  We examine whether cubic non-linearities, allowed by symmetry in the elastic
energy of a contact line, may result in a different universality class at
depinning. Standard linear elasticity predicts a roughness exponent zeta=1/3
(one loop), zeta=0.39 (numerics) while experiments give zeta of about 0.5.
Within functional RG we find that a non-local KPZ-type term is generated at
depinning and grows under coarse graining. A fixed point with zeta=0.45 (one
loop) is identified, showing that large enough cubic terms increase the
roughness. This fixed point is unstable, revealing a rough strong-coupling
phase. Experimental study of contact angles theta near pi/2, where cubic terms
vanish in the energy, is suggested.
