The “Lotus Effect” Explained: Two Reasons Why Two Length Scales of Topography Are Important

Abstract

Surfaces containing 4 × 8 × 40 μm 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 θ_A/θ_R = 176°/156°. Arguments are made that the pinning of the receding contact line by the post tops (with θ_A/θ_R = 104°/103°) is responsible for the hysteresis. The second level of topography raises the contact angles of the post tops and the macroscopic sample to θ_A/θ_R = >176°/>176° 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.