Frequency Dependence of Ultrasonically Induced Birefringence of Rodlike Particles

Abstract

Ultrasonically induced birefringence of two rodlike particles of the hematite sols and the poly(tetrafluoroethylene) latex was investigated as a function of volume fraction, ultrasonic intensity, and frequency. The negative birefringence was observed for both samples, and the birefringence was proportional to the volume fraction and the ultrasonic intensity. The frequency dependence of the birefringence was observed in the wide ultrasonic frequency range from 5 to 225 MHz and did not agree with the Oka theory. The modified Oka theory for disclike particles was extended in view of application for rodlike particles. The birefringence measured was several tens of times larger than that calculated from the modified Oka theory for rodlike particles. The orientational relaxation time of the two particles estimated from the trace of the transient birefringence can be expressed in the form of the Debye-Einstein equation. IntroductionThe double refraction is induced in liquids and solutions containing a certain amount of nonspherical particles as a result of the particle orientation due to ultrasonic waves. This was called the phenomenon of the ultrasonically induced double refraction. 1 The early theoretical studies of the birefringence have been reviewed by Hilyard and Jerrard. 2 The theory developed by Oka 3 indicated that large disclike rigid particles align by hydrodynamic torque which is produced by the radiation pressure due to the passage of the ultrasonic wave. The normal of disclike particles is parallel to the ultrasonic field, and the sign of the birefringence of disclike particles is negative.The measurements of the birefringence of large rigid disclike particles were carried out on bentonite 4 and gold sols. 5,6 These experimental results show that the birefringence is proportional to the ultrasonic intensity as predicted by the Oka theory. However, the frequency dependence of the birefringence has been scarcely investigated because the measurements of the ultrasonic intensity at different frequencies are difficult. Recently, Ou-Yang et al. 7 made use of the Raman-Nath diffraction effect to measure the ultrasonic intensity at the frequency range from 1 to 19 MHz and investigated the ultrasonically induced birefringence of gold sols as a function of solvent viscosity, particle size, and ultrasonic intensity and frequency. The birefringence increased with increasing ultrasonic frequency and decreasing viscosity, and the sign of the birefringence was positive. They modified the Oka theory to explain the sign of the birefringence and the dependence of the birefringence on the frequency and viscosity.For large rigid rodlike particles, no theoretical study of the ultrasonically induced birefringence has been reported. Experimental investigations on the birefringence of V 2 O 5 sols 1, 8,9 have been carried out. Measurements 8 by Petralia on V 2 O 5 sols indicated that the birefringence depended on the viscosity and the sign of the birefringence was positive. In our previous paper, 9 the ultrasonically induced bire...

On the frequency dependence of ultrasonically induced birefringence in isotropic phase of liquid crystal: 5CB (p-n-pentyl p′-cyanobiphenyl)

“…3,4 In their experiment, however, only the relative values of the ultrasonic intensity were measured and they examined Eq. ͑30͒ in terms of the temperature dependence of the birefringence at three different frequencies.…”

“…[3][4][5] In this study, the /4 plate was not used to detect the birefringence signal in a way of square law detection. In this case, the root mean square of the birefringence ⌬n rms is obtained from the following relation:…”

“…[1][2][3][4][5][6][7][8] For large particles of anisotropic shape such as rodlike and planar colloidal particles, ultrasonic radiation pressure produces a stationary torque on the particles and induces reorientational order as a whole, because the orientational relaxation frequency of particles is much smaller than that of ultrasound frequency. In this case, the induced birefringence is proportional to the ultrasonic intensity.…”

“…[1][2][3][4][5] The dynamical properties of large particles can be investigated by following the decay of the birefringence after rapid cessation of ultrasonic irradiation. [1][2][3][4][5] For neat liquids, however, a sinusoidal velocity gradient can directly produce sinusoidal orientational order. The reorientational order produced is not uniform but propagates as waves and the induced birefringence should be proportional to the ultrasonic amplitude, that is, the square root of the ultrasonic intensity.…”

Dynamic Viscoelastic and Multiple Scattering Effects in Fiber Suspensions

“…Brodeur (1991) presented a theoretical investigation on the motion of fluid-suspended fibers under the influence of a stationary ultrasonic wave field. Yasuda et al (1996)] investigated ultrasonically induced birefringence of two rod-like particles as a function of volume fraction, ultrasonic intensity, and frequency. Lofqvist (1999) examined how ultrasonic attenuation and phase velocities can be used to characterize paper fiber suspensions.…”