The use of ultrasonic standing waves to enhance optical particle sizing equipment

Fluid dynamics modelling augmented with routines to simulate acoustic forces on aerosol particles has been used to investigate the potential of combining ultrasonic standing wave fields with optical particle analysis equipment. Simulations of particle dynamics in airstreams incorporating acoustic forces predict that particles in the 1-10 microns diameter range may be effectively focused to the velocity nodes of the standing wave field. Particles move to the velocity nodes within tens of milliseconds for acoustic frequencies of 10-100 kHz and at an acoustic energy density of 100 Jm-3. Larger particles are predicted to move to the velocity antinodes within similar times; however, there is a crossover region at approximately 15-20 microns particle diameter where longer times are predicted due to the competing forces driving particles to the vibration node and antinode. With sufficient transverse flow velocities the models predict that disturbances due to acoustic streaming can be overcome and a useful degree of focusing achieved for the aerosol particles. Results from a model demonstrating sampling and acoustic focusing of 3-9 microns aerosol particles to a 200 microns wide analysis area are presented.     

Particle Manipulation by Ultrasonic Standing Wave Fields to complement dynamic light scattering experiments

Dynamic light scattering is a widely used technique for the sizing of colloidal suspensions. It is capable of measuring particles across the size range from approximately 1 nm to several microns. However the larger particle sizes tend to pose problems for the interpretation of the scattered light signal either by virtue of their light scattering efficiency relative to the smaller species present or the departure of the scattered light signal from Gaussian statistics. Rapid removal of such particles in-situ could extend the use of dynamic light scattering particularly in on-line analysis or laboratory automated measurement. In this paper a method is demonstrated for the in-situ removal of larger particles in suspension by means of ultrasonic standing waves and concurrent dynamic light scattering measurement. The theory behind ultrasonic particle manipulation and its effect on the motion of the particles is discussed. Data from a scattering cell designed to incorporate the ultrasonic technology is presented showing that dynamic light scattering measurements may be carried out under such conditions. Varying the energy density of the ultrasonic field allows particles greater than a defined cut-off diameter to be removed from the measurement region. Theory shows that the minimum cut-off size may be as small as 100 nm. Results presented here demonstrate complete removal at a lower diameter threshold of approximately 2000 nm.     

Dye-sensitized charge injection into amorphous SiO2 films

Results and analyses are reported on the dye-sensitized steady-state photo-conduction through metal-dye-SiO₂-metal (MDIM) structures, and on the photo-induced discharge of MDIM capacitors. In all the experiments the dye used was hydrogen phthalocyanine. The induced photo-current is substantial when the dye contact is negatively biased but is is negligible under positive bias, i.e. it appears that electron currents are fairly readily induced but not hole currents. Measurements as a function of temperature have included thermally-stimulated dielectric relaxation experiments and the results provide evidence for separate regimes of bulk- and electrode-dominated processes. The voltage dependence of the dark current and of the photo-current obeys an equation of the Poole-Frenkel type. The rate of discharge of MDIM capacitors increase markedly on illumination but, unlike the photo-current, there is no significant dependence on polarity. The photo-induced discharge rate varies with wavelength and its spectral dependence matches the photo-conduction spectrum of the dye, as does the dye-sensitized photo-current.