Applications of surface acoustic waves in microfluidics

Introduction to Surface Acoustic Waves

An acoustic surface wave is a mechanical wave that propagates on the surface of a solid material, the main types of which includeRayleigh waves, Lamb waves, B-G waves, and Love waves..

Rayleigh waves were first introduced by British physicist Lord Rayleigh in 1885 during his study of seismic waves, and have been widely used in the field of acoustic surface wave technology.

Rayleigh waves are characterized by a variety of vibration modes, both longitudinal vibrations perpendicular to the surface and transverse vibrations parallel to the surface, possessing a combination of transverse and longitudinal vibrational components, which provides a unique advantage for their application in science and engineering.

Although Rayleigh waves were discovered as early as the end of the 19th century, they were not widely used due to the limitations of technology and manufacturing processes at that time.

With the rapid development of microelectromechanical systems (MEMS) technology and the emergence of new piezoelectric materials, surface acoustic wave (SAW) technology made significant advances in the mid-to-late 20th century. The surface acoustic wave (SAW) technology has made significant progress in the mid-to-late 20th century.

Especially in 1965, the research of R.M. White and F.W. Votmer of the University of California, Berkeley, achieved a key breakthrough in their successful development of the first surface acoustic wave device, which opened up a new era of surface acoustic wave applications in modern science and technology.

In recent years, researchers have introduced acoustic surface wave technology into the field of microfluidic chips.SAW microfluidic chips offer multiple advantages, such as ease of preparation, cost-effectiveness, biocompatibility, efficient fluidic drive and non-contact particle manipulation.

Acoustic streaming, caused by surface acoustic waves, can be used to manipulate the movement of fluids. As the surface wave energy increases, the fluid vibrates, moves, jets and atomizes.

Due to its ability to precisely manipulate fluids at the micro- and nanoscale, acoustic surface wave technology has received great attention in the field of microfluidics, especially in dealing with suspended particles in fluids. Suspended particles in microfluids can be precisely controlled by acoustic radiation force, which opens up new avenues for research and applications in the field of microfluidics.

Applications of surface acoustic waves in microfluidics

(1) Mixing:

Based on the acoustic flow phenomenon of mixing, acoustic surface waves can generate high-frequency vibrations, and such vibrations can rapidly mix the fluid in the microfluidic channel, leading to rapid flow and turbulence within the liquid, and thus mixing within the droplets.

A team of researchers has developed a highly efficient Focused Surface Acoustic Wave (FSAW) basedMicromixer chip,the mixer utilizes FSAW to create a strong fluid disturbance that promotes rapid, homogeneous mixing of the solution.

Another research team developed a mixer that achieves more efficient active mixing by exciting acoustic surface waves to generate two interacting flow fields.

(2) Droplet drive:

The pressure gradient generated by the acoustic surface wave can be used to propel a liquid droplet. When the acoustic wave is delivered to the interface of the liquid in contact with the substrate, the droplet will move in the direction of wave propagation under the force of the acoustic wave.

SAW technology enables contactless actuation of droplets, reducing contamination and damage.Research at Monash University has shown that acoustic surface waves can rapidly propel micro-droplets across a flat substrate at speeds of 1 to 10 cm/s.

As the input power increases, the droplets may undergo state changes such as ejection and atomization.In 2015, academics in Australia proposed an innovative microfluidic platform that utilizes asymmetrically arranged fork-finger transducers at the ends of the droplets, enabling two droplets to collide and merge. Droplets in asymmetric electrodes are also separated in the region of acoustic surface wave action.

(3) Pumping:

Pumping is achieved through the formation of surface waves in the liquid medium and these waves create flow in the liquid.SAW technology enables contactless fluid pumping without the need for external devices, simplifying the complexity of microfluidic systems.

In 2012, a team of German researchers successfully developed a microfluidic platform utilizing SAW actuation, which demonstrated excellent actuation pressure, efficiency, and fast response when pumping a wide range of fluids such as red blood cell suspensions. It was shown that as the input frequency increases, the flow characteristics within the flow channel decrease, but the fluid pumping rate increases accordingly.

(4) Droplet heating:

Surface acoustic waves generate thermal energy during propagation, and this thermal energy can be used to heat liquid droplets. By adjusting the frequency and amplitude of the acoustic waves, precise heating of droplets can be achieved, especially for biological samples that require rapid heat treatment.

In 2015, researchers at the Italian Institute of Technology constructed a special SAW-heated experimental platform and used a thermal sensing camera to monitor the temperature changes of a liquid droplet. The experiment showed that the droplets could be rapidly heated and stabilized in a short period of time under the action of SAW. As the SAW energy increases, the heating rate of the droplet accelerates accordingly.

(5) Droplet atomization:

When acoustic surface waves contact the surface of a droplet, they produce strong vibrations that break the droplet into smaller particles or atomize it.This type of nebulization produces extremely small droplets and is suitable for drug inhalation, cell culture, and other samples that require nebulization..

In 2015, a research team from the University of Electronic Science and Technology (UEST) used ZnO as the substrate material for the SAW device and fed a low-frequency, high-power RF signal to the fork-finger transducer. The results showed that the tiny droplets experienced significant atomization in a short period of time, and the change process was captured by a high-speed camera.

(6) Particle manipulation:

Acoustic surface wave devices are typically located on either side of a microfluidic channel and are capable of generating high frequency acoustic waves. These waves propagate along the substrate surface and into the fluid medium, forming pressure nodes and antinodes.

Particles in a fluid are subject to acoustic radiation forces that arise from the differences in acoustic properties between the particles and the surrounding fluid. Depending on the size, density and compressibility of the particles, they respond differently to these forces.

Typically, particles move to the pressure node where the pressure is minimized, a phenomenon known as acoustic trapping. Fine manipulation of particles, including capturing, transporting and arranging these tiny particles, can be achieved through precise control.

In 2008, research at Monash University in Australia showed that under low-power SAW, particles tend to form circular wave-like structures on the surface of liquid droplets.

With the gradual increase of the SAW power, the colloidal particles begin to aggregate into island-like structures, showing strong localized agglomeration characteristics. When the SAW power is further increased, the flow characteristics inside the droplets become more significant, leading to the rupture of the island-like structure and the dispersion of the particles.

This flow characteristic shows that the distribution and structure of the particles inside the droplet is significantly affected by the SAW power intensity. As the power continues to increase, the flow phenomenon inside the droplet becomes more intense and stable, which ultimately leads to the permanent dispersion of particles in the droplet.

This phenomenon reveals the potential of SAW technology for efficient mixing and particle control in microfluidics, where fine tuning of the particle structure and dynamics inside the droplet can be achieved by adjusting the power of the SAW.