Molecular rheotaxis directs DNA migration and concentration against a pressure-driven flow

Abstract

In-line preconcentration techniques are used to improve the sensitivity of microfluidic DNA analysis platforms. The most common methods are electrokinetic and require an externally applied electric field. Here we describe a microfluidic DNA preconcentration technique that does not require an external field. Instead, pressure-driven flow from a fluid-filled microcapillary into a lower ionic strength DNA sample reservoir induces spontaneous DNA migration against the direction of flow. This migratory phenomenon that we call Molecular Rheotaxis initiates in seconds and results in a concentrated DNA bolus at the capillary orifice. We demonstrate the ease with which this concentration method can be integrated into a microfluidic total analysis system composed of in-line DNA preconcentration, size separation, and single-molecule detection. Paired experimental and numerical simulation results are used to delineate the parameters required to induce Molecular Rheotaxis, elucidate the underlying mechanism, and optimize conditions to achieve DNA concentration factors exceeding 10,000 fold.

Particle trapping in merging flow junctions by fluid-solute-colloid-boundary interactions

“…Driven by the interactions between the wall diffusioosmosis, particle diffusiophoresis, and the inlet fluid flow, the vortices enable robust trapping of colloidal particles near the junction. We emphasize that the reported particle trapping is a completely different mechanism from the trapping enabled by the inertial vortex breakdown [6][7][8][9][10][11] or the solute-mediated trapping enabled by the diffusiophoresis [14,15], as the current trapping is enabled by the combination of both the solute-mediated and the inertial effects. Since the particle trapping is driven by the presence of solute gradients, the trapping can happen in other flow geometries as well.…”

“…Three-dimensional (3D) particle visualization and numerical simulations are utilized to visualize the vortex so as to provide a rigorous understanding of the particle trapping phenomenon. The reported trapping mechanism is unique from the inertial trapping enabled by vortex breakdown [6][7][8][9][10][11], or the solute-mediated trapping enabled by diffusiophoresis [14,15], as the current trapping is facilitated by both the solute and the inertial effects, suggesting a new mechanism for particle trapping in flow networks.…”

“…The coupling of particle dynamics to other additional physics such as externally imposed temperature or chemical concentration gradients can also cause the particles to deviate from the fluid streamlines and results in interesting particle dynamics [12,13]. Recent examples have shown that for merging flows in channel junctions [14,15] and flow networks [14], suspended particles can also be trapped near the junction when the merging streams contain different solutes. Solute gradients formed at the junction allow the particles to migrate against the flow via diffusiophoresis, leading to trapping.…”

“…Solute gradients formed at the junction allow the particles to migrate against the flow via diffusiophoresis, leading to trapping. While this solute-mediated particle trapping contributes to the rapid clogging of porous media [14], it is also proven to be useful for on-chip applications such as DNA preconcentration [14,15], bioseparation [16], and surface characterization [17].…”

Transverse migration and microfluidic concentration of DNA using Newtonian buffers

“…7 Many methods of processing DNA on microfluidic platforms have utilized immobilization or capture. [8][9][10] General methods for concentrating analytes that utilize gradients in the buffers or fields 11,12 have been used to preconcentrate DNA, including temperature gradient focusing, 13 rheotaxis, 14 and isotachophoresis. 15 Electrokinetic trapping of DNA using membranes [16][17][18][19] and dielectrophoretic trapping of DNA in a microfluidic channel equipped with an array of electrodes have also been demonstrated.…”