Highlights. Little fake triangle highlights on the goop. That'd probably help a good bit.

Refraction, if you can figure it out, would be a good way to suggest the goop. When an object is partly immersed in a clear liquid, the part that's immersed looks different. That part of WHN could seem to be larger, with an abrupt discontinuity where the goop ends.

Waviness in the lines might also help - the way ripples in water can distort images slightly as seen through the water.

-BC

Eeee goop n..n