A technique for measuring velocity gradients in laminar flows by homodyne light scattering is presented. A theory which describes the light-scattering spectrum is derived that includes the effects of different types oflinear flow fields, particle diffusion and the intensity profile in the scattering volume. The conditions which must be satisfied in order that the theory describe the experimental situation are outlined and complementary experiments are performed which both verify the theory and apply the technique. Verification is provided using the flow in a Couette device, and the flow due to single rotating cylinder in a large bath of fluid. The technique is then applied to measure the spatial variation of the shear rate in a four-roll mill.

Measurements have been made at the critical mixing composition of the system 2,6-lutidine-water for a (Tc-T) range of 0.001°–7.5°C for the intensity and Rayleigh linewidth and of 0.007°–27.4°C for the shear viscosity. We find that Ic−1(0) ∝ (ε)(1.26± 0.02),ξs=(2.0± 0.2)(ε)−(0.61± 0.08) Å,D=(0.290± 0.020)(ε)(0.554± 0.015)× 10−5cm2/sec,ξΓ=(2.92± 0.19)(ε)−(0.567± 0.015) Å,where ε =(Tc-T)/Tc,  Ic(0) is the intensity extrapolated to zero angle, ξΓ the correlation length from intensity measurements, D the mutual diffusion coefficient, and ξΓ the correlation length obtained from fitting the Kawasaki equation to linewidth measurements with the above value of D. We find that the Ornstein-Zernike-Debye theory is valid for (Tc-T) >0.03°C and the Kawasaki mode-mode coupling theory gives a good over-all description of the behavior of the linewidth of the Rayleigh line. The Kadonoff-Swift-Kawasaki result γ - psi = ν seems to be valid with ν=νs=νΓ. We also find that the excess shear viscosity does not exhibit a simple power law dependence on (Tc-T) as the critical temperature is approached.

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