Imagine a world where fluids, those seemingly simple substances we often take for granted, can behave in ways that challenge our understanding of physics. This is the fascinating realm that Thamires Lima, a researcher at Drexel University, has been exploring. Her work with thick, viscous liquids has led to a groundbreaking discovery: some simple fluids can fracture, a phenomenon previously thought to be exclusive to elastic complex fluids.
The Unexpected Crack
While conducting experiments with a blend of hydrogen and carbon, Lima heard a sharp crack. This was not an expected outcome; the fluid was supposed to stretch, not snap. The crack was a loud, clear indication that something extraordinary was happening. Arnold Mathijssen, a fluid physicist, described it as a surprising event, given that viscosity typically rearranges molecules without causing a fracture.
A Brittle Break Explained
Lima and her team realized that the fracture was a "brittle fracture," similar to what happens when you drop a piece of glass or porcelain. Even solids with elasticity have tiny defects, and when stressed beyond a critical point, it becomes more favorable for the solid to grow a crack than to store the stress elastically. This phenomenon, observed in complex fluids like polymer melts, was thought to be exclusive to elastic materials.
Challenging Conventional Wisdom
The hydrocarbon blend Lima worked with was a simple fluid, lacking the elasticity of complex fluids. Brato Chakrabarti, a physicist, questioned the traditional theory: "If there is no elasticity, how can a crack initiate or grow?" This led the researchers back to the work of Daniel D. Joseph, who suggested in the 1990s that any liquid could fracture under sufficient tearing stress, regardless of its elasticity.
The Role of Cohesive Energy
Nicolas J. Alvarez, a professor at Drexel, wonders if the breaking point of a liquid is related to its cohesive energy, the force that holds molecules together. Simple fluids, when stressed, can form intermolecular voids or bubbles through a process called cavitation. If enough bubbles form rapidly, they could theoretically crack the liquid.
Crack Propagation and Speed
The researchers found that once a crack nucleates in a simple fluid, it propagates extremely fast due to the fluid's non-elastic nature. In their experiments, cracks in simple fluids reached velocities of approximately 500 to 1,500 meters per second, much faster than in complex fluids. This rapid propagation is attributed to the fluid's ability to dissipate energy, with no molecular chains to slow the crack down.
Critical Stress and Viscosity
Surprisingly, both complex and simple fluids tended to fracture at the same critical measure of stress: 2 megapascals. The researchers varied the viscosity of the hydrocarbon blend and found that only the least viscous liquid did not fracture. The critical stress level is proportional to the viscosity times the strain rate, indicating a strong relationship between these factors.
Future Explorations
Lima plans to use more transparent liquids to capture the crack formation process and probe the liquid's surface with high-resolution microscopes. Alvarez is interested in exploring the implications of fluid fractures in fiber spinning, inkjet printing, brain injury protection, and soft robotics. He believes that understanding fluid fracture is a paradigm shift, challenging long-held beliefs in the field.
This research opens up a new avenue of exploration, revealing the hidden complexities of simple fluids and their potential applications in various fields.
