Remnants of the disappearing critical point in chain-forming patchy fluids

For a standard model of patchy colloidal fluids with patch number M = 2, where chain formation (polymerization) occurs, we show that Wertheim theory predicts critical behavior at vanishing density and temperature. The analysis is based on determining lines in the phase diagram of maximal correlation length and compressibility. Simulation studies identify the latter line and confirm our prediction of Fisher-Widom crossover, i.e., the asymptotic decay of the pair correlation function changes from monotonic to damped oscillatory as the density is increased. For M > 2, it is known that phase separation occurs with a true critical point. Our results support the notion that a "disappearing" critical point occurs in the limit M = 2 and we uncover its remnants.