Previous Studies

5.2.1

Laser Power Input for DWS

Laser beam is generated by photons which are simulated and emitted from a laser device. The stimulation emission is a procedure in which the energy is extracted from a transition in an atom or molecule. Considering a visible laser, the violet radiation has the highest energy; and the red rays have the lowest power. The laser applied in our experiment has a wavelength of 488nm and is visible laser light with blue and green colour and a moderate energy. In the experiment, the light source illuminated the scattering medium, which was regarded as one of the most important components in DWS system.

The laser power in a conventional DWS experiment is not limited to a narrow range. Different laser powers have been employed in the studies. Some researchers have developed methods for laser power selection based on the research purpose. As noted by Chu[116] and

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Yang et al.[102], the laser for particle sizing using light scattering method must have sufficient power, being stable under all expected conditions; otherwise, it could cause distortion in the measurement. Therefore, an appropriate laser power has to be carefully considered and estimated. It cannot be too high or too low. It must be adequate enough to meet the illuminating requirements and avoid unexpected results. Depending on the objectives and experimental set-up, a variety of laser power has been applied in DWS in previous studies.

In using DWS to study complex dry mechanism of film formation, the instrument of Horus, emitting 0.9mW laser radiation, was employed as the light source[117]; similar to our experiment, a COMS video camera was used for scattered light detection in that study. In investigation of inter-particle interactions in sodium caseinate-stabilized emulsions during acidification, 100mW laser power was used by Liu et al.[118]; in their experiment, a photomultiplier was used as the light detector. In a typical DWS setup, which could be run in backscattering and transmission geometry, the time autocorrelation of multiply scattered light was measured by Scheffold[40], and the high laser power of 2W with incident wavelength of 488nm was used. In DWS with a point source and backscattering geometry done by Rega et al., the laser light was powered to 100mW with 530nm wavelength used as the light source[119]; in their experiment, a photomultiplier was used as the detector. The particles in a randomly inhomogeneous turbid medium were studied by Skipetrov et al using DWS[41], in which a laser beam generated at wavelength of 514nm and powered to 1W was introduced into the medium through a narrow fiber-optic light guide. In another work, Navabpour et al. investigated the influence of medium concentration on particle sizing using DWS[44]; in their study, the experiment was designed to exclude the scattered light polarised in the same plane as the incident light, and geometrically excluded the paths shorter thanl*. For this, a laser beam with 532nm wavelength and 100mW of power input was used.

From the previous studies, it can be concluded that the laser power is dependent on the type of light detector and the research objectives.

5.2.2

Laser Power and Particle Sizing

In particle sizing experiment, it has been reported that there is a relation between measured particle size and the incident laser power. In the study of Kondrat’ev et al[120], they used a

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laser power with a variable interval 0.07-2 W to investigate particles with the radius of 90nm;they observed the variation in the autocorrelation function with increasing the incident laser power and pointed that the variation was resulted from the laser-induced motion of scattering particles. In the research of determining nano-particles size in a suspension with low concentration, Kuyper et al.[43] focused a laser light to an avalanche photodiode that has high sensitivity to light source; particles with different diameters were studied for a number of tests. Their analysis showed that in the power changed from 300 to 900μW, there was a linear increase in the measured diffusion time. They then concluded that the measured particle size using DWS appeared to be power-dependent; for particles with a moderate size, low excitation power was sufficient to provide an accurate size measurement, but low power laser illumination could be a problem for smaller and less bright particles. Harada et al.[115] found the similar results; when using an Argon laser beam to illuminate a sample cell, they found that, with the laser power adjusted from 0 to 60mW and under improper laser power, the resultant particle size was larger than it should be, and the increase in particle size or laser radiation resulted in larger difference between true and measured sizes.

The previous studies have proved that laser light power is critical for particle sizing, especially for small particles, because small particles scatter less light than large particles. Under some conditions, the measured particle size could not be reliable. Yang et al. [102] have argued that the laser power effect came from the refractive index change of the medium; because the laser power caused local heating and resulted in thermal blooming. The optical index change induced errors in particle sizing. In order to accurately measure the size of micro- and nano- particles, the impact of laser power on the measurement were researched.