We use dynamically controlled annular beam optical tweezers to orientate non-spherical swimming E.coli. Typically elliptically shaped particles in a Gaussian optical trap will align themselves with the direction of beam propagation. This orientation makes determining the dynamics of swimming particles difficult, as most optical tweezers systems are only able to capture information in the focal plane, but not the axial direction. We use simulations and experimental measurements to show that we are able to orientate motile E.coli from a vertical to horizontal position with a spatial light modulator in as little as one intermediate step without reduction in trap stiffness.
Using absolutely calibrated optical tweezers, we make quantitative measurements of the motility force of Escherichia coli (E. coli) by measuring the change in momentum of the deflected beam. By tracking the position of the particle, in addition to the optical force measurements, it should be possible to simultaneously calculate the motility force and drag. In a simple Gaussian beam optical trap away from the sample chamber E. coli tends to align and swim along the beam axis, which can make tracking the particle position and measuring the force difficult. We use a 3-D optical force detection system to measure the absolute force on the particle, allowing us to measure the motility force of E. coli in a simple Gaussian beam. By using a line-shaped trap, it is possible to align E. coli transverse to the beam axis, facilitating easy particle position measurement. The investigated methods are not specific to E. coli and could be applied to other motile organisms, the study of wall effects and bio-films.