Explosive Boiling of Water Films Adjacent to Heated Surfaces: A Microscopic Description

Nucleation Process in Explosive Boiling Phenomena of Water on Micro-Platinum Wire

The maximum temperature limit at which liquid boils explosively is referred to as the superheat limit of liquid. Through various experimental studies on the superheating limit of liquids, rapid evaporation of liquids has been observed at the superheating limit. This study explored the water nucleation process at the superheat limit achieved in micro-platinum wires using a molecular interaction model. According to the molecular interaction model, the nucleation rate and time delay at 576.2 K are approximately 2.1 × 1011/(μm3μs) and 5.7 ns, respectively. With an evaporation rate (116.0 m/s) much faster than that of hydrocarbons (14.0 m/s), these readings show that explosive boiling or rapid phase transition from liquid to vapor can occur at the superheat limit of water. Subsequent bubble growth after bubble nucleation was also considered.

Heat Transfer in Gold Interfaces Capped with Thiolated Polyethylene Glycol: A Molecular Dynamics Study

Reverse nonequilibrium molecular dynamics simulations were used to study heat transport in solvated gold interfaces which have been functionalized with a low-molecular weight thiolated polyethylene glycol (PEG). The gold interfaces studied included (111), (110), and (100) facets as well as spherical nanoparticles with radii of 10 and 20 Å. The embedded atom model (EAM) and the polarizable density-readjusted embedded atom model (DR-EAM) were implemented to determine the effect of metal polarizability on heat transport properties. We find that the interfacial thermal conductance values for thiolated PEG-capped interfaces are higher than those for pristine gold interfaces. Hydrogen bonding between the thiolated PEG and solvent differs between planar facets and the nanospheres, suggesting one mechanism for enhanced transfer of energy, while the covalent gold sulfur bond appears to create the largest barrier to thermal conduction. Through analysis of vibrational power spectra, we find an enhanced population at low-frequency heat-carrying modes for the nanospheres, which may also explain the higher mean interfacial thermal conductance (G) value.

Tracking the Explosive Boiling Dynamics at the Alcohol/MXene Interface

We demonstrate the real-time tracking of explosive boiling dynamics at the alcohol/MXene interface by monitoring the photoinduced lattice dynamics of MXene nanosheets dispersed in different alcohols. As revealed by ultrafast spectroscopy, the explosive boiling experiences three cascading stages, i.e., the starting initiation (0-1 ns), the following phase explosion (1-6 ns), and the eventual termination (>6 ns). More importantly, the occurrence conditions of explosive boiling are rationally evaluated via photothermal modeling, echoing well to our experimental observations and further suggesting that ∼17-25 layers of alcohol molecules undergo phase transition from liquid to vapor, a result that can hardly be attained by other physicochemical means. Additionally, useful insights into thermal conduction/diffusion and transient acoustic pressure related to the early stage of explosive boiling are provided. This paradigmatic study enriches the fundamental understanding (on a microscopic level) about the elusive dynamics of explosive boiling at the liquid-solid interface.

Curvature and temperature-dependent thermal interface conductance between nanoscale-gold and water

<p>Plasmonic gold nanoparticles (AuNPs) can convert laser irradiation into thermal energy for a variety of applications. Although heat transfer through the AuNP-water interface is considered an essential part of the plasmonic heating process, there is a lack of mechanistic understanding of how interface curvature and the heating itself impact interfacial heat transfer. Here, we report atomistic molecular dynamics simulations that investigate heat transfer through nanoscale gold-water interfaces. We simulated four nanoscale gold structures under various applied heat flux to evaluate how gold-water interface curvature and temperature affect the interfacial heat transfer. We also considered a case in which we artificially reduced wetting at the gold surfaces by tuning the gold-water interactions to determine if such a perturbation alters the curvature and temperature dependence of the gold-water interfacial heat transfer. We first confirmed that interfacial heat transfer is particularly important for small particles (diameter {less than or equal to} 10 nm). We found that the thermal interface conductance increases linearly with interface curvature regardless of the gold wettability, while it increases non-linearly with the applied heat flux under normal wetting and remains constant under reduced wetting. Our analysis suggests the curvature dependence of the interface conductance coincides with changes in interfacial water adsorption, while the temperature dependence may arise from temperature-induced shifts in the distribution of water vibrational states. Our study advances the current understanding of interface thermal conductance for a broad range of applications.

Evaporation of liquid nanofilms: A minireview

Evaporation of virus-loaded droplets and liquid nanofilms plays a significant role in the pandemic of COVID-19. The evaporation mechanism of liquid nanofilms has attracted much attention in recent decades. In this minireview, we first introduce the relationship between the evaporation process of liquid nanofilms and the pandemic of COVID-19. Then, we briefly provide the frontiers of liquid droplet/nanofilm evaporation on solid surfaces. In addition, we discuss the potential application of machine learning in liquid nanofilm evaporation studies, which is expected to be helpful to build up a more accurate molecular model and to investigate the evaporation mechanism of liquid nanofilms on solid surfaces.

Capture of acidic gas molecules in metallic nanopillar array surfaces

To effectively capture the acidic fluid molecules in industrial exhaust, this study employed molecular dynamics to simulate the dynamic adsorption behavior of a mixture of carbon monoxide (CO), carbon dioxide (CO₂), hydrogen sulfide (H₂S), and water (H₂O) molecules in gold (Au) nanoslits. We systematically examined the self-diffusion coefficient (D_Z), average adsorption energy (E_a,av), and static adsorption amount (N_sa) of individual ingredients and a mixture of the adsorbates under various temperatures (T), concentrations (c), and array slit widths (d). The simulation results indicate that Au(110) has better capture capabilities with regard to H₂O and H₂S, followed by CO₂ and then finally CO. Among the various slit structures, the design of array structures with slit widths 8.15 × 5.76 Å (case C) resulted in the highest average adsorption energy and static adsorption amount for all of the adsorbates. This is due to the fact that an appropriate slit width can increase the self-diffusion coefficient of the gas molecule and provide more stable adsorption sites to capture the adsorbates. Compared to the smooth surface structure, the nanopillar array structures significantly increased the self-diffusion coefficients and the adsorption energy of specific molecules. The comprehensive molecular model is helpful to predict atomistic-level adsorption behaviors for acidic gas molecules.

Photothermal behaviour of titanium nitride nanoparticles evaluated by transient X-ray diffraction

The photothermal properties of metal nitrides have recently received significant attention owing to diverse applications in solar energy conversion, photothermal therapies, photoreactions, and thermochromic windows. Here, the photothermal response of titanium nitride nanoparticles is examined using transient X-ray diffraction, in which optical excitation is synchronized with X-ray pulses to characterize dynamic changes in the TiN lattice. Photoinduced diffraction data is quantitatively analyzed to determine increases in the TiN lattice spacing, which are furthermore calibrated against static, temperature-dependent diffraction patterns of the same samples. Measurements of 20 nm and 50 nm diameter TiN nanoparticles reveal transient lattice heating from room temperature up to ∼175 °C for the highest pump fluences investigated here. Increasing excitation intensity drives sublinear increases in lattice temperature, due to increased heat capacity at the higher effective temperatures achieved at higher powers. Temporal dynamics show that higher excitation intensity drives not only higher lattice temperatures, but also unexpectedly slower cooling of the TiN nanoparticles, which is attributed to heating of the solvent proximal to the nanoparticle surface.

The role of polarizability in the interfacial thermal conductance at the gold-water interface

We have studied the interfacial thermal conductance, G, of the flat Au(111)-water interface using non-equilibrium molecular dynamics simulations. We utilized two metal models, one based on the embedded atom method (EAM) and the other including metallic polarizability via a density readjusting EAM. These were combined with three popular water models, SPC/E, TIP4P, and TIP4P-FQ, to understand the role of polarizability in the thermal transport process. A thermal flux was introduced using velocity shearing and scaling reverse non-equilibrium molecular dynamics, and transport coefficients were measured by calculating the resulting thermal gradients and temperature differences at the interface. Our primary finding is that the computed interfacial thermal conductance between a bare metal interface and water increases when polarizability is taken into account in the metal model. Additional work to understand the origin of the conductance difference points to changes in the local ordering of the water molecules in the first two layers of water above the metal surface. Vibrational densities of states on both sides of the interface exhibit interesting frequency modulation close to the surface but no obvious differences due to metal polarizability.

Core-Satellite Gold Nanoparticle Complexes Grown by Inert Gas-Phase Condensation

Spontaneous growth of complexes consisted of a number of individual nanoparticles in a controlled manner, particularly in demanding environments of gas-phase synthesis, is a fascinating opportunity for numerous potential applications. Here, we report the formation of such core-satellite gold nanoparticle structures grown by magnetron sputtering inert gas condensation. Combining high-resolution scanning transmission electron microscopy and computational simulations, we reveal the adhesive and screening role of H2O molecules in formation of stable complexes consisted of one nanoparticle surrounded by smaller satellites. A single layer of H2O molecules, condensed between large and small gold nanoparticles, stabilizes positioning of nanoparticles with respect to one another during milliseconds of the synthesis time. The lack of isolated small gold nanoparticles on the substrate is explained by Brownian motion that is significantly broader for small-size particles. It is inferred that H2O as an admixture in the inert gas condensation opens up possibilities of controlling the final configuration of the different noble metal nanoparticles.

Simulation of the interaction of a high energy laser beam with the sea surface in the short wavelength infrared

The knowledge of the interaction of a high energy laser beam with a dynamic sea surface is of great practical interest in maritime environments. The components transmitted into the sea and reflected at the sea surface have to be considered. The calculation of energy transfer into the sea is fundamental to the prediction of upper-ocean heating and temperature-dependent optical properties of the sea, which in turn influence its reflectance characteristics. In addition, the spatial energy (or power) distribution of the laser beam reflected at the dynamic sea surface is also of high significance. For the estimation of the laser light energy reflected into a specific spatial direction, several parameters need to be considered, e.g., wind speed, wind direction, and fetch. The calculated amount of light energy reflected into a specific direction varies statistically and depends largely on the dynamics of the wavy sea surface. A 3D simulation of a dynamic sea surface is presented interacting with a high energy laser beam in the short wavelength infrared spectral band. The simulation computes the upper-ocean heating, the temperature-dependent Fresnel reflectances, and the absorption in seawater considering the laser geometric configuration. For the reflectance calculations, a bistatic configuration of the laser source and receiver is regarded, where the receiver positions are on a virtual hemisphere having the laser spot center as the center point. The specular reflection of the laser beam at the sea surface is modeled by an analytical statistical bidirectional reflectance distribution function of the sea surface. The simulation is restricted to sea surfaces heated to the boiling point to avoid complex phase transition effects between water and gas. For a high energy laser beam focused on a small laser spot on the evolving wavy sea surface, the maximum expected reflected laser power is calculated for the specular forward- and back-reflection direction for glints. The probability of occurrence and temporal occurrence of those glint events is estimated for both directions.

Effects of nanoparticle heating on the structure of a concentrated aqueous salt solution

The effects of a rapidly heated nanoparticle on the structure of a concentrated aqueous salt solution are studied using molecular dynamics simulations. A diamond-like nanoparticle of radius 20 Å is immersed in a sodium-chloride solution at 20% above the experimental saturation concentration and equilibrated at T = 293 K and P = 1 atm. The nanoparticle is then rapidly heated to several thousand degrees Kelvin, and the system is held under isobaric-isoenthalpic conditions. It is observed that after 2-3 ns, the salt ions are depleted far more than water molecules from a proximal zone 15-25 Å from the nanoparticle surface. This leads to a transient reduction in molality in the proximal zone and an increase in ion clustering in the distal zone. At longer times, ions begin to diffuse back into the proximal zone. It is speculated that the formation of proximal and distal zones, and the increase in ion clustering, plays a role in the mechanism of nonphotochemical laser-induced nucleation.

Photoinduced nanobubble-driven superfast diffusion of nanoparticles imaged by 4D electron microscopy

Dynamics of active or propulsive Brownian particles in nonequilibrium status have recently attracted great interest in many fields including artificial micro/nanoscopic motors and biological entities. Understanding of their dynamics can provide insight into the statistical properties of physical and biological systems far from equilibrium. We report the translational dynamics of photon-activated gold nanoparticles (NPs) in water imaged by liquid-cell four-dimensional electron microscopy (4D-EM) with high spatiotemporal resolution. Under excitation of femtosecond laser pulses, we observed that those NPs exhibit superfast diffusive translation with a diffusion constant four to five orders of magnitude greater than that in the absence of laser excitation. The measured diffusion constant follows a power-law dependence on the laser fluence and a linear increase with the laser repetition rate, respectively. This superfast diffusion of the NPs is induced by a strong random driving force arising from the photoinduced steam nanobubbles (NBs) near the NP surface. In contrast, the NPs exhibit a superfast ballistic translation at a short time scale down to nanoseconds. Combining with a physical model simulation, this study reveals a photoinduced NB propulsion mechanism for propulsive motion, providing physical insights into better design of light-activated artificial micro/nanomotors. The liquid-cell 4D-EM also provides the potential of studying other numerical dynamical behaviors in their native environments.

Multiscale modeling of plasmonic enhanced energy transfer and cavitation around laser-excited nanoparticles

Nanoscale bubbles generated around laser-excited metallic nanoparticles are promising candidates for targeted drug and gene delivery in living cells. The development of new nanomaterials for efficient nanobubble-based therapy is however limited by the lack of reliable computational approaches for the prediction of their size and dynamics, due to the wide range of time and space scales involved. In this work, we present a multiscale modeling framework that segregates the various channels of plasmon de-excitation and energy transfer to describe the generation and dynamics of plasmonic nanobubbles. Detailed comparison with time-resolved shadowgraph imaging and spectroscopy data demonstrates that the bubble size, dynamics, and formation threshold can be quantitatively predicted for various types of nanostructures and irradiation parameters, with an error smaller than the experimental uncertainty. Our model in addition provides crucial physical insights into non-linear interactions in the near-field that should guide the experimental design of nanoplasmonic materials for nanobubble-based applications in nanomedicine.

Methods for Generation and Detection of Nonstationary Vapor Nanobubbles Around Plasmonic Nanoparticles

Laser pulse-induced vapor nanobubbles are nonstationary nanoevents that offer a broad range of applications, especially in the biomedical field. Plasmonic (usually gold) nanoparticles have the highest energy efficacy of the generation of vapor nanobubbles and such nanobubbles were historically named as plasmonic nanobubbles. Below we review methods (protocols) for generating and detecting plasmonic nanobubbles in liquids. The biomedical applications of plasmonic nanobubbles include in vivo and in vitro detection and imaging, gene transfer, micro-surgery, drug delivery, and other diagnostic, therapeutic, and theranostic applications.

Light Source Effects on Aerosol Photoacoustic Spectroscopy Measurements

Photoacoustic spectroscopy measurements of flame-generated soot aerosol coated with small amounts of water yielded absorption enhancements that were dependent on the laser used: quasi-continuous wave (Q-CW, ≈ 650 ps pulse duration and 78 MHz repetition rate) versus continuous wave (CW). Water coating thickness was controlled by exposing the aerosol to a set relative humidity (RH). At ≈ 85 % RH, the mass of the soot particles increased by an amount comparable to a monolayer of water being deposited and enhanced the measured absorption by 36 % and 15 % for the Q-CW and CW lasers, respectively. Extinction measurements were also performed using a cavity ring-down spectrometer (extinction equals the sum of absorption and scattering) with a CW laser and negligible enhancement was observed at all RH. These findings demonstrate that source choice can impact measurements of aerosols with volatile coatings and that the absorption enhancements at high RH previously measured by Radney and Zangmeister (2015) [1] are the result of laser source used (Q-CW) and not from an increase in the particle absorption cross section.

Exploring the phase explosion of water using SOM-mediated micro-bubbles

We study the growth of a homogeneously nucleated micro-bubble amidst convective flows in water superheated close to the critical point.

Substrate-Mediated Laser Ablation under Ambient Conditions for Spatially-Resolved Tissue Proteomics

Numerous applications of ambient Mass Spectrometry (MS) have been demonstrated over the past decade. They promoted the emergence of various micro-sampling techniques such as Laser Ablation/Droplet Capture (LADC). LADC consists in the ablation of analytes from a surface and their subsequent capture in a solvent droplet which can then be analyzed by MS. LADC is thus generally performed in the UV or IR range, using a wavelength at which analytes or the matrix absorb. In this work, we explore the potential of visible range LADC (532 nm) as a micro-sampling technology for large-scale proteomics analyses. We demonstrate that biomolecule analyses using 532 nm LADC are possible, despite the low absorbance of biomolecules at this wavelength. This is due to the preponderance of an indirect substrate-mediated ablation mechanism at low laser energy which contrasts with the conventional direct ablation driven by sample absorption. Using our custom LADC system and taking advantage of this substrate-mediated ablation mechanism, we were able to perform large-scale proteomic analyses of micro-sampled tissue sections and demonstrated the possible identification of proteins with relevant biological functions. Consequently, the 532 nm LADC technique offers a new tool for biological and clinical applications.

Oscillatory Dynamics and In Vivo Photoacoustic Imaging Performance of Plasmonic Nanoparticle-Coated Microbubbles

Microbubbles bearing plasmonic nanoparticles on their surface provide contrast enhancement for both photoacoustic and ultrasound imaging. In this work, the responses of microbubbles with surface-bound gold nanorods-termed AuMBs-to nanosecond pulsed laser excitation are studied using high-speed microscopy, photoacoustic imaging, and numerical modeling. In response to laser fluences below 5 mJ cm(-2) , AuMBs produce weak photoacoustic emissions and exhibit negligible microbubble wall motion. However, in reponse to fluences above 5 mJ cm(-2) , AuMBs undergo dramatically increased thermal expansion and emit nonlinear photoacoustic waves of over 10-fold greater amplitude than would be expected from freely dispersed gold nanorods. Numerical modeling suggests that AuMB photoacoustic responses to low laser fluences result from conductive heat transfer from the surface-bound nanorods to the microbubble gas core, whereas at higher fluences, explosive boiling may occur at the nanorod surface, producing vapor nanobubbles that contribute to rapid AuMB expansion. The results of this study indicate that AuMBs are capable of producing acoustic emissions of significantly higher amplitude than those produced by conventional sources of photoacoustic contrast. In vivo imaging performance of AuMBs in a murine kidney model suggests that AuMBs may be an effective alternative to existing contrast agents for noninvasive photoacoustic and ultrasound imaging applications.

Molecular dynamics investigation of separation of hydrogen sulfide from acidic gas mixtures inside metal-doped graphite micropores

Separation of hydrogen sulfide from acidic gas mixtures is demonstrated using metal-doped graphene micropores via a prototype model.

Directed assembly of nanodiamond nitrogen-vacancy centers on a chemically modified patterned surface

Nitrogen-vacancy (NV) centers in nanodiamond (ND) particles are an attractive material for photonic, quantum information, and biological sensing technologies due to their optical properties-bright single photon emission and long spin coherence time. To harness these features in practical devices, it is essential to realize efficient methods to assemble and pattern NDs at the micro-/nanoscale. In this work, we report the large scale patterned assembly of NDs on a Au surface by creating hydrophobic and hydrophilic regions using self-assembled monolayer (SAM). Hydrophobic regions are created using a methyl (-CH3) terminated SAM of octadecanethiol molecules. Evaporating a water droplet suspension of NDs on the SAM patterned surface assembles the NDs in the bare Au, hydrophilic regions. Using this procedure, we successfully produced a ND structures in the shape of dots, lines, and rectangles. Subsequent photoluminescence imaging of the patterned NDs confirmed the presence of optically active NV centers. Experimental evidence in conjunction with computational analysis indicates that the surface wettability of the SAM modified Au surface plays a dominant role in the assembly of NDs as compared to van der Waals and other substrate-ND interactions.

Laser pulse duration is critical for the generation of plasmonic nanobubbles

Plasmonic nanobubbles (PNBs) are transient vapor nanobubbles generated in liquid around laser-overheated plasmonic nanoparticles. Unlike plasmonic nanoparticles, PNBs' properties are still largely unknown due to their highly nonstationary nature. Here we show the influence of the duration of the optical excitation on the energy efficacy and threshold of PNB generation. The combination of picosecond pulsed excitation with the nanoparticle clustering provides the highest energy efficacy and the lowest threshold fluence, around 5 mJ cm(-2), of PNB generation. In contrast, long excitation pulses reduce the energy efficacy of PNB generation by several orders of magnitude. Ultimately, the continuous excitation has the minimal energy efficacy, nine orders of magnitude lower than that for the picosecond excitation. Thus, the duration of the optical excitation of plasmonic nanoparticles can have a stronger effect on the PNB generation than the excitation wavelength, nanoparticle size, shape, or other "stationary" properties of plasmonic nanoparticles.

Transient enhancement and spectral narrowing of the photothermal effect of plasmonic nanoparticles under pulsed excitation

The transient 100-fold enhancement and spectral narrowing to 2 nm of the photothermal conversion by solid gold nanospheres under near-infrared excitation with a short laser pulse is reported. This non-stationary effect was observed for a wide range of optical fluences starting from 10 mJ cm(-2) for single nanospheres, their ensembles and aggregated clusters in water, in vitro and in vivo.

Desolvation of macromolecules by ultrafast heating: A molecular-dynamics study

Using molecular-dynamics simulation, we investigate the consequences of ultrafast laser-induced heating of a water droplet containing a solvated polymer, using the example of a 1 ps laser irradiation. We study the isolation process and the properties of the isolated polymer as a function of the polymer size, the droplet size, and the temperature to which the droplet is heated. We find that the isolation process occurs on a time scale of a few ten ps. The final polymer temperature increases linearly with the heating. Polymers embedded in larger droplets acquire higher temperatures, while larger polymers are less heated. In spite of the ultrafast heating, the isolated polymer remains in its coiled conformation.

Transient photothermal spectra of plasmonic nanobubbles

The photothermal efficacy of near-infrared gold nanoparticles (NP), nanoshells, and nanorods was studied under pulsed high-energy optical excitation in plasmonic nanobubble (PNB) mode as a function of the wavelength and duration of the excitation laser pulse. PNBs, transient vapor nanobubbles, were generated around individual and clustered overheated NPs in water and living cells. Transient PNBs showed two photothermal features not previously observed for NPs: the narrowing of the spectral peaks to 1 nm and the strong dependence of the photothermal efficacy upon the duration of the laser pulse. Narrow red-shifted (relative to those of NPs) near-infrared spectral peaks were observed for 70 ps excitation laser pulses, while longer sub- and nanosecond pulses completely suppressed near-infrared peaks and blue shifted the PNB generation to the visual range. Thus, PNBs can provide superior spectral selectivity over gold NPs under specific optical excitation conditions.

The Langevin Hull: Constant pressure and temperature dynamics for non-periodic systems

We have developed a new isobaric-isothermal (NPT) algorithm which applies an external pressure to the facets comprising the convex hull surrounding the system. A Langevin thermostat is also applied to the facets to mimic contact with an external heat bath. This new method, the "Langevin Hull", can handle heterogeneous mixtures of materials with different compressibilities. These systems are problematic for traditional affine transform methods. The Langevin Hull does not suffer from the edge effects of boundary potential methods, and allows realistic treatment of both external pressure and thermal conductivity due to the presence of an implicit solvent. We apply this method to several different systems including bare metal nanoparticles, nanoparticles in an explicit solvent, as well as clusters of liquid water. The predicted mechanical properties of these systems are in good agreement with experimental data and previous simulation work.

Plasmonic nanobubbles as transient vapor nanobubbles generated around plasmonic nanoparticles

We have used short laser pulses to generate transient vapor nanobubbles around plasmonic nanoparticles. The photothermal, mechanical, and optical properties of such bubbles were found to be different from those of plasmonic nanoparticle and vapor bubbles, as well. This phenomenon was considered as a new complex nanosystem-plasmonic nanobubble (PNB). Mechanical and optical scattering properties of PNB depended upon the nanoparticle surface and heat capacity, clusterization state, and the optical pulse length. The generation of the PNB required much higher laser pulse fluence thresholds than the explosive boiling level and was characterized by the relatively high lower threshold of the minimal size (lifetime) of PNB. Optical scattering by PNB and its diameter (measured as the lifetime) has been varied with the fluence of laser pulse, and this has demonstrated the tunable nature of PNB.

Evaporation of solvent molecules by ultrafast heating: effect on conformation of solvated protein

Using molecular dynamics simulation, we compare two cases of ultrafast heating of a small water droplet containing a solvated protein (echistatin). If the water temperature after irradiation is above the critical temperature, explosive boiling liberates the protein within some 10 ps of its hydration shell, while its temperature remains relatively low. By comparing with the case where the water shell is heated to the same final temperature, but without complete evaporation, we demonstrate that the protein conformation is governed by the hydration shell rather than by the protein temperature.

Photothermal Properties of Gold Nanoparticles

This paper describes our recent time-resolved spectroscopy studies of the properties of gold particles at high laser excitation levels. In these experiments, an intense pump laser pulse rapidly heats the particle, creating very high lattice temperatures — up to the melting point of bulk gold. These high temperatures can have dramatic effects on the particle and the surroundings. The lattice temperature created is determined by observing the coherently excited the vibrational modes of the particles. The periods of these modes depend on temperature, thus, they act as an internal thermometer. We have used these experiments to provide values for the threshold temperatures for explosive boiling of the solvent surrounding the particles, and laser induced structural transformations in non-spherical particles. The results of these experiments are relevant to the use of metal nanoparticles in photothermal therapy, where laser induced heating is used to selectively kill cells.

Heat transfer from nanoparticles: a corresponding state analysis

In this contribution, we study situations in which nanoparticles in a fluid are strongly heated, generating high heat fluxes. This situation is relevant to experiments in which a fluid is locally heated by using selective absorption of radiation by solid particles. We first study this situation for different types of molecular interactions, using models for gold particles suspended in octane and in water. As already reported in experiments, very high heat fluxes and temperature elevations (leading eventually to particle destruction) can be observed in such situations. We show that a very simple modeling based on Lennard-Jones (LJ) interactions captures the essential features of such experiments and that the results for various liquids can be mapped onto the LJ case, provided a physically justified (corresponding state) choice of parameters is made. Physically, the possibility of sustaining very high heat fluxes is related to the strong curvature of the interface that inhibits the formation of an insulating vapor film.