Turn-induced isotachophoretic focusing in microfluidic channels

Area of Science:

• Analytical Chemistry
• Microfluidics
• Separation Science

Background:

• Multifunctional microfluidic platforms often employ isotachophoresis (ITP) for efficient sample stacking.
• ITP in conventional on-chip capillary electrophoresis (CE) typically shows different band behavior compared to serpentine channels.
• Serpentine channels are integral to ITP-based sample stacking in microfluidic devices.

Purpose of the Study:

• To investigate the distinct sample band behavior during ITP in serpentine microfluidic channels.
• To elucidate the underlying physical mechanisms responsible for sample focusing on the outer wall of channel turns.
• To identify key parameters controlling the outward focusing effect for optimized microfluidic ITP applications.

Main Methods:

• Development of a simplified analytical model for ITP in serpentine channels at the infinite Peclet number limit.
• Comparison of model predictions with full numerical simulations.
• Validation of theoretical and simulation results using experimental measurements.

Main Results:

• Observed reduction in band skew and outward focusing of sample bands on the outer wall of serpentine turns.
• Identified a wall-normal electric field, arising from the skewed leading buffer-trailing buffer (LB-TB) interface, as the driving force for outward focusing.
• Demonstrated that outward focusing is controllable by the ratio of LB to TB conductivities, channel width to turn radius ratio, and Peclet number.

Conclusions:

• The outward focusing phenomenon in ITP within serpentine microfluidic channels is a predictable and controllable effect.
• Optimizing buffer conductivity ratios and channel geometry can enhance sample focusing for improved ITP performance.
• Maximizing band skew, dependent on non-dimensional parameters, further amplifies the outward focusing effect.