Excitable particles in an Optical Torque Wrench
We have shown that a birefringent particle held in rotation in the Optical Torque Wrench (OTW) behaves as an excitable system.
The neuron is probably the best known example of excitable system: ready to fire, it waits for input perturbations (voltage coming from other neurons) to overcome a threshold. Once this happens, it gives in output a voltage spike, always identical, that propagates along its axon and will eventually stimulate other neurons or muscle cells.
A trapped birefringent particle, forced in rotation by the linear polarization of the trapping laser, behaves similarly. Perturbations of the surrounding liquid (transient increase in drag) can overcome a threshold and make the particle to slip away from the rotating polarization, which can catch the axis of the particle again after 180deg rotation. This slipping event is measured as an always identical spike in the torque signal.
The neuron is probably the best known example of excitable system: ready to fire, it waits for input perturbations (voltage coming from other neurons) to overcome a threshold. Once this happens, it gives in output a voltage spike, always identical, that propagates along its axon and will eventually stimulate other neurons or muscle cells.
A trapped birefringent particle, forced in rotation by the linear polarization of the trapping laser, behaves similarly. Perturbations of the surrounding liquid (transient increase in drag) can overcome a threshold and make the particle to slip away from the rotating polarization, which can catch the axis of the particle again after 180deg rotation. This slipping event is measured as an always identical spike in the torque signal.
The picture shows the analogy we want to underline between two systems which have in common their dynamical behavior, despite their totally different microscopic description. With experience in different scientific communities, I became aware that drawing such analogies is something not very well accepted by everybody. Nonetheless this is an important lesson I have learned from the community of non-linear dynamical systems, which I fully adopt: very different systems, that do not have anything in common at certain lower scale descriptions (a neuronal cell and a mechano-optical system for example), can display at a higher scale an identical dynamical behavior. Is it not the role of science to unify different phenomena under a common description, interpretation, understanding? The rigorous and mathematical non-linear dynamical point of view, sometimes, can provide such description. When this happens, I think we should consider it a step forward. It is just a matter of scale we decide to adopt.