Microscopic 3D tracking of DEHS particles — determination of the mean square displacement

Particles suspended in a medium move randomly, i.e. the Brownian motion. The Brownian motion is difficult to measure, in particular the ballistic regime of the motion of a Brownian particle (at very short time scales t<<τ_p). However, even at larger time-scales, at t>>τ_p, referred to as the diffusive regime, the mean square displacement (MSD) of the particles is challenging to capture, since it is still small. Conducting particle MSD measurements in a gas rather than in a liquid is advantageous, since the MSD values in gases are about one order of magnitude larger. For this study, we employ particle imaging to determine the movement of di(2-ethylhexyl) sebacate (DEHS) seeding particles with an average diameter of around 0.35 micrometer in a confined, microscopic domain. More specifically, the 3D particle tracking velocimetry (3D-PTV) technique is used, where the spatial particle location is triangulated from at least two different views. The particle displacements are determined by means of particle tracking, using time-resolved or double-frame recordings. Unlike other microscopic single camera 3D particle imaging approaches, which require fluorescent seeding particles (and are therefore restricted to liquid flow analyses) to remove background reflections from the interfaces of the confined measurement domains, 3D-PTV uses a different approach: The issue of light reflections on the channel surfaces is overcome by a forward-scattering illumination set-up, where the camera views are at an off-axis angle of around 30° relative to the laser beam. In this way, the laser intensity can be relatively low, while the sub-micrometer DEHS seeding particles are still detectable on the sensor. Thus, microscopic 3D-PTV enables us to measure the mean square displacement of DEHS particles in air, yielding a 3D analysis of the random particle movements.