Magnetic fluids (ferrofluids) consist of magnetic nanoparticles (diameter ~10nm) which are dispersed in a liquid, often with the use of surfactants. They were first developed by NASA to address the unique requirements of moving liquid fuel in microgravity conditions. With a help of a holographic optical tweezers, interaction of magnetic nanoparticles with strongly focused laser beam was observed. When the light intensity was high enough, magnetic nanoparticles were removed from the beam center and they formed a dark ring. Creation process lasts less than 330μs and cannot be observed precisely even with ultrafast camera. Such rings exist when the laser beam is affecting the sample and disappear (with a lifespan of 10’th second range) after the laser is switched off. Moreover, when several rings are created simultaneously, complex interactions between them can be observed. In this work, the results of our experiments will be presented with hypotheses about the physical background of such a behavior.
The stiffness of a single optical trap is a basic parameter of optical manipulators and detecting the displacement of trapped object belongs to fundamental measurements . The trap stiffness can be determined by position measurements of oscillating (with high frequency) dielectric microsphere. Usually the signal is captured by simple detector like quadrant photodiode . However, this method is hard to apply when working in multitrap mode, especially in case of holographic optical tweezers. In multitrap mode, widely used method is video image processing focused on determining the center of mass of an object . In this contribution we present the method for position measurment based on video sequence analysis from fast camera. The position is obtained by correlating the video frame with templates of known positions of that object. This algorithm can work on several objects, finding their positions independently. It has also scalable accuracy up to sub-pixel level.