Fluid Mechanics


"The highest excellence is like that of water."

- Lao Tzu, Tao Te Ching (~ 450 B.C.)

Fluid  mechanics is one of the oldest branches of physics, the study of which can be traced back to the dawn of science in ancient Greece. Yet, the dynamics of fluid still elude a complete theoretical understanding with many important unsolved problems. For example, turbulence is generally regarded as one of the most difficult unsolved problems. A famous apocryphal story goes that Heisenberg was once asked what he would ask God, given the opportunity. His reply was: "When I meet God, I am going to ask him two questions: Why relativity? And why turbulence? I really believe he will have an answer for the first." As the most abundant material on Earth, fluid is tightly related to almost every aspect of natural and industrial processes. Hence, understanding the dynamics of fluid flows in different circumstances is of great imporance and the study of fluid mechanics is one of the most active research areas in physics, chemical engineering and materials science. 

Our research focuses on experimental study of fluid flows in coating and transportation processes with a special emphasis on the instability and singularity developed in fast fluid flows. Specifically, we are interested in the singular dynamics of liquid drop during impact, pattern formation arising from the interaction between turbulent flows and sediment granular bed and also the dynamics of complex fluids under shear.  We have developed various state-of-the-art experimental techniques including high-speed photography, holographic imaging and confocal imaging to visualize fast three-dimensional fluid flows ranging from centimeter scales down to micron/nanometer scales.  

(1) Liquid drop impacts on solid and granular surfaces
(2) Polymeric fluids under shear
(3) Turbulence-sedimentation interactions

(1) Liquid drop impacts on granular and solid surfaces

When a granular material is impacted by a sphere, its surface deforms like a liquid yet it preserves a circular crater like a solid. Although the mechanism of granular impact cratering by solid spheres is well explored, our knowledge on granular impact cratering by liquid drops is still very limited. Here, by combining high-speed photography with high-precision laser profilometry, we investigate liquid-drop impact dynamics on granular surface and monitor the morphology of resulting impact craters. Surprisingly, we find that despite the enormous energy and length difference, granular impact cratering by liquid drops follows the same energy scaling and reproduces the same crater morphology as that of asteroid impact craters. Inspired by this similarity, we integrate the physical insight from planetary sciences, the liquidmarblemodel from fluid mechanics, and the concept of jamming transition from granular physics into a simple theoretical framework that quantitatively describes all of the main features of liquid-drop imprints in granular media. Our study sheds light on the mechanisms governing raindrop impacts on granular surfaces and reveals a remarkable analogy between familiar phenomena of raining and catastrophic asteroid strikes.

The video shown above from our research has won 2014 APS Division of Fluid Dynamics "The Gallery of Fluid Motion" Award. The image taken from our high-speed video shown below has been selected as back scatter by Physics Today and as PNAS featured image. Our research has been reported by Discover Channel, Popular Science, CBC news (Canada), SPIEGEL magazine (Germany), De Kennis van Nu (The Netherlands).


3.9 mm water drop impacts on a granular bed composed of 90 um glass beads

  • L. Gordillo, T.-P. Sun, and X. Cheng, "Dynamics of drop impact on solid surfaces: evolution of impact force and self-similar spreading", J. Fluid Mech. 840, 190 (2018).
  • Granular impact cratering by liquid drops: Understanding raindrop imprints through an analogy to asteroid strikes, R. Zhao, Q. Zhang, H. Tjugito, X. Cheng, Proc. Natl. Acad. Sci. USA 112, 342 (2015).
  • Q. Zhang, M. Gao, R. Zhao, and X. Cheng, “Scaling of liquid-drop impact craters in wet granular media”, Phys. Rev. E 92, 042205 (2015). (Selected as “Editors’ Suggestion”)
  • R. Zhao, Q. Zhang, and X. Cheng, “Raindrop imprints and asteroid strikes”, Phys. Today 68(3), 72 (2015).
  • R. Zhao, Q. Zhang, H. Tjugito, and X. Cheng, “Raindrop impact on a sandy surface”, Phys. Fluids 27, 091111 (2015).

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