  At impact of a liquid drop on a solid surface an air bubble can be entrapped.
Here we show that two competing effects minimize the (relative) size of this
entrained air bubble: For large drop impact velocity and large droplets the
inertia of the liquid flattens the entrained bubble, whereas for small impact
velocity and small droplets capillary forces minimize the entrained bubble.
However, we demonstrate experimentally, theoretically, and numerically that in
between there is an optimum, leading to maximal air bubble entrapment. Our
results have a strong bearing on various applications in printing technology,
microelectronics, immersion lithography, diagnostics, or agriculture.
