Modulating thermocapillary flow using light for logic operation at the air-water interface

Motion of microscale objects can be controlled at an air-water interface by using the Marangoni effect - the generation of convective flows arising from a gradient in the surface tension. We present an example of precisely designing collective particle movement by modulating the surface tension through spatiotemporal control of light intensity. Two modes of particle control are employed - optical trapping and interparticle repulsion. Particle trapping is realized by generating a dark pattern with higher surface tension inside an illuminated region. Interparticle repulsion is realized by the interacting thermocapillary flows arising from the temperature gradients around the particles. Using both effects, the movement of an ‘output’ particle can be engineered inside a trap under particular conditions of trap geometry and distance from ‘input’ particles. We design AND, OR, and NOT gates to perform logic operation at the air-water interface. Additionally, we identify a critical distance between the particles at which a particle assembly becomes unstable causing a particle to escape its trap because of the repulsion from its neighbors. This critical distance increases with the system size, presenting a fundamental limitation in the dense packing of a large number of logic gates.