The optical manipulation of nanoparticles has multiple applications, but presents great difficulties because the optical forces are very weak. A study by the Autonomous University of Madrid (UAM) has shown that not only the volume of the particle matters, as the theoretical models predict, but also the force can be increased by varying the particle load. The results are published in the Nano Letters magazine.
Since it was shown that light can exert force on bodies, micro and nanometric objects have been trapped and manipulated using laser beams focused by a lens. This technique is known as optical tweezers and is currently widely used, for example, in medical and biological studies, since it allows manipulating both cells and parts of them in a very precise and delicate manner. Specifically, the optical manipulation of very small objects, with a size 100 times smaller than the diameter of a human hair, is very interesting and complicated at the same time. In those size scales, any small disturbance can destabilize the trapped object and cause it to be released from the optical trap (laser focus) because the optical forces are very weak. To get an idea, the optical forces exerted on nanoparticles are a billion billion times smaller than the force that an ant can exert. The most important destabilizing force is thermal. Normally, the optical trapping experiments are carried out in an aqueous medium. The particles are dispersed in water, so that, if they pass close to the optical trap, they can fall inside it and get trapped.
The aqueous medium is a set of water molecules that move and interact with each other. When a particle is between the water molecules, it suffers collisions with them. The force exerted on the particle due to these shocks depends on the temperature: the higher it is, the more numerous these collisions will be and the greater the thermal force. Even at room temperature, the thermal force is comparable to optics. For this reason it is so complicated to trap nanoscale particles and a large number of researchers have made multiple studies, both theoretical and experimental, to determine what the optical force depends on and thus find out how it can be increased.
Theoretical models establish that the force increases linearly when the volume of the trapped nanoparticle increases. For this reason the force is so weak when the particle has a very small size. However, in a previous experimental study conducted by the research group Fluorescence Imaging Group of the Autonomous University of Madrid (UAM), in collaboration with scientists from the University of Verona (Italy), it was discovered that what the theoretical models establish does not It is quite true: particles with the same volume could be trapped with greater or lesser force depending on their surface. These preliminary results led the group to carry out a more exhaustive study in collaboration with researchers from different research centers in Wroclaw, Poland, which has recently been published in the journal Nano Letters. In this case, particles with different sizes and potential zeta values were used. When comparing the new results with those obtained in the previous study, the researchers were able to clearly determine that the optical strength does not increase linearly with the volume, as the theoretical models predict.
In sum, the study allows us to conclude that the optical forces exerted on nanoscale particles can be controlled through a good choice of the characteristics of the particle. In addition, it demonstrates that the theoretical models must be revised to take into account other parameters, not only the volume.
Bibliographic reference: Rodríguez-Sevilla, P .; Prorok, K .; Bednarkiewicz, A .; Marquis, M. I .; García-Martín, A .; García Solé, J .; Haro-González, P .; Jaque, D. 2018. Optical Forces at the Nanoscale: Size and Electrostatic Effects. Nano Lett. DOI: 10.1021 / acs.nanolett.7b04804