Collection Efficiency, often referred to as "Catch Rate", is a product of two factors:
Collision efficiency - determined by aerodynamics, and
Adhesion efficiency - determined by the adhesive qualities of the component surface.
Specifically, it is the fraction of the liquid water in the direct path of an aircraft component (aerofoil, antenna, windscreen etc) which is deposited as ice on that component whilst flying in icing conditions. Collection efficiency varies directly with both droplet size and aircraft speed and inversely with the geometric size of the collecting surface.