Experimental investigation of the mechanism for non-photochemical laser induced nucleation
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The aim of this thesis was to discover the mechanism for non-photochemical laser-induced nucleation (NPLIN), which is a technique for inducing nucleation of crystals with laser light without absorption. The mechanism of the optical Kerr effect (OKE) was suggested by Garetz et al. [Physical Review Letters 77, 3475–3476 (1996)] to give an explanation for NPLIN. Since the feasibility of the OKE mechanism for NPLIN has been questioned, a series of experiments on NPLIN of aqueous supersaturated urea were carried out to quantify the relationship between crystal alignment and laser polarization. Digital imaging of crystal growth during laser irradiation showed that nascent needle-shaped crystals of urea were not aligned with the direction of the electric field of the laser. Additionally, work on glycine was aimed at verifying the possibility to control the polymorph of the obtained crystal via the laser polarization. However, our finding shows that the probability of γ-glycine is more likely to increase with increasing supersaturation; and the influence of laser polarization on the resulting morphologies is not strong as reported by Sun et al. [Crystal Growth & Design 6, 684–689 (2006)]. Furthermore, in another work on NPLIN of L-histidine, based on Sun et al. [Crystal Growth & Design 8, 1720–1722 (2008)], we were unable to reproduce the results as stated in Sun’s published paper. We find their results exhibit a large uncertainty when recalculated through the Wilson score interval for binomial distributions. On account of these revised uncertainties, it is unlikely that laser polarization gives polymorphism control. Comparison with the nucleation probability of unfiltered samples in the work of urea and glycine shows that the number of filtered samples nucleated as a result of NPLIN was largely decreased. Moreover, experiments on NPLIN of NaCl and KCl also exhibited that the number of filtered samples nucleated was significantly lower than that of unfiltered samples. This downward tendency in nucleation probability after filtration cannot be explained by Garetz’s OKE mechanism. On account of this, an alternative mechanism named particle-heating mechanism was proposed, and supported by experiments on NPLIN of sodium acetate. Sodium acetate experiments showed that the crystallization of sodium acetate can be induced by a single pulse of a nanosecond laser (1064 nm) with minimum power of 0.1 J cm−2. As discovered by Oliver et al. [D. Oliver, PhD Thesis, Edinburgh University, 2014], anhydrous or trihydrate sodium acetate can be formed under the effects of different organic and inorganic additives, such as poly- (methacrylic acid) and disodium hydrogen phosphate. We demonstrate that crystalline growth velocities and crystal morphology can be influenced by these additives. We find that high levels of additive cause only nucleation of bubbles. By counting the number of crystals, which is approximately consistent to the number of bubbles observed, video microscopy of laser-induced crystallization of sodium acetate has revealed that the general mechanism of NPLIN is most likely to be caused by a particle-heating mechanism. Chapter 8 of the thesis describes a number of solute molecules that were tested using NPLIN, but failed. In terms of improvements for future work or a perspective on NPLIN, detailed suggestions have been described in Chapter 9, which also gives a summary of all work presented.