2.12.3 Critical Opalescence
The lab can support the observation and measurement of critical
opalescence in a binary fluid mixture, methanal-cyclohexane. The equipment includes a
moderately well controlled optical bath (a fish tank containing paraffin oil), laser
source and photodiod es. Precision thermometry is done with a GPIB interfaced DMM (e.
g., a Keithley 193) as a 6-1/2 digit resistance reading of a stable thermistor in the
bath. A similar thermistor provides the temperature sensing for the control circuit.
A careful measurement of the optical transmission of the sample as it
clouds up near the critical point provides the opportunity to follow the divergence of the
critical fluctuations in concentration in this system. The cross over from diverging with
the exponent to a weak logarithmic
divergence can be seen.
Extensions and improvements of the experiment would be to use a
photomultiplier to measure the scattered light intensity itself at some angle like 90°.
One could see it diverging far from Tc and cross-over into a saturation
when the correlation range becomes longer than the scattering Fou rier component
wavelength as Tc is approached. The experiment is a quantitative test of
the understanding of fundamental scaling relations for binary mixtures and the
Ornstein-Zernike theory, which connects them to critical opalescence. The lab at present has a stable 10 ml sample sealed in an all glass cell.
- A. C. Mowery and D. T. Jacobs,
"Undergraduate experiment in critical phenomena: Light scattering in a binary fluid
mixture," Am. J. Phys. 51, 542 (1983). Good description of the basic
- D.T. Jacobs, "Turbidity in the
binary fluid mixture methanol-cyclohexane," Phys. Rev. A 33, 2605 (1986).
- V. G. Puglielli, and N. C. Ford, Jr.,
"Turbidity measurements in SF6 near its critical point," Phys. Rev.
Lett. 25, 143 (1970). Useful information on Ornstein Zernike Theory; and
measurement of .
- R.B. Kopelman, R.W. Gammon, and M.R.
Moldover, " Turbidity very near the critical point of mehthanol-cyclohexane
mixtures," Phys. Rev. A 29, 2048 (1984).
- K.C. Zhang, M.E. Briggs, R.W.
Gammon, "The susceptibility critical exponent for a nonaqueous ionic binary mixture
near a consolute point," J. Chem. Phys. 97, 8692 (1992).
- K. Hamano, N. Kuwahara, and M. Kaneko,
"Scatterng light intensity in the strongly opalescent region of the system
polystyrene-diethyl malonate," Phys. Rev. A 21, 1312 (1980). Independent
measurement of .
- A. L. Sengers, R. Hocken, J. V.
Sengers, "Critical Point Universality and Fluids," Physics Today No. 12, 42
- H. E. Stanley, Introduction to Phase Transitions and Critical
Phenomena (New York: Oxford University Press, 1971). QD501.S78.
- M. E. Fisher, "Correlation
Functions and the Critical Region of Simple Fluids," Jour. Math. Phys. 5 944
- Moore, J. H., Davis, C. C., Coplan, M. A., Building Scientific
Apparatus, 2nd Edition, London: Addison Wesley & Co., 1989. Has a good discussion
of temperature control. Q185.M66.
- YSI Super-Stable Thermistor descriptions
of bridge resistance measurement, temperature probe details, spec sheets for 5K
thermistor, least square fitting results for 45 to 55 °C for T(R).
- YSI Thermistor Info and Hg
thermometer emergent stem corrections.
- United Detector
Planar-Diffused Silicon Photodiodes spec sheets including the photodiode used for
detection in this experiment.
- P. G. Witherell and M. E. Faulhaber,
"The Silicon Solar Cell as a Photometric Detector," Appl. Optics 9, 73
- Properties of Common Liquids.
Useful for calculating heat capacity and time constant of bath.
M. E. Fisher, "Renormalization group theory: Its basis and
formulation in statistical physics," Reviews of Modern Physics, 70,
653 April (1998)
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