Apparatus include a test cell body having a first exterior surface, a second exterior surface, a cavity extending between the first exterior surface and the second exterior surface, and a first textured wetted plate and a second textured wetted plate disposed within the cavity. A fluid flow gap is defined between the first textured wetted plate and the second textured wetted plate. An illuminator is disposed between the second textured wetted plate and the second exterior surface, and a viewing window formed within the first exterior surface. The first textured wetted plate and the second textured wetted plate may be transparent.