Using hollow tubes of varying diameters and high-speed imaging, KAUST researchers captured the hidden shapes of the thin liquid film left behind after water flowed out. Credit: 2025 KAUST
When water drains from the bottom of a vertical tube, it is followed by a thin film of liquid that can adopt complex and beautiful shapes. KAUST researchers have now studied exactly how these “fluted films” form and break up, developing a mathematical model of their behavior that could help improve the performance, safety, and efficiency of industrial processes.
The paper is published in the journal Physical Review Letters.
“At first glance, water draining from a tube seems like an everyday process driven by gravity,” says Abhijit Kushwaha, a member of the team behind the work. “It is only with high-speed imaging that we can slow down time enough to capture the hidden choreography of this process.”
For the study, the team used hollow tubes of varying diameters, filled with water to different heights. As the researchers allowed the water to flow out, a high-speed camera captured the shapes formed over a period of about a hundred milliseconds.
This revealed a curious effect for certain combinations of tube diameter and water height. As the liquid fell, a thin film of water dragged against the tube walls and descended more slowly. Once the main water column exited the tube, this film emerged and formed a fleeting, tulip-shaped bubble. In some cases, the fluted film quickly retracted into the tube; in others, it stretched until the water column broke away from it.
The formation of fluted films depends on a delicate balance of gravity, surface tension, inertia, and viscosity, explains Kushwaha. If the water column is too short or the tube is too narrow, the film does not form. Conversely, the widest tubes produce a cylindrical film that breaks away from the tube to create a crown shape.
The no-film (i) behavior is shown in Fig. 1(a). It occurs when the fluid does not completely drain from the tube. This behavior is observed for the smallest R and H, where surface tension retains all liquid inside the tube, and is identified as “static slug” [Fig. 1(f), Supplemental Movie 1]. Credit: Physical Review Letters (2025). DOI: 10.1103/PhysRevLett.134.224001
The researchers created a mathematical model to predict the behavior of these films based on a few simple parameters, such as tube radius and water height. “This can inform better design and control strategies in any system where thin liquid films play a vital role—from industrial reactors to microelectronics to biological systems, such as the lungs,” explains Tadd Truscott, who leads the research.
For example, devices called falling-film evaporators are widely used in industries like food processing, pharmaceuticals, and power generation to concentrate liquids or remove solvents. These systems feature thin films of liquid that evaporate as they flow down the walls of heated tubes. If these films break or become uneven, heat transfer efficiency can be reduced, or equipment can be damaged.
“Our research helps improve understanding of when and how such films might rupture or behave unexpectedly, offering insights that could be used to design more reliable systems,” Truscott says. “This could also be relevant to cooling rocket engines or applying protective coatings to surfaces.”
The team plans to study how other fluids behave in a broader range of tubes. “Ultimately, our goal is to develop a predictive framework that helps scientists and engineers understand, design, and optimize systems where thin films play a hidden but crucial role,” Kushwaha adds.
More information:
Abhijit K. Kushwaha et al, Transient Fluted Films behind Falling Water Columns, Physical Review Letters (2025). DOI: 10.1103/PhysRevLett.134.224001
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Falling water forms beautiful fluted films (2025, August 20)
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