Logo Logo
Switch language to English
Kirchner, Silke (2015): Focused optical beams for driving and sensing helical and biological microobjects. Dissertation, LMU München: Fakultät für Physik



A novel and interesting approach to detect microfluidic dynamics at a very small scale is given by optically trapped particles that are used as optofluidic sensors for microfluidic flows. These flows are generated by artificial as well as living microobjects, which possess their own dynamics at the nanoscale. Optical forces acting on a small particle in a laser beam can evoke a three dimensional trapping of the particle. This phenomenon is called optical tweezing and is a consequence of the momentum transfer from incident photons to the confined object. An optically confined particle shows Brownian motion in an optical tweezer, but is prevented from long term diffusion. A careful analysis of the motion of the confined particle allows a precise detection of microfluidic flows generated by an artificial or living source in the close vicinity of the particle. Thus, the particle can be used as a sensitive optofluidic detector. For this aim, several optical tweezers at different wavelengths are integrated into a dark-field microscope, combined with a high speed camera, to achieve a precise detection of the motion of the center-of-mass of the trapped particle. With this unique experimental system, a gold sphere is used as an optofluidic nanosensor to analyze for the first time the microfluidic oscillations generated by a biological sample. Here, a freely swimming larva of Copepods serves as the living source of flow. However, even if the trapping laser wavelength is off-resonant to the plasmon resonance of the flow detector, a finite heating of the gold nanoparticle occurs which reduces the sensitivity of detection. To increase the sensitivity of the optofluidic detection, a non-absorbing, dielectric microparticle is introduced as the optofluidic sensor for the microflows. It enables a quantitative, two dimensional mapping of the vectorial velocity field around a microscale oscillator in an aqueous environment. This paves the way for an alternative and sensitive detection approach for the microfluidic dynamics of artificial and living objects at a very small scale. To this aim and as a first step, an optically trapped microhelix serves as a model system for the mechanical and dynamical properties of a living microorganism. An optical tweezer is implemented for initiating a light-driven rotation of the chiral microobject in an aqueous environment and the optofluidic detection of its flow field is established. The method is then adopted for the measurement of the microfluidic flow generated by a biological system with similar dynamics, in this case a bacterium. The experimental approach is used to quantify the time-dependent changes of the flow generated by the flagella bundle rotation at a single cell level. This is achieved by observing the hydrodynamic interaction between a dielectric particle and a bacterium that are both trapped next to each other in a dual beam optical tweezer. This novel experimental technique allows the extraction of quantitative information on bacterial motility without the necessity of observing the bacterium directly. These findings can be of great relevance for an understanding of the response of different strains of bacteria to environmental changes and to discriminate between different states of bacterial activity.