fluid diode.

Researchers will also put the fluid diode in a circular and spiral shape to show the macroscopic phenomenon of spontaneous, long-distance directional flow of liquid. This transmission can even overcome gravity.

Applications
Fluid diodes can construct logic gates of fluids, even logic gate arrays, a "logic circuit" of fluids. The application of such "fluid logic circuits" in the field of microfluidic control will greatly accelerate the development of pharmaceutical, electronic cooling and other industries. Secondly, fluid control may also be used for heat dissipation. A large amount of cost and energy can be saved if the coolant can be returned spontaneously to the evaporation end. Secondly, the spontaneous transport of liquids may also be used in aerospace. Under microgravity conditions, controlling the direction of fluid movement often requires more energy input, and even a cup of coffee needs to be sucked. Topological Fluid Diodes allow astronauts to drink coffee without sucking in space! Finally, it is assumed that due to the universal applicability of fluid diodes to a variety of liquids/fluids. Controlling the directional movement of magnetic fluids can produce more powerful products if other forms of fluids are introduced, such as magnetic fluids-fluid diodes/logic gates.