(179c) Trapping Pathogens with Secondary Stagnation Flows

A new micro-fluidic concentration technique is shown to utilize convection to concentrate particles from dilute suspensions from a large sample volume (~0.2 ml), making it useful for chip scale detection kits. Rapid (~15 minute) and large concentration is achieved to dramatically increase the sensitivity and speed of biosensors. A high AC potential is applied to the needle electrode of a needle-plate electrode configuration to generate an intense air flow to drive the desired flow field within the liquid sample. The sharp geometry of the needle tip leads to field divergence causing corona discharge effects. Under such conditions an ionic wind is generated at the needle electrode, which can act as a point source of momentum on the liquid and generate a planar interfacial vortex flow on the interface, the magnitude of the induced velocity depending upon the electric field intensity. The topological characteristics of the interfacial flow field and the bulk liquid flow field depend upon the exact location of the needle with reference to the interface as well as to the inclination of the needle. In the case when the needle is placed centrally above the liquid surface in a vertical position, a symmetrical group of vortices, similar to that produced by a point source of momentum, is generated. For concentrating particles at a single location, a single vortex is preferred, so the needle is placed off center at an angle of 45 degrees, such that instead of generating symmetric vortex pairs, a single dominating primary vortex is produced, as a combination of the effect of an off center momentum source and bounding side walls. The bulk flow field generated under such conditions is very similar to the flow generated by contacting a rotating disk with a liquid surface and at low Reynolds numbers the liquid velocity is only in the azimuthal direction. Hence the particles suspended in the liquid revolve at the same radial position, with no concentration effects. However, at the finite Reynolds numbers of our micro-flow, centrifugal forces generate secondary inertial currents that flow radially outwards near the free liquid surface at the top, where viscous dissipation is lower, and by flow balance, radially inwards near the substrate surface on which the liquid volume is placed. A secondary converging stagnation flow is hence added to the primary vortex, with the stagnation point at the center of the primary interfacial vortex. The suspended particles follow the fluid streamlines and hence can reach the center of the vortex close to the substrate in a spiral trajectory. Near the stagnation point at the vortex center, the convective forces on the particles are reduced due to the low velocity very close to the stationary substrate. A downward acting body force, such as gravitational force or DEP, can then draw and accumulate the particles at that location and prevent them from being re-suspended into the liquid sample. Thus, this converging flow field coupled with an external force creates a stable spiral trap for particle focusing at the converging stagnation point of the flow field. There exists a balance between the resuspending and trapping forces which depends on the particle size, liquid velocity, and the depth and shape of the substrate. Depending upon the sample properties, concentration takes place within a voltage window applied on the needle electrode, where the converging stagnation flow at the substrate can concentrate the particles without resuspending them. The trapping time and the lateral size of the mound of trapped particles at the stagnation point are likewise highly voltage dependent for a given reservoir geometry and sample. We quantify these observations with a scaling theory on the particle dynamics and estimate the criteria favorable for particle trapping within a small area on the substrate. More detailed trajectory calculation is also carried out by using an eigen-function expansion of the outer flow field away from the stagnation point and an inner expansion of the converging stagnation flow. Coupled with spectroscopic detection methods, this concentration technique has been shown to be useful for rapid and sensitive pathogen detection in very dilute samples.