On the precision of optical imaging to study free surface dynamics at high frame rates

The precision of optical imaging to study free surface dynamics is analyzed. The damping of a liquid bridge free surface oscillation is used to validate this method. Images are acquired with a digital camera at relatively high frame rates and processed by several techniques. Oscillations with amplitudes of about 20 times smaller than the pixel size are measured, which allows one to reach the nanometer scale in the analysis. The experimental results presented in this paper constitute the first quantitative validation of optical imaging to study free surface dynamics at the nanometer scale. As a secondary goal, we propose an image processing technique based on the local thresholding criterion to determine the free surface position with sub-pixel resolution. This yields more precision (less noise) than the standard technique when considering very small oscillations. Further improvement of the results is obtained by a simple smoothing technique.