Microchip injection and separation anomalies due to pressure effects.

Area of Science:

• Microfluidics
• Analytical Chemistry
• Separation Science

Background:

• Routine electroosmotically driven capillary electrophoresis (CE) separations on microfluidic chips can exhibit anomalies in peak shape, migration time, and baseline drift.
• These anomalies are often attributed to pressure-driven backflow, opposing electroosmotic flow (EOF), with meniscus surface tension (Laplace pressure) identified as a potential cause.

Purpose of the Study:

• To isolate and characterize pressure effects from electroosmotic flow (EOF) and siphoning in microfluidic devices.
• To investigate the contribution of meniscus surface tension to bulk flow anomalies in microchip capillary electrophoresis (CE).

Main Methods:

• Pressure flow was measured in the absence of an electric field to isolate pressure effects from EOF.
• Experiments were conducted with the microfluidic chip tilted to differentiate meniscus effects from siphoning.
• Flow rates were compared between pressure-driven backflow, electroosmotic flow (EOF), and predicted siphoning effects.

Main Results:

• Observed pressure flow ranged from 0.4 to 0.8 mm/s, comparable to the experimentally generated EOF of 0.6 mm/s at 150 V/cm.
• Pressure flow was 10-20 times greater than predicted siphoning flow based solely on liquid level differences.
• Siphoning effects were found to have a negligible impact on meniscus flow under the studied microchip conditions.

Conclusions:

• Meniscus surface tension (Laplace pressure) is a significant driver of pressure-driven backflow in microfluidic CE separations.
• This pressure-driven backflow is a primary cause of observed anomalies, rather than siphoning effects.
• Findings are critical for microfluidic and array-based technologies utilizing microliter volumes and similar reservoir geometries.