Application Cases

Experiment Name: Application of Broadband Power Amplifier in Non-Contact Droplet Manipulation on Superhydrophobic Surfaces and High-Throughput Surface-Enhanced Raman Scattering Measurement

Research Direction: Acoustic Levitation

Experimental Equipment: ATA-1220E Broadband Power Amplifier, Signal Generator, Ultrasonic Transducer, Stage, etc.

Experimental Content:
In this study, we introduce an innovative non-contact acoustic tweezer (CAT) for droplet manipulation on superhydrophobic surfaces. This technology generates an ultrasonic standing wave between the ultrasonic transducer and the superhydrophobic substrate, enabling droplet manipulation without physical contact. We demonstrate that small droplets as small as 20 μL can be manipulated three-dimensionally in mid-air, while larger droplets up to 500 μL can be captured and manipulated in-plane. Experimental results confirm that CAT can effectively manipulate droplets of different compositions and volumes on various superhydrophobic substrates, providing an efficient, versatile, and cross-contamination-free liquid handling solution for applications such as high-throughput surface-enhanced Raman scattering (SERS).

Experimental Procedure:
All droplet manipulation experiments were conducted using a custom platform equipped with a manual xyz stage. A set of 3D-printed molds was used to fix an ultrasonic transducer (10 mm diameter, 38 kHz resonant frequency) onto the z-axis of the stage. To power the ultrasonic transducer, a 38 kHz sinusoidal signal was generated by a function generator and amplified by a power amplifier. The superhydrophobic surface was placed on top of the xyz stage, allowing manual adjustment of the distance between the ultrasonic transducer and the superhydrophobic surface to generate a standing wave for reliable droplet manipulation.

The figure below illustrates the workflow of the high-throughput SERS experiment. Droplets containing highly diluted aqueous analytes interacted with carbon and silver nanoparticles. Under the controlled movement of the acoustic tweezers, the particles were separated from the substrate. Due to the plasmonic properties of the silver nanoparticles, the sensitivity of Raman measurements was enhanced. After measurement, the first droplet was removed, and the second droplet was manipulated to the desired detection point using the acoustic tweezers, enabling the analysis of different droplets.

Experimental Results:

The figure below demonstrates the handling of multiple droplets on superhydrophobic paper, showcasing the precise and selective droplet manipulation capability of CAT.

(A) Formation of three droplet patterns on a superhydrophobic surface using non-contact ultrasound.
(B) Selective transport of a red droplet surrounded by six green droplets.
(C) Merging of two droplets using ultrasound to enhance mixing.
(D) Sequential transport and merging of multiple droplets.
(E) Sequential coalescence of three droplets containing vinegar, litmus, and sodium carbonate using CAT.

The figure below shows the Raman signals of two droplets containing 10 μM and 1 mM Rhodamine 6G (R6G), respectively. The second droplet, with a concentration of 1 mM R6G, exhibited R6G peaks in the SERS spectrum. However, the first droplet, with a concentration of 10 μM R6G, produced a spectrum with relatively low noise, making it difficult to detect the typical R6G peaks. Furthermore, the signal from the blank substrate obtained after removing both droplets (shown as the black line in the figure below) indicates that no residual material remained on the superhydrophobic surface once the droplets were removed by the acoustic tweezers. The entire detection process was completed within one minute. This result demonstrates the successful application of acoustic tweezers for droplet-based SERS on superhydrophobic substrates, enabling high-throughput measurements.

Specifications of the ATA-1220E Broadband Power Amplifier Used in the Experiment: