Crystallization is one of the most widely used purification and separation processes applied in a multitude of industries such as pharmaceuticals, food & beverages, agriculture, and fine chemicals. However, the initial step of the crystallization process, nucleation, is still poorly understood and highly stochastic. As a result, most crystallization processes lack proper control over the properties of the crystals produced. Among many techniques for achieving better control over the nucleation process, the application of non-photochemical laser induced nucleation (NPLIN) has gathered significant interest. This is because of its potential to improve product quality in crystallization processes by directly controlling the nucleation rate, both spatially and temporally. Additionally, NPLIN can induce crystallization in solutions that would otherwise take a long time to nucleate, offering a unique advantage over traditional methods. However, despite its promising capabilities, NPLIN is not widely used in practice yet. The fundamental mechanism behind NPLIN is not fully understood, making it unclear how it should be applied effectively in practice and for which systems NPLIN could be beneficial.

This Ph.D. project aims to delve into the fundamental mechanisms of NPLIN, by examining how specific laser and solution parameters influence nucleation kinetics, leveraging innovative experimental setups. Laser parameters being studied include laser-exposed volume, laser irradiation position, laser intensity, and laser wavelength, and solution parameters include supersaturation levels, solution filtration, and the presence of impurities or dopants, particularly nanoparticles.

The thesis begins with a comprehensive review of the experimental and computational literature on NPLIN. It then presents a detailed study on the effect of the laser-exposed volume and laser irradiation position on the nucleation probability within partly illuminated supersaturated aqueous potassium chloride solutions. An increase in the laser-exposed volume resulted in a higher nucleation probability and a higher number of crystals per nucleated sample. Furthermore, laser irradiation, particularly through the air/solution interface, not only enhances nucleation probability but also influences the formation of different crystal morphologies. These observations are partly explained by the Nanoparticle Heating mechanism and the Dielectric Polarization model (Chapter 2).

The research then transitions to a microfluidic platform, which allows for high-throughput and crystallization detection using the deep learning method. This innovative approach addresses the need for large data sets in NPLIN research, which has been a significant challenge due to the manual nature of traditional experiments. The study examines the effects of laser intensity, wavelength, supersaturation, solution filtration, and intentional doping on nucleation probability in supersaturated potassium chloride solutions. Higher laser intensities and increased supersaturation significantly enhance nucleation probabilities. The laser wavelength effect was only observed for 355 nm at higher laser intensities. Solution filtration suppresses the NPLIN effect, whereas the addition of nanoparticles as dopants into the solution not only increases the NPLIN probabilities but also affects the crystal morphology. The results highlight the importance of impurities in the solution and support the hypothesis that nanoparticle or impurity heating could be the key mechanism in understanding NPLIN (Chapter 3).

The study finally investigated the effects of solution filtration, laser intensity, and nanoparticle properties including nanoparticle concentration and material on NPLIN probability in supersaturated aqueous urea solutions. The study highlights the significant role of impurities in NPLIN, demonstrating that doping with different nanoparticle materials leads to varied nucleation probabilities. In particular, gold nanoparticles were found to enhance nucleation more effectively than silica nanoparticles. Additionally, it was observed that NPLIN probabilities followed a Poisson distribution to changes in nanoparticle concentration and laser intensity respectively. The findings in this chapter enhance our understanding of the critical role of impurities in comprehending the NPLIN mechanism (Chapter 4).@en

Oscillatory flow in confined spaces is central to understanding physiological flows and rational design of synthetic periodic-actuation based micromachines. Using theory and experiments on oscillating flows generated through a laser-induced cavitation bubble, we associate the dynamic bubble size (fluid velocity) and bubble lifetime to the laser energy supplied—a control parameter in experiments. Employing different channel cross-section shapes, sizes and lengths, we demonstrate the characteristic scales for velocity, time and energy to depend solely on the channel geometry. Contrary to the generally assumed absence of instability in low Reynolds number flows (<1000), we report a momentary flow distortion that originates due to the boundary layer separation near channel walls during flow deceleration. The emergence of distorted laminar states is characterized using two stages. First the conditions for the onset of instabilities is analyzed using the Reynolds number and Womersley number for oscillating flows. Second the growth and the ability of an instability to prevail is analyzed using the convective time scale of the flow. Our findings inform rational design of microsystems leveraging pulsatile flows via cavitation-powered microactuation.

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Correction to: Scientific Reportshttps://doi.org/10.1038/s41598-024-68971-x, published online 13 August 2024 In the original version of this Article a previous rendition of Figure 2B, Figure 4 and Figure 5D was published. The original Figure 2, 4 and 5 and accompanying legends appear below. (Figure presented.) (Figure presented.) (Figure presented.) (A) Representative bubble dynamics for different channel geometries. (B) Universal motion of bubbles within channels with different size, shape and length. The dashed line represents the developed theory, Eq. (2). The marker colors represent the hydraulic diameters (d_h), the shapes represent the cross-section and the facecolor represent the lengths (L). The graphical marker symbols and colors established here are followed throughout this article. The black arrow represents the region of deviation(s) from the expected dynamics. The threshold laser energy absorbed for bubble formation estimated from experiments (E_th,exp) against theory (E_th,theory) presented in Eq. (5). (A,B) Representative dynamic bubble size curves illustrating the emergence of instabilities. The zones of the instabilities are highlighted using a shaded rectangular area. The arrows represent if the instabilities occur before or after X_max. (A) Illustrates the experimental data for different d_h with similar oscillation time. The instabilities emerge with increasing d_h. (B) Illustrates the data for d_h = 200 µm with increasing laser energies. The instabilities disappear with increasing E_abs. (C) Flow stability diagram with the transition line at W_o = 734. The markers represent the experiments and the lines represent the analytical estimate. The numbers correspond to the channel hydraulic diameters (in µm) with the dashed and solid lines representing the channel lengths L = 25 and 50 mm, respectively. (D) The dimensionless convective timescale against the L/d_h aspect ratio. The partition line is a linear relation between the x and y axes with 45 × 10⁻⁶ as the slope and the origin as the intercept. The original Article has been corrected.

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We demonstrate that a cavitation bubble initiated by a Nd:YAG laser pulse below breakdown threshold induces crystallization from supersaturated aqueous solutions with supersaturation and laser-energy-dependent nucleation kinetics. Combining high-speed video microscopy and simulations, we argue that a competition between the dissipation of absorbed laser energy as latent and sensible heat dictates the solvent evaporation rate and creates a momentary supersaturation peak at the vapor-liquid interface. The number and morphology of crystals correlate to the characteristics of the simulated supersaturation peak.

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Herein, we study the influences of the laser-exposed volume and the irradiation position on the nonphotochemical laser-induced nucleation (NPLIN) of supersaturated potassium chloride solutions in water. The effect of the exposed volume on the NPLIN probability was studied by exposing distinct milliliter-scale volumes of aqueous potassium chloride solutions stored in vials at two different supersaturations (1.034 and 1.050) and laser intensities (10 and 23 MW/cm²). Higher NPLIN probabilities were observed with increasing laser-exposed volume as well as with increasing supersaturation and laser intensity. The measured NPLIN probabilities at different exposed volumes are questioned in the context of the dielectric polarization mechanism and classical nucleation theory. No significant change in the NPLIN probability was observed when samples were irradiated at the bottom, top, or middle of the vial. However, a significant increase in the nucleation probability was observed upon irradiation through the solution meniscus. We discuss these results in terms of mechanisms proposed for NPLIN.

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Crystallization abounds in nature and industrial practice. A plethora of indispensable products ranging from agrochemicals and pharmaceuticals to battery materials are produced in crystalline form in industrial practice. Yet, our control over the crystallization process across scales, from molecular to macroscopic, is far from complete. This bottleneck not only hinders our ability to engineer the properties of crystalline products essential for maintaining our quality of life but also hampers progress toward a sustainable circular economy in resource recovery. In recent years, approaches leveraging light fields have emerged as promising alternatives to manipulate crystallization. In this review article, we classify laser-induced crystallization approaches where light-material interactions are utilized to influence crystallization phenomena according to proposed underlying mechanisms and experimental setups. We discuss nonphotochemical laser-induced nucleation, high-intensity laser-induced nucleation, laser trapping-induced crystallization, and indirect methods in detail. Throughout the review, we highlight connections among these separately evolving subfields to encourage the interdisciplinary exchange of ideas.

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Optofluidic devices have revolutionized the manipulation and transportation of fluid at smaller length scales ranging from micrometers to millimeters. We describe a dedicated optical setup for studying laser-induced cavitation inside a microchannel. In a typical experiment, we use a tightly focused laser beam to locally evaporate the solution laced with a dye resulting in the formation of a microbubble. The evolving bubble interface is tracked using high-speed microscopy and digital image analysis. Furthermore, we extend this system to analyze fluid flow through fluorescence-Particle Image Velocimetry (PIV) technique with minimal adaptations. In addition, we demonstrate the protocols for the in-house fabrication of a microchannel tailored to function as a sample holder in this optical setup. In essence, we present a complete guide for constructing a fluorescence microscope from scratch using standard optical components with flexibility in the design and at a lower cost compared to its commercial analogues.@en

Non-photochemical laser-induced nucleation (NPLIN) has emerged as a promising primary nucleation control technique offering spatiotemporal control over crystallization with potential for polymorph control. So far, NPLIN was mostly investigated in milliliter vials, through laborious manual counting of the crystallized vials by visual inspection. Microfluidics represents an alternative to acquiring automated and statistically reliable data. Thus we designed a droplet-based microfluidic platform capable of identifying the droplets with crystals emerging upon Nd:YAG laser irradiation using the deep learning method. In our experiments, we used supersaturated solutions of KCl in water, and the effect of laser intensity, wavelength (1064, 532, and 355 nm), solution supersaturation (S), solution filtration, and intentional doping with nanoparticles on the nucleation probability is quantified and compared to control cooling crystallization experiments. Ability of dielectric polarization and the nanoparticle heating mechanisms proposed for NPLIN to explain the acquired results is tested. Solutions with lower supersaturation (S = 1.05) exhibit significantly higher NPLIN probabilities than those in the control experiments for all laser wavelengths above a threshold intensity (50 MW/cm²). At higher supersaturation studied (S = 1.10), irradiation was already effective at lower laser intensities (10 MW/cm²). No significant wavelength effect was observed besides irradiation with 355 nm light at higher laser intensities (≥50 MW/cm²). Solution filtration and intentional doping experiments showed that nanoimpurities might play a significant role in explaining NPLIN phenomena.

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