Liquid Drops, Fingers, and Stars

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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. This effect is known as the Leidenfrost effect, and can be easily visualized when small water drops float across a hot frying pan.  Pictured at left is a water drop floating above an aluminum surface at T = 285C. The reflection of the drop can be seen in the metal surface.  Underneath the drop, there is a small vapor pocket created by the evaporation of the liquid. The video on the right shows large-amplitude, star-shaped oscillations that spontaneously form in larger drops, where the diameter is ~ 3 cm.

Geometry of the Vapor Layer Under a Leidenfrost Drop
J. C. Burton, A. L. Sharpe, R. C. A. van der Veen, A. Franco, and S. R. Nagel. Physical Review Letters 109, 074301 (2012).

We study the fundamental physics liquid/solid contact or "wetting" using  both optical and 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 miniscule amount of a less-volatile miscible liquid (solute) in the bulk (solvent); contact line instabilities in the form of ``fingers'' develop (right). Fluids can also form fingers when a low-viscosity fluid penetrates a high-viscosity fluid in a confined geometry (left). Here the two fluids are confined in a thin channel less than 1 mm thick. When the channel thickness changes (light to dark region in the pictures), 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.

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. Our tool of choice is ultra-fast video cameras, although we also use other optical and electrical methods to probe the dynamics.

Coalescence of Bubbles and Drops in an Outer Fluid
J. D. Paulsen, R. Carmigniani, A. Kannan, J. C. Burton, and S. R. Nagel. Nature Comm. 5, 3182 (2014).
Simulations of Coulombic Fission of Charged Inviscid Drops
J. C. Burton and P. Taborek. Physical Review Letters 106, 144501, (2011).