Centimeter-scale objects in liquid can be manipulated using the mutual attraction of two arrays of air bubbles in the presence of sound waves.

Assembling small components into structures is a fiddly business often encountered in manufacturing, robotics, and bioengineering. Some existing approaches use magnetic, electrical, or optical forces to move and position objects without physical contact. Now a team has shown that acoustic waves can create attractive forces between centimeter-scale objects in water, enabling one such object to be accurately positioned above another [1]. The scheme uses arrays of tiny, vibrating air bubbles that provide the attractive force. This acoustic method requires only simple equipment and could provide a cheap, versatile, and gentle alternative technique for object manipulation.

Researchers are developing techniques that use acoustic waves to position objects such as colloidal particles or biological cells. Attractive forces are produced by the scattering of sound waves from the objects being manipulated. One limitation of this approach, however, is that positioning is more accurate with waves of higher frequency (and thus smaller wavelength), but higher frequencies are also more strongly absorbed and attenuated by many materials.