ABSTRACT
There are many variations of optical trapping systems to suit a wide
range of applications. Many systems are based on the pioneering
work of Ashkin et al. (Ashkin, 1970; Ashkin and Dziedzic, 1971)
but it was ever since the first realisation of a single-beam gradient-
force trap, termed “Optical Tweezers” (Ashkin et al., 1986), that
they have been developed as a useful tool for both the application
and measurement of sensitive forces. The pico-Newton range of
forces that can be applied using optical tweezers are well suited
for studying biological systems (Block, 1995). They are particularly
useful in microrheology where micron sized dielectric spheres are
introduced into a fluid and their motion tracked using a high-
speed video camera (Cicuta and Donald, 2007; Yao et al., 2009a),
allowing the viscous and elastic properties of the fluid to be
obtained. Microrheology techniques can be defined as either passive
or active (Brau et al., 2007). Passive microrheology relies only
on the thermal (Brownian) motion of the sphere whereas active
microrheology relies on an external force being applied to the sphere
and monitoring its response. Mechanical properties of objects can
be measured through application of forces to the object itself, or
to dielectric spheres attached to the object. The inherent scale
of optical tweezers makes them readily applicable to microfluidic
systems (Leach et al., 2006; Mushfique et al., 2008a,b) and they are
suited to performing measurements on small volumes and in non
conventional geometries (e.g. interior of a biological cell).
