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Optical `Atomizers'

In addition to manipulating particles, optical tweezers can permanently alter soft materials. If intense laser radiation is even partially absorbed by the sample, the resulting heating can be dramatic and, in the case of biological organisms, lethal. Minimizing radiation damage to biological systems is a central concern not only in studying the elastic properties of biological materials, but also for such practical applications as in vitro fertilization [57]. Beyond simple heating, optical tweezers exert a more subtle influence on extended dielectric structures which may explain some nonthermal damage suffered by biological samples.

When optical tweezers are focused on extended structures such as a membrane, radiation pressure tends to draw more material inward to the focal spot. The resulting increase in the local surface tension leads to beautiful effects [58, 59, 60, 61] which recently were reviewed in these pages [62]. These include pearling instabilities in cylindrical vesicles [58], unbinding of bilayer membranes [59], tensioning of flaccid vesicles [60], and expulsion of small inner vesicles from tensioned parents [60, 61]. The increase in surface tension caused by optical tweezers has been invoked to explain at least some of the dynamical and configurational changes in these systems [63, 64, 65].

Moroz and coworkers [61] recently have explained why some of these effects are irreversible. The suggest that optical tweezers can convert bilayer membranes into micelles or tiny vesicles too small to be imaged. The excess osmotic pressure due to the micelle population inside a tweezer-tightened vesicle drives the expulsion of daughter vesicles and helps to maintain the parent vesicle's tension afterward. Such `membrane atomization' possibly occurs also during tweezing of related materials and may provide a mechanism for subtle non-thermal damage in biological specimens.

David G. Grier
Mon Feb 17 21:35:47 CST 1997