Pushing the limit: investigation of hydrodynamic forces on a trapped particle kicked by a laser pulse

Abstract

We introduce a new optical technique where a train of short optical pulses is utilized to disturb a trapped microscopic particle. Using fast (250 kHz) and accurate (nm) detection of the position of the particle, accurately synchronized to the repetition rate of the laser pulses, we can coherently superimpose the displacement caused by each individual laser pulse. Thereby we are able to both bypass the influence from the Brownian motion of the trapped particle and to simultaneously increase the ability to localize its average trajectory by √n, where n is the number of repetitive pulses. In the results presented here we utilize a train of 1200 pulses to kick a 5 μm polystyrene sphere and obtain a spatial resolution corresponding to 0.09 nm and a time resolution of 4 μs. The magnitude of the optical force pushing the particle corresponds to ∼ 10(4)g and enables an investigation of both the hydrodynamical drag and the inertial effects caused by the particle and the surrounding liquid. Our results enables a more accurate testing of the existing extended models for the hydrodynamic drag and we discuss the observed agreement between experiments and theory.

Probing the Local Response of Glass-forming Liquids by Laser Excitations

“…Previous studies showed the magnitude of the scattering force also linearly increased with A (Fig. S1b) [43]; and its influencing range was roughly the size of the laser beam [42]. Although the scattering force was in z-direction, in our case of 2D confined system, the particle collided with the glass substrate frequently and thus generated significant xycomponent of the force.…”

“…When the colloidal particles were strictly confined between two glass walls, the pushing force in z-direction will be able to generate xycomponent due to the collisions between the colloidal particles and the glass wall. Moreover, the scattering force was proved to be localized and the magnitude of which was proportional to A [42,43]. The spatial separation between light spots was more than 20 σ s , much larger than the DLS in the deepest non-linear region.…”

“…As a type of optical forces, the scattering force represents the momentum transfer from the external radiation field to the colloidal particle by scattering and absorption. When a short pulse was imposed to colloidal particles, it tends to 'kick' them away from the center of the beam [42,43]. Previous studies showed the magnitude of the scattering force also linearly increased with A (Fig.…”

Pushing nanoparticles with light — A femtonewton resolved measurement of optical scattering forces

“…The switching process (on/off) is controlled by blocking the laser output in intervals with a chopper wheel. By applying a post-elimination technique 19,20 for thermal noise based on a lock-in strategy, a force sensitivity of 2.4 fN is obtained. This value is lower than the force sensitivity obtained before in the context of optical tweezers applications in liquids.…”

“…For the present experimental configuration based on the periodic repetition of a mechanical stimulus, it is possible to bypass the Brownian noise. 20 By averaging the tracked time series with respect to the chopping period, the spatial displacement is revealed with subpixel resolution. In Figure 2 with all the following events.…”