GELS AND FLUIDS
Despite the ubiquitous nature of fluids and soft materials, many mysteries remain regarding their behavior. In the Burton Lab, we explore a wide range of projects of soft systems such as hydrogels, frictional fluids, and droplets.
GELS AND FLUIDS
Interfacial Phenomena: Coalescence, Charging, and Singularities
Proposed redox reaction behind nanopure water and metal interface contact potential
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Charging at fluid-solid interfaces has been a long-standing open question in electrification, as the mechanism behind triboelectric charging and flow electrification is poorly understood. Waterfalls and hydrocarbon pipelines are known to develop large potentials (and potentially dangerous discharge events) when fluids move across these surfaces—which could have wild implications for extraterrestrial contexts like the methane lakes of Titan. We worked to carefully characterize thousands of individually formed, high-resolution droplet charges and masses across varied fluids, contact materials, flow rates, and applied potentials to explore this charging behavior and underlying mechanism. The contact material has a domineering influence on droplet charge, where predictable charging is seen with metallic surfaces (attributed to a contact potential) while any introduction of plastics gives rise to erratic charging behavior.
Singularities arise in many areas of physics and have the unique ability to organize dynamics in a large region of phase space. In order to understand these singularities, we must study how nature is able to pass through the singular point. In the laboratory, the breakup of a drop or bubble into pieces is an example of a finite-time singularity. The fluid's topology changes because what began as one mass of fluid, ends up as many individual pieces. Quantities such as the fluid velocity and pressure are becoming very large near the singularity where two masses separate, while the size of the connecting neck region shrinks to zero diameter. The exact manner in which these quantities diverge or shrink depends on the fluid parameters, which define universality classes for the singularities, just as in critical phenomena and thermodynamic phase transitions. We study fluid singularities in a variety of systems including bubbles, droplets, and floating liquid puddles.
Relevant Publications:
- The electrostatic charge on exuded liquid drops
- Coalescing Clusters Unveil New Regimes of Frictional Fluid Mechanics
- Coalescence of Bubbles and Drops in an Outer Fluid
- The Inexorable Resistance of Inertia Determines the Initial Regime of Drop Coalescence
- Viscous to Inertial Crossover in Liquid Drop Coalescence
Hydrogels
"The Tomato," a dyed PAAM hydrogel slab
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Measurement of a hydrogel sphere's surface contact area
with fluorescent microparticles
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Hydrogels are hydrophilic, cross-linked polymer networks that contain over 90% water by weight. Think of Orbeez, boba pearls, and absorbent diapers; their uses extend to agricultural soil management, contact lenses, tissue engineering, and even cosmetics. Using tribological and rheological methods, we can probe the gels' contracting and swelling responses to applied compression, shear, and strain. The video on the top left corner shows the gradual swelling of a hydrogel as it fills with water over the course of 12 hours. We aim to explore the physical properties of hydrogels during sliding with the focus of stiff gels such as polyacrylamide and poly-acrylic acid. The ultra-low friction between hydrogels and its substrate during sliding is highly dependent on sliding velocities. At low sliding velocities, friction is inversely proportional to the characteristic pore size of hydrogels and is controlled by fluid flow through the hydrogel network. In contrast, at high sliding velocities, there appears a lubricating fluid film that forms between hydrogel and the substrate. Understanding the frictional behavior of hydrogels is highly relevant regarding the design of gels in industrial use.
Besides having ultra-low friction, stiff hydrogels (modulus of ~10-100 kPa) also swell upon mechanical shear. Such swelling may manifest as a volume increase in a stress-controlled environment, or a normal force increase in a strain-controlled environment. Soft hydrogels and biopolymers are known to shrink when sheared. However, cartilages, which structurally are similar to stiff hydrogels, have shown swelling behavior upon sliding. Utilizing rheological measurements, we can confirm that bulk swelling is indeed present and persist over hours in stiff gels, and may explain rehydration and mechanical rejuvenation in biological tissues.
Relevant Publications:
The Leidenfrost Effect
When a drop of liquid is placed on a very hot surface, an amazing phenonmeon occurs. Above a certain temperature, instead of boiling rapidly, the drop will levitate on a thin cushion of vapor created by the evaporation of the liquid. This effect is known as the Leidenfrost effect, and can be easily visualized when small water drops float across a hot frying pan. The video shows large-amplitude, star-shaped oscillations that spontaneously form in larger drops, where the diameter is ~3 cm.
Relevant Publications:
Viscous Fingering
Poster Submission to the 2023 APS Division of Fluid Dynamics Gallery of Fluid Motion, by
Tabitha C. Watson
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We study the fundamental physics liquid/solid contact or "wetting" using high-speed video techniques. When a volatile liquid drop is placed on a wetting surface, it rapidly spreads and evaporates. The spreading dynamics and drop geometry are determined by a balance between thermal and interfacial forces, including Marangoni effects. However, this spreading behavior is drastically altered when drops contain a minuscule amount of a less-volatile miscible liquid (solute) in the bulk (solvent); contact line instabilities in the form of "fingers" develop. Fluids can also form fingers when a low-viscosity fluid penetrates a high-viscosity fluid in a confined geometry. Here the two fluids are confined in a thin channel less than 1 mm thick. When the channel thickness changes, the finger can become unstable and break. The resulting "bubbling" of the fluid leads to an increase in the pressure, and can occur in similar, three-dimensional systems where oil/water or even magma flows through porous materials, such as rock.