Photophoretic Velocimetry for Colloid Characterization and Separation in a Cross-Flow Setup

Abstract

We introduce photophoretic velocimetry as a new technique for characterization of particulate matter on the basis of optical particle properties. Complementary to well-established techniques, we could show that, by measuring the photophoretic velocity of the single particles, it is possible to distinguish particles of different sizes as well as particles of one size but different refractive indices. The difference in photophoretic migration of particles can be applied to the separation of particles. Polystyrene, melamine, and SiO2 microparticles (0.3-10 mum) suspended in purified water were used as test samples for validation of a cross-flow setup. The particles were pushed perpendicular to a uniform, pulsation-free fluid flow by a focused He-Ne laser (lambda = 633 nm, P = 47 mW, I(max) = 14.0 kW cm(-2)) providing a well-defined Gaussian-shaped flux distribution. The migration behavior was observed by means of a video camera system, and the velocities and displacements were calculated by using an adapted particle imaging velocimetry code as an approach to automatic characterization. The photophoretic displacement depends on both flow conditions and particle properties and can be applied for separation means.

Assessment of cross-type optical particle separation system

“…The retention distance for the Poiseuille profile v = v (x, z) was a factor of two smaller than the distance obtained under the assumption v = U o (constant). The actual local particle flow velocity was higher than the mean flow velocity, which reduced the residence time of the particle in the region of influence of the laser beam (Helmbrecht et al 2007;Lee et al 2011). Figure 6c shows two trajectories for particles injected at different x-positions and the same z-positions.…”

“…(Dholakia et al 2008;Hart and Terray 2003;Ladavac et al 2004;MacDonald et al 2003;Wang et al 2005;Yang et al 2009;Hoi et al 2009). Recently, a crosstype optical particle separator (COPS), which uses the scattering force to make a particle move across flow streamlines, was developed and characterized (Helmbrecht et al 2007;Kim et al , 2008bHoi et al 2010). When a particle flowing along the flow direction passes through a laser beam, the particle is displaced laterally due to the scattering force exerted perpendicular to the flow direction.…”

“…When a particle flowing along the flow direction passes through a laser beam, the particle is displaced laterally due to the scattering force exerted perpendicular to the flow direction. This lateral displacement is called the displacement distance or retention distance (Helmbrecht et al 2007;. The COPS has advantages, especially for cell separation, over other optical particle manipulation methods, e.g., optical tweezers, because it uses a loosely focused laser beam, minimizing the energy density, which can result in radiation-induced cell damage (Lee et al 2011).…”

“…The COPS configuration has wide applications in microfluidic platforms such as cell handling, particle ligand change, etc., as well as particle separation (Hsu et al 2010;Hakem et al 2010;Maruyama et al 2011). The particle behavior during COPS was theoretically estimated and experimentally evaluated in terms of two factors: size and refractive index of the particle (Helmbrecht et al 2007;Kim et al , 2008b. However, the effect of the flow velocity profile in a microfluidic channel was not considered in previous studies.…”

Nanoparticles in the Water Cycle

“…Other minimum invasive methods of determining particle size are static light scattering (Schurtenberger and Newman, 1993) and single particle counting (Knollenberg and Veal, 1992;Walther et al, 2006). A new very gentle method for the characterization and separation of particles of a size from several hundred nanometers is photophoretic velocimetry in a flowing solvent (Helmbrecht et al, 2007). It allows properties such as the size or the refractive index of the single particles to be analyzed.…”