Testing Different Tracer Types in Particle Image Velocimetry Measurements in Electrohydrodynamics

A powerful technique used to visualize fluid flows is the Particle Image Velocimetry (PIV) method. It allows having instantaneous velocity fields with a single measurement shot. However, this technique requires seeding the fluid with fine tracer particles. These particles are illuminated by a Laser source and images are captured in order to obtain a velocity field pattern of the global scene. In electrohydrodynamics, in addition to classical fluid mechanics problems, further complications emerge as a result of adding these tracer particles. The major problem in such cases is particle charging which is caused by the presence of an electric field. These particles accumulate electric charges on their surface and may affect the electric field distribution in the device. In addition, the particles can move faster than the surrounding fluid, and therefore, PIV measurements become inaccurate because the assumption of identical velocity for both the fluid and the particles is not valid anymore. Moreover, the seeding particles can stick together, form filaments, or adhere to surrounding solid walls, thus blocking charge transfer with the dielectric liquid. Furthermore, the particles could capture all the electric charges on their surface such that the liquid is not charged anymore, which weakens the fluid motion. The particle charging phenomenon depends on the operating fluid and the type of seeding particles used for PIV measurements. The main objective of the current work is to highlight the effect of particle type on PIV results. In this context, a hydrofluoroether dielectric liquid (HFE-7100) is tested in a cylinder-to-cylinder configuration. A DC voltage of up to 30 kV amplitude is applied between the two cylindrical electrodes. PIV measurements are then conducted and repeated with four different types of seeding particles: PMMA, PTFE, SiO
2
, and Vestosint. The obtained results were not similar and the fluid behavior does not seem to be independent of the presence of the particles. To conclude, one must be careful when selecting the appropriate tracer type for each fluid and each experiment, especially when an electric field is applied.