Biomedical subjects

John Hegseth

Publications and source records attributed to John Hegseth.

3 recordsLinked to original sources

PubMed2005

Wetting film dynamics during evaporation under weightlessness in a near-critical fluid.

By performing near-critical fluid experiments in the weightlessness of an orbiting space vehicle, we have suppressed buoyancy-driven flows and gravitational constraints on the liquid-gas interface of a large gas bubble. At equilibrium, the liquid completely wets the walls of a cylindrical cell, and the bubble is pushed to the sidewall. In these experiments the system's temperature T is increased at a constant rate past the critical temperature T(C), pushing it slightly out of equilibrium. The wetting film shows a large mechanical response to this heating, including contact lines that recede on a solid surface and a spreading bubble. Near T(C), the receding contact lines make the entire bubble appear to spread along the copper sidewall. The spreading bubble is a manifestation of the boiling crisis near the critical point. We present quantitative data of the receding contact lines that are observed prior to the near-critical boiling crisis. We analyze the receding contact lines in detail, and find that they are driven by vapor recoil from evaporation, as is the spreading bubble of the boiling crisis.

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PubMed2004

Critical temperature shift in pure fluid SF6 caused by an electric field.

A systematic study of the effect of an electric field on the critical temperature of a pure fluid is made for the first time to our knowledge. An ac electric field is applied to a spherical capacitor filled with SF6 at its critical density, while the temperature is slowly ramped down through its critical temperature T(c). By continuously observing the light transmission through the fluid during the temperature ramp, a shift in T(c), DeltaT(c), is found at various electric fields. By shining the light vertically through the fluid, we utilize the density gradient induced by the fluid's weight to compensate for the effects of density changes from electrostriction. This technique effectively keeps the system at constant critical density with respect to the observation of T(c). We observe an increase in T(c) as expected from thermodynamic stability and renormalization group theory, but quantitatively larger by an order of magnitude.

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PubMed2002

Large-scale geophysical flows on a table top.

Results from two similar experimental systems that attempt to create laboratory geophysical analogs in spherical geometry are presented. In the first system, real time holographic interferometry and shadowgraph visualization are used to study convection in the fluid between two concentric spheres when two distinct buoyancy forces are applied to the fluid. The heated inner sphere has a constant temperature that is greater than the outer sphere's constant temperature by DeltaT. In addition to the usual gravitational buoyancy from temperature induced density differences, another radial buoyancy is imposed by applying an ac voltage difference, DeltaV, between the inner and outer spheres. The resulting electric field gradient in this spherical capacitor produces a central polarization force. The temperature dependence of the dielectric constant results in the second buoyancy force that is especially large near the inner sphere. The normal buoyancy is always present and, within the parameter range explored in our experiment, always results in a large-scale cell that is axisymmetric about the vertical axis. We have found that this flow becomes unstable to toroidal or spiral rolls that form near the inner sphere and travel vertically upward when DeltaT and DeltaV are sufficiently large. These rolls start near the center sphere's equator and travel upward toward its top. In the second experimental system, the central force is applied to a highly compressible near-critical fluid in weightlessness (parabolic flight) and normal gravity. Although a geophysically similar density distribution could not be obtained in the limited time of a parabolic flight, clear influences of the central force on the fluid were observed in both weightlessness and terrestrial experiments.

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