Contact angle is a measure of static hydrophobicity, and contact angle hysteresis and slide angle are dynamic measures. Contact angle hysteresis is a phenomenon that characterizes surface heterogeneity. When a pipette injects a liquid onto a solid, the liquid will form some contact angle. As the pipette injects more liquid, the droplet will increase in volume, the contact angle will increase, but its three phase boundary will remain stationary until it suddenly advances outward. The contact angle the droplet had immediately before advancing outward is termed the advancing contact angle. The receding contact angle is now measured by pumping the liquid back out of the droplet. The droplet will decrease in volume, the contact angle will decrease, but its three phase boundary will remain stationary until it suddenly recedes inward. The contact angle the droplet had immediately before receding inward is termed the receding contact angle. The difference between advancing and receding contact angles is termed contact angle hysteresis and can be used to characterize surface heterogeneity, roughness, and mobility. Surfaces that are not homogeneous will have domains which impede motion of the contact line. The slide angle is another dynamic measure of hydrophobicity and is measured by depositing a droplet on a surface and tilting the surface until the droplet begins to slide. Liquids in the Cassie-Baxter state generally exhibit lower slide angles and contact angle hysteresis than those in the Wenzel state. A simple model can be used to predict the effectiveness of a synthetic micro- or nano-fabricated surface for its conditional state (Wenzel or Cassie-Baxter), contact angle and contact angle hysteresis. The main factor of this model is the contact line density, Λ, which is the total perimeter of asperities over a given unit area. The critical contact line density ΛC is a function of body and surface forces, as well as the projected area of the droplet. where ρ = density of the liquid droplet g = acceleration due to gravity V = volume of the liquid droplet θa = advancing apparent contact angle θa,0 = advancing contact angle of a smooth substrate γ = surface tension of the liquid w = tower wall angle If Λ > ΛC, drops are suspended in the Cassie-Baxter state. Otherwise, the droplet will collapse into the Wenzel state. To calculate updated advancing and receding contact angles in the Cassie-Baxter state, the following equations can be used. with also the