Robert Higgin Professor of Mechanical Engineering, Visiting Professor of Architecture
Ph.D. 1970, Harvard University
Phone: 1 (203) 432-4346
Fax: 1 (203) 432-7654

Professor Robert E. Apfel succumbed to cancer August 1, 2002

My interests focus on the application of ultrasonics to the characterization of materials and to the induction of physical changes in materials and in tissue (a sample result).

In the application of ultrasonics to the characterization of materials, we use relatively low amplitude acoustic waves to probe material properties by measuring parameters such as the sound velocity and acoustic nonlinearity parameter. In one study, we measured these two parameters and the density of a complex tissue material and, using a model we developed, were able to infer the volume percentages of water, fat, and protein in the material. Such information may permit non-invasive tissue characterization.

In another study, we measured the properties and the size of microparticles and sub-micron bubbles by scattering high-frequency (30 MHz) ultrasound pulses from the inclusions. In the case of bubbles, we seek to determine the circumstances in which diagnostic ultrasound may produce transient microcavitation arising from the increased power of diagnostic instrumentation used for imaging and blood flow determination. Transient microcavitation may produce subtle bioeffects which are of special concern in routine fetal examinations. Our theoretical work has led the FDA to require that certain commercial diagnostic ultrasound instrumentation display a Mechanical Index value, so that sonographers can assess the risk-benefits of certain procedures. We are also looking at therapeutic applications of ultrasound, including a new means of gene transvection.

One of our experiments on the surface properties of liquids in the presence of surfactants was conducted on the space shuttle in 1992 (my first Ph.D. advisee was a member of the crew) and in October/Novermber of 1995. In these space experiments, drops of about 25 mm in diameter are levitated acoustically. The sound field causes the drops to be deformed and to oscillate in their quadrupole mode (see the non-linear oscillation in an MPEG movie). The frequency and damping constant of the free decay of that mode can be measured and used with a model we developed to infer surface properties, such as surface tension, Gibbs elasticity, and surface dilatational viscosity. The work at 0-g can also inform our ground- based work, so that future ground based experimentation can yield reliable data. This research has been supported by NASA. We have recently electrified our levitation apparatus, which enables us to produce electrically charged drop clusters and arrays and study evaporation, combustion, and nucleation processes.

Recently, we have worked with what I have called "implosion acoustics." This term describes the process and the result when a sound wave, or other source, causes the substantial growth of a bubble in a liquid. If the liquid is undersaturated with gas, the resulting bubble can implode with great force, concentrating the energy up to 12 orders of magnitude. Light emission (in a process called sonoluminescence) and chemical change (in a process called sonochemistry) have been observed to occur. The physics and chemistry of this process need to be better understood, but several potential applications loom on the horizon, including a controllable broad band sound source, and possibly even a new form of inertial (hot) nuclear fusion. Our laboratory is examining what is needed to produce larger bubbles and what it takes to maintain the spherical symmetry of the collapsing bubble, so that the maximum energy concentration can be achieved.

Another area of interest involves the production of a new class of solid materials by a process (patented in 1995) that we call "dynamic decompression and cooling (DDC)." In this process, a melted material is doped with a significant volume fraction of a volatile liquid in the form of small, uniformly dispersed droplets. In order to keep this volatile material from boiling, the pressure is elevated. The dynamic process occurs when the pressure is suddenly released, causing the droplets to boil and to steal the latent heat of vaporization from the melt, thereby cooling it and rapidly forming a solid, lightweight foam. We are examining the microstructure of this foam material, with recent applications to polymers such as polypropylene and polyethylene.

Selected Publications

"Experimental Validation of the Dissociation Hypothesis for Single Bubble Sonoluminescence," J. Ketterling and R.E. Apfel, Phys. Rev. Lett., 821, 4991-4994 (1998). 

"Free Oscllations and Surfactant Studies of Superdeformed Drops in Microgravity," R.E. Apfel, Y. Tian, J. Jankovsky, T. Shi, X. Chen, R.G. Holt, E. Trinh, A. Croonquist, K.C. Thornton, A. Sacco Jr., C. Colemen, F.W. Leslie, and D.H. Matthiesen,  Phys. Rev. Lett., 78(10), 1912-1915 (1998).

"Investigation of Liquid Surface Rheology of Surfactant Solutions by Droplet Shape Oscillations: Experiments,"  Y. Tian, R.G. Holt, and R.E. Apfel, J. of Colloid and Interface Science, 187, 1-10 (1997).

"A Novel Multiple Drop Levitator for the Study of Drop Arrays," Y. Tian and R.E. Apfel, J. Aerosol Sci., 27(5), 721-737 (1995).

Updated: 12/22/98

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