Colloidal suspensions consist of nanometer- to micron-size particles dispersed in liquid or gas. Examples include smoke, milk and ink as well as living cells in biological fluids. In everyday environments, colloidal suspensions typically are not found isolated in protective containers but are diffused in porous media where their motion depends upon particle size, pH and orientation with respect to gravity.To understand the perfusion of wastewater in soil, for example, or bone repair in synthetic materials, it is necessary to evaluate the effect of these factors on the flow of colloidal suspensions. Two independent research teams recently reported the development of fluorescence measurement systems for the straightforward quantification of flow through porous media.Jonathan W. Bridge is a PhD candidate in the groundwater protection and restoration group at the University of Sheffield in the UK. He and his colleagues developed an apparatus to directly image colloid transport through a chamber filled with porous sand. Although high-resolution methods have been used to evaluate flow on a microscopic scale, the mesoscale — with resolution on the order of millimeters to decimeters — has been measured only in unrealistic one-dimensional column configurations.“Both of these [existing methods] are valid and produce extremely good data,” Bridge said, “but are not so useful for examining the specific effects of heterogeneity in porous media.”The apparatus is centered on a Perspex (acrylic) flow chamber with internal dimensions of 200 × 100 × 6.7 mm. One face of the flow chamber consists of a quartz view plate. Sylvania black-light tubes provide ultraviolet illumination through the quartz plate into the medium in the flow chamber, and a Hitachi 8-bit color CCD camera images the excited fluorescence. The entire apparatus is placed in a darkroom to maximize the signal-to-noise ratio (Figure 1). Figure 1. UV illumination of a sand-filled flow cell excites fluorophores within colloidal suspensions. Imaging with a CCD camera captures transport and deposition processes with a sensitivity and resolution not previously available. Images courtesy of Jonathan W. Bridge.The porous medium was pure quartz sand with a mean diameter of about 250 μm, the solute tracer was composed of disodium fluorescein-labeled water, and the colloids were 1.9-μm-diameter carboxylate-modified latex microspheres labeled with a UV-red fluorescent dye from Duke Scientific Corp. of Fremont, Calif. The colloidal suspension flowed through the chamber at an adjustable rate.Interchangeable 10-nm bandpass filters allowed separate detection of the solute and the colloids. The solute fluorescence was detected through a 530-nm filter, while the colloid excitation filter was centered at 612 nm. The apparatus provided a relatively inexpensive, robust, simple and rapid means of measuring colloid transport and deposition, Bridge said.In principle, detected fluorescence at a given pixel is directly proportional to the number of fluorophores within the detection volume. However, in the experiment, nonlinearities in signal detection, effects of the solution’s pH and scattering effects modified the linear relationship. Initial calibration provided correction coefficients, and flat-fielding corrected imaging irregularities.The calibrated system was then configured to acquire sets of three red and three green fluorescence images over separate 20-second intervals, effectively measuring the reactive colloid and conservative solute tracers simultaneously at 5- or 10-minute intervals without disturbing the flow system (Figure 2).Figure 2. With a broadband UV excitation source, multiple fluorophores within the flow cell can be imaged. By labeling colloids and solutes with different dyes, detailed information can be obtained about colloid deposition.Bridge said that the imaging method showed for the first time this discrete measurement of multiple fluorophores passing together through the sand, offering the possibility of studying the complex interactions between co-infiltrating species and spatially complex porous media.The initial experiments quantified colloid transport and deposition as functions of pH, flow rate and ionic strength. The method makes it possible to measure the interactions between colloids and different co-infiltrating species that are also fluorescent-tagged.“For example,” Bridge explained, “in agricultural environments, pathogenic bacteria such as E. coli are washed into soils along with livestock waste (organic matter). How does E. coli transport behavior in the presence of this waste differ from that in its absence?”Although pleased with the capabilities of the current system, Bridge noted that the apparatus is still under development. One change he would like to see is a self-contained lighttight box housing for the system to avoid the need for a darkroom. Similar innovations have been incorporated into a design developed at City College of New York with assistance from Eastman Kodak scientists.Environmental Science & Technology, Oct. 1, 2006, pp. 5930-5936.