Skipping the Boundary Layer: High-Speed Droplet-Based Immunoassay Using Rayleigh Acoustic Streaming

Area of Science:

• Biotechnology
• Biosensor technology
• Acoustic fluid dynamics

Background:

• Acoustic mixing of droplets offers high-speed, low-reagent biosensing.
• Current methods use high-frequency acoustic waves, limited by slow analyte advection due to hydrodynamic boundary layers.

Purpose of the Study:

• To overcome the speed limitations of acoustic droplet mixing biosensors.
• To enhance analyte advection to the sensor surface by eliminating the hydrodynamic boundary layer.

Main Methods:

• Utilized low-frequency ultrasonic excitation to generate Rayleigh acoustic streaming in droplets.
• Employed experimental validation and three-dimensional simulations to compare Rayleigh streaming with Eckart streaming.
• Applied the optimized method to a SARS-CoV-2 antibody immunoassay.

Main Results:

• Rayleigh streaming effectively eliminates the hydrodynamic boundary layer.
• Achieved a three-fold speedup in mixing compared to Eckart streaming at equivalent flow velocities.
• Reduced a SARS-CoV-2 antibody immunoassay time from 20 minutes to 40 seconds.

Conclusions:

• Low-frequency acoustic excitation driving Rayleigh streaming is a superior method for acoustic droplet mixing biosensors.
• This approach significantly enhances sensor speed and efficiency.
• Enables rapid point-of-care diagnostics and high-throughput screening.