The forced convection of a monodisperse, monoclonal suspension of bacteria through a uniform, saturated porous medium has been investigated. Bench-scale column studies were carried out to measure the removal of microorganisms from suspension due to attachment to the surfaces of the solid phase. The columns were packed with 40-μm borosilicate glass beads, and bacterial sorption was measured as a function of depth in the column using a leucine radiolabel assay. The strains A1264 and CD1 were examined separately. Colloid filtration theory was used to interpret the data, and the average, or effective, affinity of the bacteria for the glass beads was found to decrease with distance traveled through the column. It is postulated that, under these circumstances, the cell/collector affinity (that is, the collision efficiency α) varied due to intrapopulational differences in bacterial surface characteristics. A simple bimodal probability density function, consisting of two Dirac delta functions, was found to satisfactorily represent the α distribution in the original bacterial population. This form of the distribution function was supported by capillary electrophoresis measurements on the bacteria, which showed intrapopulational differences in the surface charge density under the conditions of the transport experiments. These variations in surface charge density are significant inasmuch as they give rise to substantial differences in the colloidal interaction potentials and, presumably, large differences in cell affinity for negatively charged collectors such as glass beads or quartz.
The electrophoretic mobilities of three bacterial strains were investigated by capillary electrophoresis (CE) and were compared with results obtained by microelectrophoresis (ME). The CE measurements yielded bimodal electropherograms for two of the strains, thus illustrating for the first time that surface charge variations within a monoclonal population can be probed by CE. Intrapopulation variations were not detected by ME. The mobilities of three chemically distinct types of latex microspheres were also measured. Differences between the mean mobilities obtained by CE and ME were not statistically significant (P ≤ 0.50); the standard deviations of the CE measurements were typically 2 to 10 times smaller than those obtained by comparable ME measurements. The reproducibility of CE permitted batch-to-batch mobility variations to be probed for the bacteria (one of the strains exhibited such variations), and aggregation was evident in one of the latex suspensions. These effects were not measurable with ME.
Small-scale electrohydrodynamic flows were investigated in a configuration where the motive force derives from charge generated in an electrolyte composition gradient. Using a thin Hele-Shaw cell filled with aqueous electrolyte, we studied flow through a disc-shaped bolus whose conductivity differed from that of the surrounding solution. When an ac electric field was applied, a thin layer of free charge arose from differences in conductivity between the bolus and its surroundings. The action of the field on the charge produced bulk flow, and since material interfaces were absent, the bolus deformed continuously. Deformations were prolate or oblate, depending on the conductivity of the bolus relative to the surrounding electrolyte. Modeling the flow with the Hele-Shaw formalism showed how electrical forces produce motion. To indicate the sense of the deformationsprolate or oblate with respect to the applied fieldsa discriminating function was derived. The discriminating function reduces to the Taylor-Melcher leaky dielectric model under the appropriate limiting conditions. Experiment and theory are in good agreement. The flows described here occur in the absence of rigid interfaces and equilibrium charge layers. Accordingly, they offer a means of controlling microfluidic flow and mixing.
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