Grooves spacing govern water retention during condensation

Condensation on vertical surfaces leads to fluid retention, which limits the efficiency of applications ranging from heat exchangers to atmospheric water harvesters. A common strategy is to structure the surface with grooves, yet whether grooves help drainage or worsen retention remains unclear. Here we use a high-throughput condensation setup to quantify retention on substrates patterned with parallel vertical grooves of fixed geometry (𝑑/𝑤=1) while varying the spacing 𝑠. We uncover two opposite regimes separated by the droplet detachment radius 𝑅𝑑. For large spacings (𝑠>𝑅𝑑), droplets grow and slide under gravity while grooves, acting as passive reservoirs, increase retention. For small spacings (𝑠<𝑅𝑑), grooves instead trigger active drainage, confining droplet growth and reducing retention to values even lower than on smooth surfaces. Two asymptotic models, a groove-volume reservoir model and a plateau-packing model, capture this transition and explain the scaling of retention with 𝑠. These findings show that groove spacing controls whether grooves act as drains or reservoirs, providing a simple geometric design rule for tailoring condensation retention in practical systems.