There’s a couple of possible reasons. First, you probably spilled more oil onto the plate. In the experiment, 0.81 milliliters of oil spread out until it cov

There’s a couple of possible reasons. First, you probably spilled more oil onto the plate. In the experiment, 0.81 milliliters of oil spread out until it covered a circle with diameter 84 cm. Most spills would be more than a mL of oil, and most plates are much smaller. Second, most plates aren’t flat. If you have an area of the plate that is at a lower elevation than the rest, the oil would pool up in that area. Third, even if you fill the plate with water, you could have elevation changes due to surface tension of the water. If the water is concave up, the oil would float upward and form a ring around the edge. If the water is above the surface of the plate, held in just by surface tension, the oil would float upwards and form a bubble at the center of the plate. Fourth, you could have something else on the plate that acts as an emulsifier. Whether a bit of egg, some pasta water, leftover detergent, these would break up the oil and prevent a film from forming. The easiest way to have a flat surface is to do the experiment in the center of a much larger body of water, since any effects from the surface tension would be at the edge.