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Register a new userDirect measurement of the nonconservative force field generated by optical tweezers
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The force field of optical tweezers is commonly assumed to be conservative, neglecting the complex action of the scattering force. Using a novel method that extracts local forces from trajectories of an optically trapped particle, we measure the three dimensional force field experienced by a Rayleigh particle with 10 nm spatial resolution and femtonewton precision in force. We find that the force field is nonconservative with the nonconservative component increasing radially away from the optical axis, in agreement with the Gaussian beam model of the optical trap. Together with thermal position fluctuations of the trapped particle, the presence of the nonconservative force can cause a complex flux of energy into the optical trap depending on the experimental conditions.
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Viscosity is an important property of out-of-equilibrium systems such as active biological materials and driven non-Newtonian fluids, and for fields ranging from biomaterials to geology, energy technologies and medicine. However, noninvasive viscosity measurements typically require integration times of seconds. Here we demonstrate a four orders-of-magnitude improvement in speed, down to twenty microseconds, with uncertainty dominated by fundamental thermal noise for the first time. We achieve this using the instantaneous velocity of a trapped particle in an optical tweezer. To resolve the instantaneous velocity we develop a structured-light detection system that allows particle tracking with megahertz bandwidths. Our results translate viscosity from a static averaged property, to one that may be dynamically tracked on the timescales of active dynamics. This opens a pathway to new discoveries in out-of-equilibrium systems, from the fast dynamics of phase transitions, to energy dissipation in motor molecule stepping, to violations of fluctuation laws of equilibrium thermodynamics.
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