This describes an apparatus comprising a plate with a number of elements having a hydrophilic surface arranged in an array with an overlay having a hydrophobic surface. A height of the overlay is between about 5% and 100% of the diameter of the elements. The application also discloses a wall circumferential to the plate; and a removable grid insertable into the apparatus, said grid comprising dividers enclosing a number of through-holes, said through-holes spaced in the grid to allow alignment of the through-holes of the grid over the elements in the plate when said grid is inserted into the apparatus, said dividers of the grid inserted into the apparatus form sides of wells bottomed by the plate and at least one element on said plate.