This special issue focuses on the use of light to induce motion and to manipulate objects. From the early years of quantum mechanics, we know that light particles, i.e., photons, carry both momentum and energy. This momentum can be transferred to other objects, eventually inducing an effective motion. A now famous demonstration of this effect was proposed by Ashkin in the early 80s to trap micro-particles at the focal point of a laser beam. Since then, the use of laser for manipulating small objects has been extensively studied worldwide and presently, finds numerous applications, notably in biotechnologies. In this special issue, two articles address some aspects of optical trapping. Yamamoto et al. discusses the stability of particles trapped by time-division optical tweezers and Kawamura et al. introduces laser manipulation by using liquid crystal devices with variable-focusing and beam-steering functions.
Photons carry energy inversely proportional to their wavelengths. When interacting with materials, the energy can be transferred in different manners depending on how photons interact with the material structure. For instance, this energy transfer can be used to induce phase transformations that in turn generate mechanical work. This is the case of shape memory alloys that when heated undergo a phase transformation that results in macroscopic material shape changes. Along these lines, the paper from Okamura et al. uses a laser beam to drive a shape memory alloy actuators. There, light is used to transfer heat to the material.
When interacting with liquids, light may also give rise to local flow pattern resulting from surface tension gradients. This effect, called Marangoni, can be used to manipulate components in a fluid using a laser beam. Different from optical trapping that uses radiation pressures to mechanically interact with small object, here the effect is heat-driven and can be applied to larger size components. In this special issue, the paper from Vela et al. shows how one can use Marangoni effects to move components of several tens of microns in dimensions in a fluid.
A more exotic use of light for manipulation is proposed by Mestre et al. There, a laser beam is used for fine size tuning and stabilization of single salt-water microdroplets on a superhydrophobic surface. The objective is to control whispering modes in droplet resonant cavities through the fine tuning of their dimensions.
These five articles illustrate some diverse and interesting aspects of light-driven actuation and manipulation. This is just a fragment of the many possibilities that light offers to induce motion and to manipulate things in general. We hope this special issue will be informative on the topic and stimulate novel ideas on this exciting multi-disciplinary field of research at the cross-roads between optics, mechanics, and fluidics.