[en] The micro-PIV technique represents a development of the general particle image velocimetry technique to applications in fluid mechanics phenomena at a micrometric scale. It takes advantage from the very small depth of field of long distance objectives with high magnification to carry out accurate measurements in the focusing plane. The typical configuration consists of a microscope coupled to a pulsated laser and a double exposure PIV camera. The flow is seeded using sub-micrometric fluorescent particles. The laser light is directed on the investigated flow through the epifluorescent microscope objective. The light re-emitted by the fluorescent particles is detected by the PIV camera equipped of an optical filter to select only the fluorescence wavelength. The micro-PIV technique presents a large interest in the validation of numerical codes developed in different micro-fluidic framework such as biological flows and in industrial application as the ink-jet print-head.
To investigate the micro-PIV measurements capabilities for fast moving and deforming droplets, measurement of the flow inside a jet ejected by a piezo-driven capillary up to the droplet formation by Rayleigh instability are studied in combination with PTV in order to distinguish the main liquid movement from the bulk one. Liquids differing from each other for their viscosity and their surface tension as well as piezo-element frequency in the 1 to 8 KHz range are investigated. The flow rate of the jet is adjusted by means of a pushing syringe system. Low concentration of 0.86 microns fluorescent particles is employed as seeding in order to have good signal to noise ratios. The ensemble averaging method is used to increase the height of the correlation peaks. Stroboscopic method is used to achieve several couples of frames taken in the same conditions thanks to high repeatability of piezo-driven instabilities. Moreover changing the delay of stroboscopy all the droplet formation phases can be analyzed in detail.
In the experimental configuration, optical aberrations play a role since they affect the position and shape of the particle images and as a consequence the velocity field. The two main optical aberration experienced are astigmatism and measurement plane deformation. Astigmatism cannot be avoided in the experimental configuration, as it is clearly observed on droplet images were particles above and bellow the focalisation plane appear as perpendicular lines (Fig 1a). Nevertheless cross correlation method is not sensitive on particle image shape. As the measurement plane defined by the focal plane of the microscope is located inside a curved transparent object, it deformed as it was passing through a lens. The deformation of the objective plane affects the measurements as a function of the optical configuration, droplet curvature and relative refractive index. However, in the studied configuration, deformed plane differs only from the straight one of about 8 µm.
The micro-PIV method is therefore suited to measure the instantaneous vector field inside droplets through cross-correlation methods (Fig 1b). The internal flow recirculation is observed. Measurements can be also performed in different planes inside the droplet depending on focalisation plane.
