  The theory for linear deformations of fluid microparticles in a laser beam of
Gaussian profile is presented, when the beam focus is at the particle center as
in optical trapping. Three different fluid systems are considered: water
microdroplet in air, air microbubble in water, and a special oil-emulsion in
water system used in experiments with optical deformation of fluid interfaces.
We compare interface deformations of the three systems when illuminated by a
wide (compared to particle radius) and narrow laser beams and analyse
differences. Deformations of droplets are radically different from bubbles
under otherwise identical conditions, due to the opposite lensing effect
(converging and diverging, respectively) of the two; a droplet is deformed far
more than a bubble, cetera paribus. Optical contrast is found to be of great
importance to the shape obtained when comparing the relatively low-contrast
oil-emulsion system to that of water droplets. We finally analyse the dynamics
of particle motion when the laser beam is turned on, and compare a static beam
to the case of a short pulse. The very different surface tension coefficient
implies a very different time scale for dynamics: microseconds for the
water-air interface and tens of milliseconds for the oil-emulsion. Surface
oscillations of a water microdroplet are found always to be underdamped, while
those of the oil-emulsion are overdamped; deformations of a microbubble can be
either, depending on physical parameters.
