W. Benjamin Rogers and John C. Crocker
We describe a simple scanning-line optical tweezers instrument for measuring pair interactions between micrometer-sized colloidal particles. Our instrument combines a resonant scanning mirror and an acousto-optic modulator. The resonant scanning mirror creates a time-averaged line trap whose effective one-dimensional intensity profile, and corresponding trapping potential energy landscape can be programmed using the acousto-optic modulator. We demonstrate control over the confining potential by designing and measuring a family of one-dimensional harmonic traps. By adjusting the spring constant, we balance scattering-induced repulsive forces between a pair of trapped particles, creating a flat potential near contact that facilitates interaction measurements. We also develop a simple method for extracting the out-of-plane motion of trapped particles from their relative brightness, allowing us to resolve their relative separation to roughly 1 nm.
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