Why is it difficult to wash aphids off from superhydrophobic kale?

Many varieties of the cabbage family have leaves covered with superhydrophobic epicuticular wax, which provides them with self-cleaning characteristics. Since the wax also lowers insect adhesion, rinsing of the leaves with water should be an effective way of removing the insects. Conversely, we report that superhydrophobicity of tuscan kale increases resistance of aphids to hydrodynamic removal. The exterior surface of the insects is also superhydrophobic and acts as an extension of the leaf's surface. As a result even at moderate impact velocities impinging water drops cannot penetrate under the pests. Consequently, liquid impact aids the insect's adhesion by increasing the normal compressive forces they experience. We show that on a hydrophilic arugula leaf this mechanism is absent, and aphids can be easily washed off with water, as it is able to penetrate underneath them. As for removal of aphids from Tuscan kale, we show that lower surface tension liquids, such as oils and soapy water are more effective, because they are able to wet both the plant and insect surfaces. We also show that aerodynamic removal of aphids consisting of simply exposing the invaded leaf to an air flow is most effective.