In nonphotochemical laser-induced nucleation (NPLIN), an unfocused nanosecond laser pulse with low intensity (≈MW/cm²) triggers nearly instantaneous nucleation in supersaturated solutions, a process that would typically take days or weeks when the solution is left undisturbed. Previous studies have shown that the introduction of nanoparticles into supersaturated solutions enhances the probability of NPLIN measured during a fixed time window, compared to undoped control experiments. However, the precise mechanisms driving this enhancement remain unclear hampering industrial implementation of NPLIN. In this study, we systematically investigate how the properties of doped nanoparticles─specifically their concentration and chemical composition─affect the NPLIN probability in supersaturated urea solutions. We observed that higher laser intensities resulted in elevated NPLIN probabilities at a fixed pegylated gold nanoparticle (AuNP) concentration and supersaturation, while increasing concentrations of AuNPs at a fixed laser intensity and supersaturation interestingly led to higher NPLIN probabilities. Moreover, supersaturated solutions doped with gold nanoparticles exhibited significantly higher NPLIN probabilities compared to silica nanoparticle doped solutions at comparable nanoparticle size and concentration. We interpret these experimental results based on the impurity heating hypothesis as well as recent results highlighting the role of thermocavitation. We furthermore propose a helicopter-view model based on a thermodynamic equilibrium stage sequence. Our findings highlight the significance of nanoparticle properties in the design of heteronucleants optimized for NPLIN applications.

Primary nucleation control is crucial for obtaining crystals with specific properties, such as purity, size, morphology, and polymorphic form. Non-photochemical laser-induced nucleation (NPLIN) has attracted interest due to its ability to control these properties without chemical reactions, using non-invasive methods, and allowing spatio-temporal precision. However, the exact mechanism underlying NPLIN remains debated in the literature.

This dissertation explores how micron-sized vapor bubbles, formed by laser interaction with supersaturated aqueous solutions, can trigger crystal nucleation. Despite aqueous solutions generally being transparent to laser wavelengths of 532 nm and 1064 nm, transient bubbles can still form due to energy absorption by impurities or by focusing the laser. The research begins by examining the crystallization of KCl in aqueous solutions, initiated by bubbles formed using focused laser light with nanosecond pulse width. Findings show that solute accumulation at the bubble surface exceeds the saturation limit, leading to localized supersaturation. A finite element method model, validated by experimental bubble size data, is used to estimate solute transfer and supersaturation levels. The model demonstrates a concentrated solute boundary layer around the bubble, driven by high solvent evaporation rates associated with bubble growth. The experimental results for crystallization probability and crystal count align with classical nucleation theory predictions based on the numerically estimated supersaturation at the vapor-liquid interface.

The bubble formation mechanism proposed for NPLIN is extended to other solutes like NH4Cl, NaCl, KBr, and CH4N2O. Experiments with NH4Cl and NaCl yield a general analytical relation for supersaturation in the liquid surrounding the bubble, explaining NPLIN activity for these solutes when an unfocused laser is used. The predicted bubble sizes, based on Mie theory, correlate with the minimum nucleation rate necessary for crystal formation, indicating that the bubble-driven mechanism is a key factor in NPLIN.

Since isolated bubbles are rare in irradiated volumes due to the random distribution of impurities, the study also investigates bubble-bubble interactions and their effect on crystallization. The dynamics of single laser-induced bubbles in microchannel geometries are analyzed, revealing a unified theory for bubble size and lifetime as a function of laser energy. This analysis also uncovers a transient flow instability, rare in low Reynolds number flows, which originates from the channel walls and is characterized by the Womersley number and flow timescale. The research further demonstrates crystallization using bubble pairs in microchannels with KMnO4 as a model salt. The interaction between bubbles produces microjets that alter nucleation kinetics through induced shear, enabling crystallization at lower laser energies and solution supersaturation compared to single bubbles. A numerical model based on the boundary integral element method is used to predict microjet velocities and the resulting shear, correlating these factors with crystallization probabilities.

Overall, this work advances understanding of NPLIN, suggesting that bubble formation and interactions can be harnessed to achieve targeted crystallization with lower energy inputs and reduced supersaturation, paving the way for more efficient laser-induced crystallization processes.

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.

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.

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.

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.

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.

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.