Zeta potential is the key parameter that controls electrostatic interactions in particle dispersions. Laser Doppler electrophoresis is an accepted method for the measurement of particle electrophoretic mobility and hence zeta potential of dispersions of colloidal size materials. Traditionally, samples measured by this technique have to be optically transparent. Therefore, depending upon the size and optical properties of the particles, many samples will be too concentrated and will require dilution. The ability to measure samples at or close to their neat concentration would be desirable as it would minimize any changes in the zeta potential of the sample owing to dilution. However, the ability to measure turbid samples using light-scattering techniques presents a number of challenges. This paper discusses electrophoretic mobility measurements made on turbid samples at high concentration using a novel cell with reduced path length. Results are presented on two different sample types, titanium dioxide and a polyurethane dispersion, as a function of sample concentration. For both of the sample types studied, the electrophoretic mobility results show a gradual decrease as the sample concentration increases and the possible reasons for these observations are discussed. Further, a comparison of the data against theoretical models is presented and discussed. Conclusions and recommendations are made from the zeta potential values obtained at high concentrations.
Thermocouples were used in the past, with a few exceptions, for cigarette coal temperature measurements. The main shortcomings of this technique are: the cigarette is disturbed by insertion of the thermocouple, the position of the bead does not necessarily coincide with the location of the puff, heat transfer between tobacco shred and bead is poor, heat capacity and conductivity are orders of magnitude different for tobacco and metals. Methods employing x-ray observations of fusions of metal inclusions or cinematographic optical pyrometry also possess serious shortcomings. These shortcomings are eliminated by a non-contact technique based on radiometry. This method is applicable only to the measurement of surface temperature of the cigarette coal. By measuring the intensity of the emitted radiation, the temperature can be calculated or can be obtained by calibration with an artificial blackbody. Since the coal temperature is not stable even during the 2-second duration of a puff, the instantaneous response of an infra-red detector has to be coupled with a very fast recording system. An AGA Scanning Infra-red Camera was selected for the coal temperature measurements. The electronic system of the camera generates a waveform proportionaI to the 2-5.4 AAµ band of the infra-red radiation emitted by the coal. The peak value of the video signal, representing the hottest point of the coaI, is stored. At a command, given automatically by the smoking machine, the signal is digitized and printed out. Although the peak signal decreases with very small targets, evaluation of the system indicates acceptable accuracy for target sizes down to 3 mm in width. The cigarette coal fulfills this condition.
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