A simple technique has been developed that produces holograms made of sound waves. These acoustic landscapes are used to manipulate microscale objects, and offer great potential in medical imaging and selective heating. See Letter p.518
Although sound waves oscillate, they exert a steady force on suspended objects. This force can be used to trap and manipulate particles1, droplets2 and cells3 in microfluidic and levitation systems. On page 518, Melde et al.4 report an impressively simple technique that produces complex acoustic holograms. The forces exerted within these holograms allow for stunning results in particle patterning and in controlling particle trajectories.
When a sound wave enters a fluid, an oscillation in pressure will propagate through the fluid — this pressure can vary in time and space. Both temporal and spatial domains are important for manipulating matter suspended in the fluid into patterns. In the temporal domain, the pressure that acts on a particle will vary during a single oscillation. The particle will vibrate, but the fluctuations will be too rapid to lead to movement. However, if there is also spatial variation in the pressure field, the momentum of the fluid will impart a force on the particle that is, on average, non-zero. Over many oscillation cycles, this acoustic-radiation force will cause the particle to move.
By combining this temporal effect with shaping of the pressure field in the spatial domain, acoustic-radiation forces can cause particles to migrate to predetermined locations. This allows such particles to be manipulated5, sorted6 or patterned7. For example, a standing wave — created when a sound wave reflects back and forth in a chamber — is characterized by nodes (points of minimum amplitude) and antinodes (points of maximum amplitude). Particles will cluster at either the nodes or the antinodes of the pressure field5, the exact final locations being dependent on the particles' properties6. Such fields can be produced using a single transducer (a device for converting electrical energy into the mechanical deformation of matter that causes sound, just as a speaker does at audible frequencies). However, with additional complexity come more possibilities.
If two sets of transducers are placed perpendicular to each another, their sound waves will interfere to create a pressure field that consists of a grid of nodes and antinodes7. By altering the relative phase of the sound waves from each set of transducers, the nodes and antinodes will shift, taking the trapped particles with them. This allows the trajectory of the particles to be controlled8.
Going further, arrays of transducers, in which each member has a different phase9, can produce acoustic holograms for complex particle manipulation (Fig. 1a). Such phased arrays are most prevalent in medical ultrasonic imaging, in which sound waves in the form of an 'acoustic beam' are directed across an object and then analysed to build up a scan image. When pebbles are dropped into a pond in a row, the ripples created by each pebble will interfere to form a wave pattern. By changing the time at which each pebble is dropped, the shape of this wave pattern can be controlled. In a similar way, by applying a different time (or phase) delay to each member of an array of independent, spatially distributed transducers, the shape of the acoustic beam can be designed, enabling intricate particle patterning and trajectory control9.
“The authors create an extremely detailed acoustic hologram by means of a simple experimental set-up.”
In conventional systems, there is thus a strong correlation between the complexity of the instrumentation and that of the pressure field. In stark contrast to this, the elegance of Melde and colleagues' approach lies in the fact that it can be used to create an extremely detailed acoustic hologram by means of a simple experimental set-up. In an array, signals that have different phases are applied to each transducer, but here, the same phase distribution can be produced using only one transducer (Fig. 1b). The transducer is coupled to a 3D-printed monolithic element, which is a finely contoured solid plastic block. In this set-up, a sound wave will emanate from the transducer and pass through the element into the fluid; the time taken for this to occur is determined by the element's thickness. Therefore, if the element has a varying thickness, the time at which different parts of the sound waves enter the fluid will also vary, as will their phases. In addition to the simplicity of this technique, the level of control over the beam's shape is enhanced because it is not limited by the resolution due to the transducer's size, but rather by the resolution of the 3D printer, which is about 100 times higher.
In previous work, particle trajectory was controlled by changing the phase of the transducers, but here the phase is hard-wired into the system, so that objects move along a pre-programmed path. Clearly, this demands prior knowledge of the required trajectory, but, in return, it offers a huge reduction in complexity. To demonstrate the patterning potential of their technique, Melde and colleagues arrange particles into the outline of a dove in flight (see Figure 1 of the paper4). They also demonstrate trajectory control by continuously moving particles around a circular path and using particles to draw shapes (see Figure 3 of the paper4). It is therefore easy to see how this might be used to govern the motion of cells in a Petri dish or through different chemical environments in a microfluidic network. The authors' method of producing acoustic holograms also has applications in a multitude of fields that require the production of complex acoustic beams, such as extremely high-resolution imaging and selective heating.Footnote 1
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Advanced Functional Materials (2019)
International Journal of Applied Mechanics (2019)
Lab on a Chip (2017)
Lab on a Chip (2017)