The disclosure features methods, fluid delivery platforms, and apparatus for preparing a sample on a substrate that includes a substrate handler configured to move a substrate between a first position and a second position, and a platform positioned so that when the substrate is in the second position, the platform faces the substrate, where the platform includes a fluid delivery area having a second surface formed from a hydrophilic material for which a water contact angle is 40 degrees or less, and a first surface facing the substrate when the substrate is in the second position, formed from a hydrophobic material for which a water contact angle is 100 degrees or more.