Figure 3.2: Interference images of 9.4mPa s silicone oil drops of radiusR = 1.65mm impacting a glass slide with velocity of 2.2m s−1 as seen from below. The images were taken at t = 0.08, 0.18, 0.24, and 0.46ms after impact and at three different pressures, which were chosen to illustrate the transition in Fig. 3.1: P = 55kPa below the transition pressure, 58kPa in the transition region, and 70kPa above the transition pressure. At the earliest time the image of the spreading liquid is black, indicating that it is in contact with the glass. A trapped air bubble can be seen trapped at the center of each impact [15]. At t = 0.18ms an interference pattern is seen at the edge of the 70kPa drop, indicating that the liquid is now spreading over a thin air gap and that a thin sheet has been created. The air gap is also seen in parts of the liquid edge at 58kPa. At t = 0.24ms the interference patterns grow for both the 58kPa and the 70kPa drops. In the 58kPa images, the regions of the spreading drop that had not already formed a thin sheet remain on the substrate and do not form a thin sheet until a later time. At t = 0.46ms an air gap has finally developed for the 55kPa drop, as well as the remaining regions of the 58kPa case. Close inspection reveals that the liquid locally bridges the air gap near the contact line, as described in [6]. The white scale bar is equivalent to 3.3mm, the diameter of the original drop.
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