Optical Tweezer Measurements in Biophysical Environments

Recently, optical tweezers have been used to study force fluctuations in equilibrium systems and to determine the physical properties of complex materials. We are currently implementing two calibration methods, the photon-momentum method and the active-passive method, to measure displacements and forces at the nanometer and piconewton scales. Here, we study the force fluctuations of chlamydomonas microswimmers as well as the rheological properties of xylem sap. By locally trapping a chalmydomonas, we are able to measure its beating frequency as well as the force generated to beat at that frequency. We calculated the stochastic force spectrum by estimating the power spectral density of the fluctuating force signal. We are applying a theoretical framework to extract the non-equilibrium microswimmer forces from the total force spectrum. Xylem sap is a fluid found in plants that is vital to the plant's ability to transport nutrients throughout the plant's system. The xylem sap was analyzed through an active microrheological experiment (AMR). We are quantifying xylem sap rheology to investigate the physics of negative pressure and cavitation phenomena in plants.