My research interests are the flow inside animal bodies including urination, defecation, and digestion. These fluids are widely examined from the perspective of physiology.   However, the flow properties and the impacts on the function of organs are not well-understood. I apply the knowledge of fluid mechanics, especially internal flow, to investigate the function of organs.

Duration of Urination does not change with body size

Steven Vogel, a biology professor at Duke University who was not involved in the research, was surprised. “Who would have expected urination time to be so nearly constant?” he said. “I wrote a whole book on biological fluid mechanics, even giving some space to one aspect of urination, and I never imagined.” [LINK] [Steven Vogel on Reader Expectations]

Many urological studies rely on models of animals, such as rats and pigs, but their relation to the human urinary system is poorly understood. Here, we elucidate the hydrodynamics of urination across five orders of magnitude in body mass. Using high-speed videography and flow-rate measurement obtained at Zoo Atlanta, we discover that all mammals above 3 kg in weight empty their bladders over a nearly constant duration of 21 ± 13 s. This feat is possible, because larger animals have longer urethras and thus, higher gravitational force and higher flow speed. Smaller mam- mals are challenged during urination by high viscous and capillary forces that limit their urine to single drops. Our findings reveal that the urethra is a flow-enhancing device, enabling the urinary system to be scaled up by a factor of 3,600 in volume without compromising its function. This study may help to diagnose urinary problems in animals as well as inspire the design of scalable hydrodynamic systems based on those in nature. [PDF]

The touching illustrated book by Alessandro SannaThe river


Hydrodynamics of Defecation

Animals discharge feces within a range of sizes and shapes. Such variation has long been used to track animals as well as to diagnose illnesses in both humans and animals. However, the physics by which feces are discharged remain poorly understood. In this combined experimental and theoretical study, we investigate the defecation of mammals from cats to elephants using the dimensions of large intestines and feces, videography at Zoo Atlanta, cone-on-plate rheological measurements of feces and mucus, and a mathematical model of defecation. The diameter of feces is comparable to that of the rectum, but the length is double that of the rectum, indicating that not only the rectum but also the colon is a storage facility for feces. Despite the length of rectum ranging from 4 to 40 cm, mammals from cats to elephants defecate within a nearly constant duration of 12 ±  7 seconds (N = 23). We rationalize this surprising trend by our mathematical model, which shows that feces slide along the large intestine by a layer of mucus, similar to a sled sliding down a chute. Larger animals have not only more feces but also thicker mucus layers, which facilitate their ejection. Our model accounts for the shorter and longer defecation times associated with diarrhea and constipation, respectively. This study may support clinicians use of non-invasive procedures such as defecation time in the diagnoses of ailments of the digestive system. [PDF] [The Conversation]

The poem by Bethany Brookshire and the comic by JoAnna Science.

Intestine is a blender

According to the U.S. Department of Health and Human Services, the digestive disease affects 60 to 70 million people and costs over 140 billion annually. Despite the significance of the gastrointestinal tract to human health, the physics of digestion remains poorly understood.  In this study, we ask a simple question: what sets the frequency of intestinal contractions?  We measure the frequency of intestinal contractions in rats, as a function of distance down the intestine.  We find that intestines contract radially ten times faster than longitudinally.  This motion promotes mixing and, in turn, absorption of food products by the intestinal wall.  We calculate viscous dissipation in the intestinal fluid to rationalize the relationship between the frequency of intestinal contraction and the viscosity of the intestinal contents.  Our findings may help to understand the evolution of the intestine as an ideal mixer.

The beautiful x-ray photo by Arie van ‘t Riet: Street rat in the garden.

Flying fish accelerate at 5G to leap from the water surface

Flying fish can both swim underwater and glide in the air. Transitioning from swimming to gliding requires penetration of the air-water interface, or breaking the “surface tension barrier,” a formidable task for juvenile flying fish measuring 1 to 5 cm in length. In this experimental investigation, we use high-speed videography to characterize the kinematics of juvenile flying fish as they leap from the water surface. During this process, which lasts 0.05 seconds, flying fish achieve body accelerations of 5 times earth’s gravity and gliding speeds of 1.3 m/s, an order of magnitude higher than their steady swimming speed. We rationalize this anomalously high speed on the basis of the hydrodynamic and surface tension forces and torques experienced by the fish. Specifically, leaping fish experience skin friction forces only on the submerged part of their body, permitting them to achieve much higher speeds than in steady underwater swimming. We also perform experiments using a towed flying fish mimic to determine optimality of various parameters in this process, including body angle and start position with respect to the water surface.  [Press]

Butterflies draft to oscillating flow