The "lotus effect" explained: Two reasons why two length scales of topography are important

Surfaces containing 4 x 8 x 40 pm staggered rhombus posts were hydrophobized using two methods. One, using a dimethyldichlorosilane reaction in the vapor phase, introduces a smooth modified layer, and the other, a solution reaction using methyltrichlorosilane, imparts a second (nanoscopic) length scale of topography. The smooth modified surface exhibits contact angles Of theta(A)/theta(R) = 176 degrees/156 degrees. Arguments are made that the pinning of the receding contact line by the post tops (with theta(A)/theta(R) = 104 degrees/103 degrees) is responsible for the hysteresis. The second level of topography raises the contact angles of the post tops and the macroscopic sample to theta(A)/theta(R) = > 176 degrees/> 176 degrees and eliminates hysteresis. The increase in Laplace pressure due to the increase in the advancing contact angle of the post tops is a second reason that two length scales of topography are important.