.comment-link {margin-left:.6em;}

2Physics Quote:
"Many of the molecules found by ROSINA DFMS in the coma of comet 67P are compatible with the idea that comets delivered key molecules for prebiotic chemistry throughout the solar system and in particular to the early Earth increasing drastically the concentration of life-related chemicals by impact on a closed water body. The fact that glycine was most probably formed on dust grains in the presolar stage also makes these molecules somehow universal, which means that what happened in the solar system could probably happen elsewhere in the Universe."
-- Kathrin Altwegg and the ROSINA Team

(Read Full Article: "Glycine, an Amino Acid and Other Prebiotic Molecules in Comet 67P/Churyumov-Gerasimenko"

Thursday, October 26, 2006

Brownian Motion of Ellipsoidal Particles

brownian motionBrownian motion, the tiny random movements of small objects suspended in a fluid, has served as a paradigm for concepts of randomness ranging from noise in light detectors to fluctuations in the stock market. One hundred years ago, Einstein first described rotational Brownian motion for spheres in water. In his 1906 paper, Einstein predicted that the rotation of spherical particles does not affect their translation.

On average, particles undergoing Brownian motion do not move very far. For example, in one second, the largest number of particles will stay very close, say within one micron, of their starting point; a smaller number will move between one micron and two microns; a still smaller number will move between two microns and three microns, and so on. A plot of the number of particles traveling specific distances yields the famous bell-shaped or Gaussian curve from statistics.

PerrinJean Perrin

On the other hand, the rotation of non-spherical particles may affect their translation. Since most Brownian particles are not spherical, they may experience cross-talk between translation and rotation. This idea of such a coupling was first published by French physicist Francis Perrin in the 1930s but were apparently "forgotten" by the science community. Perrin's father, Jean Perrin won the Nobel Prize in 1926 for the first experimental observations confirming Einstein's theories about Brownian motion.

In a recent research paper published in Science, a research team led by Arjun G. Yodh of University of Pennsylvania confirmed the theory's curious description of how an ellipsoid's random motions are different from those of spherical particles. They reported definitive measurement of the Brownian motion of an isolated ellipsoidal particle. UPenn researchers employed state-of-art digital imaging technology and computer image analysis for their experiments. Using a charge-couple device (CCD) camera, they recorded the orientations and positions of a single, micrometer-sized plastic ellipsoid particle suspended in water at a sequence of times. Their study of the motion produced a curve that is not Gaussian -- thus directly confirming ideas about rotational-translational coupling.

Reference: "Brownian Motion of an Ellipsoid" by Y. Han, A. M. Alsayed, M. Nobili, J. Zhang, T. C. Lubensky, and A. G. Yodh, Science 27 October 2006: pp626-630. Abstract.

Labels: ,


Post a Comment

Links to this post:

Create a Link