(625a) Insight From Simulations of Laser-Induced Nucleation

Nonphotochemical laser-induced nucleation (NPLIN) has been observed for a variety of supersaturated solutions including urea, glycine, potassium chloride, proteins, and even a liquid crystal former[1,2]. Experiments show that nanosecond pulses of an intense laser (10-1000 MW/cm²) can reduce the induction time for nucleation from weeks to seconds. The laser frequencies are in the range where the solute molecules are transparent, so many of the proposed mechanisms involve perturbed electronic degrees of freedom that result in altered intermolecular interactions. For example, the optical Kerr hypothesis suggests that the molecules may become partially aligned with their most polarizable axis along the oscillating field direction[1,2]. Another hypothesis due to Isard suggests that the growing nucleus is stabilized due to the difference in dielectric constants of the nucleating phase and the surrounding solution[3]. Simulations using a lattice gas-Potts model are used to test these hypotheses in the first computational investigations of NPLIN. Our results confirm that an optical Kerr aligning mechanism does reduce the critical nucleus size and the free energy barrier to nucleation. However, at the experimental electric field strengths, our model shows that the optical Kerr effect only reduces the free energy barrier by a small fraction of kT which is insufficient to explain the rate enhancements observed in experiments. An important factor that has not been considered in-depth in experiments is the effect of the laser pulse duration. According to the hypotheses in the literature, the nucleation barrier should revert to the unperturbed barrier once the laser pulse ends. For the extremely short pulses used in the experiments, a postcritical nucleus relative to the perturbed critical size may not have time to grow beyond the unperturbed critical nucleus size before the laser turns off. Our simulations show that this dynamical barrier crossing effect may be important for short laser pulses, so we use a Smoluchowski model of the barrier crossing process to derive a threshold pulse duration required to observe a nucleation enhancement. Our results suggest experiments that may be used to test hypotheses based on perturbed molecular interactions and to gain insight into the monomer attachment frequencies for nuclei at the critical size.

References

[1] Matic, et al, Crystal Growth & Design 2005 5 (4), 1565-1567

[2] Sun, et al, Crystal Growth & Design 2006 6 (3), 684-689

[3] Alexander and Camp, Crystal Growth & Design 2009 9 (2), 958-963