Rb. Liebert et Dc. Prieve, FORCE EXERTED BY A LASER-BEAM ON A MICROSCOPIC SPHERE IN WATER - DESIGNING FOR MAXIMUM AXIAL FORCE, Industrial & engineering chemistry research, 34(10), 1995, pp. 3542-3550
When light strikes a body, it exerts a force which arises from the mom
entum of the photons. Here we use ray optics to predict the force on a
sphere (much larger than the wavelength of light) exerted by a laser
beam of low divergence (in contrast with high divergence beams used to
form ''optical traps'') to determine if the magnitude of force attain
able is large enough to dislodge microscopic particles adhered to surf
aces. For simple dielectric microspheres (e.g. polystyrene or glass) t
he maximum axial force of about 0.6 nN/W is exerted by an ideal unifor
m beam, concentric with the sphere, when the beam radius is comparable
to the sphere radius. Donut and Gaussian beams produce slightly small
er maximum forces. Focusing the beam to a waist size much smaller than
the sphere and aiming it at the edge of the sphere can produce axial
forces of 5 nN/W, independent of beam shape. Unfortunately, this eccen
tricity also generates a torque which would stress the adhesive nonuni
formly. The absolute maximum force of 8.9 nN/W (2/v, where v is the sp
eed of light through the surrounding medium) can be obtained without t
orque using a concentric beam if the sphere is constructed of a highly
reflective material. Our results suggest that radiation forces can be
used to detach microscopic particles adhered to surfaces.