Force scattering

While neat-resonant light exerts both scattering forces and dipole forces on single atoms, similar forces ate also exerted on larger dielectric objects. 

In optical tweezer experiments, the optical scattering force is used to trap particles, but the force can also be used to control the shape of liquid droplets26. An infrared laser with 43-mW power focused onto a microdroplet on a superhydrophobic surface enabled up to 40% reversible tuning of the equatorial diameter of the droplet26. Such effects must naturally also be taken into account when exciting laser modes in droplets in experiments with levitated drops. 

Fig. 18.15 Advanced methods for single particle analysis in LC ARROW chips, (a) Nanopore added to reservoir for single particle entry into LC ARROW (b) Optical dual beam particle trap based on balancing the scattering force due to counter propagating beams...

Optical traps have also been used to retain cells [176,866-868,1174]. For instance, manipulation of polystyrene beads by optical gradient force (attractive) and scattering force (repulsive) was achieved in a PDMS chip (see Figure 8.26). A polystyrene bead was first retained by an optical trap. Then the bead was moved and released so that it flowed to a desired channel downstream [1174]. 

Although metal nanoparticles experience strong radiation pressure and scattering forces due to their high absorption and polarizability, stable trapping of such particles has been demonstrated in both two and three dimensions [74, 85, 86]. As for cells and dielectric particles, a NIR laser is usually used to trap metal nanoparticles while a separate laser, or the trapping laser itself, excites the... 

Fig. 2 Schematic diagram of laser optical tweezers (LOT) system. (Top panel) An infrared laser beam was steered to trap and move pm-size particles in the focal plane. At a sufficiently large laser intensity, the gradient force dominates over the scattering force. (Lower panel) Membrane tethers are extracted from live cells. Dielectric beads (0.5 pm in diameter) were conjugated with antibodies against integiins and attached to the cell membrane. LOT traps one bead and pulls it away from the cell, as indicated by the arrow. Thin membrane tethers extending from the beads to the cell body appear. Using conventional brightfield microscopy, the tether length and its diameter are measured which, in turn, provide an estimate of the plasma membrane tension.

This is done by treating the particle as a point dipole and using Maxwell s equations to solve for the electric field within the beam. It is convenient to analyze the trapping force in terms of two separate components the scattering force, Fscm, and the gradient force, Fy. The scattering force arises due to absorption and reradiation by the dipole, whereas the gradient force arises due to the interaction between the induced dipole and the electric field. The dipole moment can be calculated by... 

The total trapping force acting on the trapped sphere is the sum of the trapping and scattering forces ... 

The scattering force and absorption force are given as follows ... 

In Eq. 10 the first term in brackets represents the scattering force and the second the absorption. 

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