Communication: Bubbles, crystals, and laser-induced nucleation

Effect of acidic polymers on the morphology of non-photochemical laser-induced nucleation of potassium bromide

Non-photochemical laser-induced nucleation (NPLIN) in supersaturated potassium bromide (KBr) solutions with the addition of acidic polymers is reported here for the first time. Upon absorbing the incident laser, crystallites are immediately induced along the laser pathway in the solution, eventually growing into needle-shaped crystals of varying sizes. When comparing induction time, nucleation probability, and crystal habits with spontaneous nucleation, the results suggest that NPLIN creates a distinct morphological pathway, transforming cubic crystals into needle-like structures. Additionally, it improves crystallization probability and growth rate. This paper aims to realize control from crystal nucleation to crystal growth by adding acidic polymers to the process of laser-induced nucleation, potentially influencing crystal morphology modification in NPLIN. With 19 wt% acidic polymers added to the solution as additives, control over both crystal growth and morphological modifications was observed: cubic KBr crystals with square patterns were produced through laser irradiation, and there was a varying reduction in both the number and growth rate of the crystals. The influence of acidic polymers on the solution environment was analyzed to determine the reasons for the variations in crystal quantity and growth speed. The underlying mechanisms responsible for the changes in crystal shape were also discussed.

Effect of Laser-Exposed Volume and Irradiation Position on Nonphotochemical Laser-Induced Nucleation of Potassium Chloride Solutions

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.

Design and Validation of a Droplet-based Microfluidic System To Study Non-Photochemical Laser-Induced Nucleation of Potassium Chloride Solutions

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.

A Review of Laser-Induced Crystallization from Solution

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.

Fast Formation of Hydrate Induced by Micro-Nano Bubbles: A Review of Current Status

Hydrate-based technologies have excellent application potential in gas separation, gas storage, transportation, and seawater desalination, etc. However, the long induction time and the slow formation rate are critical factors affecting the application of hydrate-based technologies. Micro-nano bubbles (MNBs) can dramatically increase the formation rate of hydrates owing to their advantages of providing more nucleation sites, enhancing mass transfer, and increasing the gas–liquid interface and gas solubility. Initially, the review examines key performance MNBs on hydrate formation and dissociation processes. Specifically, a qualitative and quantitative assembly of the formation and residence characteristics of MNBs during hydrate dissociation is conducted. A review of the MNB characterization techniques to identify bubble size, rising velocity, and bubble stability is also included. Moreover, the advantages of MNBs in reinforcing hydrate formation and their internal relationship with the memory effect are summarized. Finally, combining with the current MNBs to reinforce hydrate formation technology, a new technology of gas hydrate formation by MNBs combined with ultrasound is proposed. It is anticipated that the use of MNBs could be a promising sustainable and low-cost hydrate-based technology.

Molecular Dynamics Study of Clathrate-like Ordering of Water in Supersaturated Methane Solution at Low Pressure

Using molecular dynamics, the evolution of a metastable solution for “methane + water” was studied for concentrations of 3.36, 6.5, 9.45, 12.2, and 14.8 mol% methane at 270 K and 1 bar during 100 ns. We have found the intriguing behavior of the system containing over 10,000 water molecules: the formation of hydrate-like structures is observed at 6.5 and 9.45 mol% concentrations throughout the entire solution volume. This formation of “blobs” and the following amorphous hydrate were studied. The creation of a metastable methane solution through supersaturation is the key to triggering the collective process of hydrate formation under low pressure. Even the first stage (0–1 ns), before the first fluctuating cavities appear, is a collective process of H-bond network reorganization. The formation of fluctuation cavities appears before steady hydrate growth begins and is associated with a preceding uniform increase in the water molecule’s tetrahedrality. Later, the constantly presented hydrate cavities become the foundation for a few independent hydrate nucleation centers, this evolution is consistent with the labile cluster and local structure hypotheses. This new mechanism of hydrogen-bond network reorganization depends on the entropy of the cavity arrangement of the guest molecules in the hydrate lattice and leads to hydrate growth.

Clean production and characterization of nanobubbles using laser energy deposition

We have demonstrated the production of laser bulk nanobubbles (BNB) with ambient radii typically below 500 nm. The gaseous nature of the nanometric objects was confirmed by a focused acoustic pulse that expands the gas cavities to a size that can be visualized with optical microscopy. The BNBs were produced on demand by a collimated high-energy laser pulse in a "clean" way, meaning that no solid particles or drops were introduced in the sample by the generation method. This is a clear advantage relative to the other standard BNB production techniques. Accordingly, the role of nanometric particles in laser bubble production is discussed. The characteristics of the nanobubbles were evaluated with two alternative methods. The first one measures the response of the BNBs to acoustic pulses of increasing amplitude to estimate their rest radius through the calculation of the dynamics Blake threshold. The second one is based on the bubble dissolution dynamics and the correlation of the bubble's lifetime with its initial size. The high reproducibility of the present system in combination with automated data acquisition and analysis constitutes a sound tool for studying the effects of the liquid and gas properties on the stability of the BNBs solution.

A survey on fractionation: the optimal control of distilling in batch and semibatch configurations

Since the middle of the last century, discussion about the operation of discontinuous fractionation to meet multifarious goals, such as product purity and recovery rate, by monitoring process variables including reflux or/and heat duty, is been on. The engineering practice showed intolerable events to occur; hereof the operation must be supervised, which makes it difficult to be in agreement with the batch distillation objectives. Hence, to uphold the effectuation of new operating policies into the industrial “know-how” techniques, different optimal control strategies can be conceived. The objective of this work is to offer a literature survey on the investigations of optimal control functioning for selected simple distillation column configurations employed in batch/semibatch distillation of homogeneous/reactive mixtures, as well as the approaches used in this regard. Available optimal control schemes have been reviewed in detail, emphasizing its major assets.

Local Light-Controlled Generation of Calcium Carbonate and Barium Carbonate Biomorphs via Photochemical Stimulation

Photochemical activation is proposed as a general method for controlling the crystallization of sparingly soluble carbonates in space and time. The photogeneration of carbonate in an alkaline environment is achieved upon photo‐decarboxylation of an organic precursor by using a conventional 365 nm UV LED. Local irradiation was conducted focusing the LED light on a 300 μm radius spot on a closed glass crystallization cell. The precursor solution was optimized to avoid the precipitation of the photoreaction organic byproducts and prevent photo‐induced pH changes to achieve the formation of calcium carbonate only in the corresponding irradiated area. The crystallization was monitored in real‐time by time‐lapse imaging. The method is also shown to work in gels. Similarly, it was also shown to photo‐activate locally the formation of barium carbonate biomorphs. In the last case, the morphology of these biomimetic structures was tuned by changing the irradiation intensity.

On-Demand Bulk Nanobubble Generation through Pulsed Laser Illumination

We demonstrate the temporally and spatially controlled nucleation of bulk nanobubbles in water through pulsed laser irradiation with a collimated beam. Transient bubbles appear within the light exposed region once a tension wave passes through. The correlation between illumination and cavitation nucleation provides evidence that gaseous nanobubbles are nucleated in the liquid by a laser pulse with an intensity above 58  MW/cm^{2}. We estimate the radius of the nanobubbles through microscopic high-speed imaging and by solving the diffusion equation to be below 420 nm for ∼80% of the bubble population. This technique may provide a novel approach to test theories on existence of stable bulk nanobubbles.

Urea-induced supramolecular self-assembly strategy to synthesize wrinkled porous carbon nitride nanosheets for highly-efficient visible-light photocatalytic degradation

Graphitic carbon nitride (g-C₃N₄) has attracted immense interest as a promising photocatalyst. To facilitate its versatile applications in many fields, new low-cost strategies to synthesize outstanding g-C₃N₄ need to be further developed. Although supramolecular preorganization has been considered as a promising candidate, the utilized supramolecules like melamine–cyanuric acid (MCA) are typically synthesized by expensive triazine derivatives. Herein, wrinkled porous g-C₃N₄ nanosheets were successfully fabricated by hydrothermal-annealing of supramolecular intermediate MCA synthesized by the cheap precursors dicyandiamide and urea. During the formation of MCA, urea could act as a facile agent to react with dicyandiamide to form melamine and cyanuric acid firstly and then assemble into MCA through hydrogen bonds. In addition, urea could serve as a porogen and decompose to generate bubbles for conducive formation of micro-size MCA self-templates and thus wrinkled porous g-C₃N₄ nanosheets could be obtained. The nanostructure and photocatalytic performance of g-C₃N₄ were optimized by modulating microstructures and physicochemical properties of MCA, which could be conveniently controlled by urea addition and hydrothermal duration. The obtained wrinkled porous g-C₃N₄ nanosheets exhibit highly-efficient visible-light photocatalytic degradation compared with traditional MCA-derived g-C₃N₄, which could remove 98.3% of the rhodamine B in 25 min. The superior photocatalytic activity is mainly attributed to the urea-induced larger specific surface area, better light harvesting ability, faster transfer and more advanced separation efficiency of the photogenerated electron–hole pairs. This research provides a new strategy for preparing high-performance porous g-C₃N₄ from the self-assembled supramolecule MCA synthesized by low-cost precursors.

Wrinkled porous g-C₃N₄ nanosheets were fabricated by supramolecular MCA self-templates. Due to the reactant and porogen agent urea, g-C₃N₄ could be modulated with efficient electron–hole pairs and thus superior photocatalytic degradation performance.

Laser-induced nucleation promotes crystal growth of anhydrous sodium bromide

It is demonstrated that laser-induced nucleation enables preferential crystallization of metastable anhydrous solids from solution.

Using optical tweezing to control phase separation and nucleation near a liquid-liquid critical point

About 20 years ago, it was shown that lasers can nucleate crystals in super-saturated solutions and might even be able to select the polymorph that crystallises. However, no theoretical model was found explaining the results and progress was slowed down. Here we show that laser-induced nucleation may be understood in terms of the harnessing of concentration fluctuations near a liquid-liquid critical point using optical tweezing in a process called laser-induced phase separation (LIPS) and LIPS and nucleation (LIPSaN). A theoretical model is presented based on the regular solution model with an added term representing optical tweezing while the dynamics are modelled using a Kramers diffusion equation, and the roles of heat diffusion and thermophoresis are evaluated. LIPS and LIPSaN experiments were carried out on a range of liquid mixtures and the results compared to theory.

Low-intensity ultrasound induced cavitation and streaming in oxygen-supersaturated water: Role of cavitation bubbles as physical cleaning agents

A number of acoustic and fluid-dynamic phenomena appear in ultrasonic cleaning baths and contribute to physical cleaning of immersed surfaces. Propagation and repeated reflection of ultrasound within cleaning baths build standing-wave-like acoustic fields; when an ultrasound intensity gradient appears in the acoustic fields, it can in principle induce steady streaming flow. When the ultrasound intensity is sufficiently large, cavitation occurs and oscillating cavitation bubbles are either trapped in the acoustic fields or advected in the flow. These phenomena are believed to produce mechanical action to remove contaminant particles attached at material surfaces. Recent studies suggest that the mechanical action of cavitation bubbles is the dominant factor of particle removal in ultrasonic cleaning, but the bubble collapse resulting from high-intensity ultrasound may be violent enough to give rise to surface erosion. In this paper, we aim to carefully examine the role of cavitation bubbles from ultrasonic cleaning tests with varying dissolved gas concentration in water. In our cleaning tests using 28-kHz ultrasound, oxygen-supersaturated water is produced by oxygen-microbubble aeration and used as a cleaning solution, and glass slides spin-coated with silica particles of micron/submicron sizes are used to define cleaning efficiency. High-speed camera recordings and Particle Image Velocimetry analysis with a pressure oscillation amplitude of 1.4 atm at the pressure antinode show that the population of cavitation bubbles increases and streaming flow inside the bath is promoted, as the dissolved oxygen supersaturation increases. The particle removal is found to be achieved mainly by the action of cavitation bubbles, but there exists optimal gas supersaturation to maximize the removal efficiency. Our finding suggests that low-intensity ultrasound irradiation under the optimal gas supersaturation in cleaning solutions allows for having mild bubble dynamics without violent collapse and thus cleaning surfaces without cavitation erosion. Finally, observations of individual bubble dynamics and the resulting particle removal are reported to further support the role of cavitation bubbles as cleaning agents.

Effect of nucleants in photothermally assisted crystallization

Laser-induced crystallization is emerging as a promising technique to crystallize biomolecules like amino acids and proteins. The use of external materials as nucleants and novel seeding methods open new paths for protein crystallization. We report here the results of experiments that explore the effect of nucleants on laser-based crystallization of microlitre droplets of small molecules, amino acids, and proteins. The role of parameters like solute concentration, droplet volume, type and size of the nucleant, and laser power, are systematically investigated. In addition to crystallization of standard molecules like NaCl, KCl, and glycine, we demonstrate the crystallization of negatively (l-histidine), and positively (l-aspartic acid) charged amino acids and lysozyme protein. Single crystal X-ray diffraction and Raman spectroscopy studies unequivocally indicate that the nucleants do not alter the molecular structure of glycine, hydrogen bonding patterns, and packing. Localized vaporization of the solvent near the nucleant due to photothermal heating has enabled us to achieve rapid crystallization - within 3 s - at laser intensities of 0.1 MW cm^-2, significantly lower than those reported earlier, with both saturated and unsaturated solutions. The outcome of the current experiments may be of utility in tackling various crystallization problems during the formation of crystals large enough to perform X-ray crystallography.

Supersaturation dependence of glycine polymorphism using laser-induced nucleation, sonocrystallization and nucleation by mechanical shock

Nucleation of glycine by laser, ultrasound and mechanical shock exhibits a transition from the alpha to the gamma polymorph with increasing supersaturation.

Polarization independence of laser-induced nucleation in supersaturated aqueous urea solutions

Imaging reveals no alignment of urea crystal axis with the electric field direction, contrary to current understanding of laser-induced nucleation.

Molecular-dynamics simulations of urea nucleation from aqueous solution

Despite its ubiquitous character and relevance in many branches of science and engineering, nucleation from solution remains elusive. In this framework, molecular simulations represent a powerful tool to provide insight into nucleation at the molecular scale. In this work, we combine theory and molecular simulations to describe urea nucleation from aqueous solution. Taking advantage of well-tempered metadynamics, we compute the free-energy change associated to the phase transition. We find that such a free-energy profile is characterized by significant finite-size effects that can, however, be accounted for. The description of the nucleation process emerging from our analysis differs from classical nucleation theory. Nucleation of crystal-like clusters is in fact preceded by large concentration fluctuations, indicating a predominant two-step process, whereby embryonic crystal nuclei emerge from dense, disordered urea clusters. Furthermore, in the early stages of nucleation, two different polymorphs are seen to compete.

A new experimental setup for high-throughput controlled non-photochemical laser-induced nucleation: application to glycine crystallization

Non-photochemical laser-induced nucleation (NPLIN) has been a growing field of study since 1996, and more than 40 compounds including organics, inorganics and proteins have now been probed under various conditions (solvents, laser types, laser beamsetc.). The potential advantages of using this technique are significant, in particular polymorphic control. To realize these benefits, the objective is a carefully designed experimental setup and highly controlled parameters, for example temperature and energy density, in order to reduce the uncertainty regarding the origin of nucleation. In this paper, a new experimental setup designed to study NPLIN is reported. After a full technical description of the present setup, the different functionalities of this device will be illustrated through results on glycine. Glycine crystals obtained through NPLIN nucleate at the meniscus and exhibit different morphologies. The nucleation efficiency, as a function of the supersaturation of the solution used and the laser beam energy density, has also been established for a large number of samples, with all other parameters held constant.