Plasmonic Bubble Nucleation and Growth in Water: Effect of Dissolved Air

Submegahertz Nucleation of Plasmonic Vapor Microbubbles near a Solid Vertical Boundary

Laser triggered and photothermally induced vapor bubbles have emerged as promising approaches to facilitate optomechanical energy conversion for numerous applications in microfluidics and nanofluidics. Here, we report an observation of spontaneously triggered periodic nucleation of plasmonic vapor bubbles near a rigid sidewall with readily tuned nucleation frequency from 0.8 kHz to over 200 kHz. The detailed collapsing process of the vapor bubbles was experimentally and numerically investigated. We find that the lateral migration of residual bubbles toward the sidewall refreshes the laser spot area, terminates the subsequent steady bubble growth, and leads to the repeatable bubble nucleation. A mathematic model regarding the Kelvin impulses was derived. It shows that the competition between the rigid boundary induced Bjerknes force and laser irradiation caused thermal Marangoni force on collapsing bubbles governs the process. The model also leads to a criterion of γζ<0.34 for repeatable bubble nucleation, where γ is the normalized distance and ζ thermal Marangoni coefficient. This study demonstrates nucleation of violent vapor bubbles at extreme high frequencies, providing an approach to remotely realize strong localized flows in microfluidics and nanofluidics.

Roles of vaporization and thermal decomposition in the dynamic evolution of laser-induced bubble on the surface of a submerged metal plate

This paper aims to explain when the vaporization or thermal decomposition prevails during laser-induced bubble growth and how they influence bubble morphology. Bubbles were generated by irradiating a 304 stainless steel plate submerged in degassed water using millisecond lasers with a pulse width of 0.4 ms and powers of 1.6 kW and 3.2 kW, respectively. The dynamic evolution of bubbles was recorded by a high-speed camera. Moreover, the numerical models were developed to obtain a vaporization model and a decomposition model by incorporating the source terms due to the vaporization and decomposition mass fluxes into the governing equations, respectively. The simulated dynamic bubble evolution is consistent with the experimental results. When the laser power is 1.6 kW, a thin-layer bubble is formed, which gradually shrinks and eventually disappears after the laser stops irradiating. When the laser power is 3.2 kW, a spherical bubble is formed, and its volume decreases significantly after the laser stops irradiating. Subsequently, it remains relatively stable during the observation period. The fundamental reason for the difference between the bubble morphologies obtained from the vaporization model and the decomposition model lies in the presence of a condensation zone in the gas phase. When water vaporization or thermal decomposition dominates, the temperatures obtained from the models align with the decomposition ratios at varying temperatures reported in the literature. Our findings are significant for understanding the dynamic behavior of bubbles, with implications for various laser processing underwater.

Vapor Nanobubbles around Heated Nanoparticles: Wetting Dependence of the Local Fluid Thermodynamics and Kinetics of Nucleation

Plasmonic nanobubbles are composite objects resulting from the interaction between light and metallic nanoparticles immersed in a fluid. Plasmonic nanobubbles have applications in photothermal therapies, drug delivery, microfluidic manipulations, and solar energy conversion. Their early formation is, however, barely characterized due to the short time and length scales relevant to the process. Here, we investigate, using molecular dynamics (MD) simulations, the effect of nanoparticle wettability on both the local fluid thermodynamics and the kinetics of nanobubble generation in water. We first show that the local onset temperature of vapor nucleation decreases with the nanoparticle/water interfacial energy and may be 100 K below the water spinodal temperature in the case of weak nanoparticle/water interactions. Second, we demonstrate that vapor nucleation may be slower in the case of weak water/nanoparticle interactions. This result, which is qualitatively at odds with the predictions of isothermal classical nucleation theory, may be explained by the competition between two antagonist effects: while, classically, hydrophobicity increases the vapor nucleation rate, it also penalizes interfacial thermal transfer, slowing down kinetics. The kinetics of heat transfer from the nanoparticle to water is controlled by the interfacial thermal conductance. This quantity turns out not only to decrease with the nanoparticle hydrophobicity but also drops down prior to phase change, yielding even longer nucleation times. Such conclusions were reached by considering the comparison between MD and continuous heat transfer models. These results put forward the role of nanoparticle wettability in the generation of plasmonic nanobubbles observed experimentally and open the path to the control of boiling using nanopatterned surfaces.

Laser Controlled Manipulation of Microbubbles on a Surface with Silica-Coated Gold Nanoparticle Array

Microbubble generation and manipulation play critical roles in diverse applications such as microfluidic mixing, pumping, and microrobot propulsion. However, existing methods are typically limited to lateral movements on customized substrates or rely on specific liquids with particular properties or designed concentration gradients, thereby hindering their practical applications. To address this challenge, this paper presents a method that enables robust vertical manipulation of microbubbles. By focusing a resonant laser on hydrophilic silica-coated gold nanoparticle arrays immersed in water, plasmonic microbubbles are generated and detach from the substrates immediately upon cessation of laser irradiation. Using simple laser pulse control, it can achieve an adjustable size and frequency of bubble bouncing, which is governed by the movement of the three-phase contact line during surface wetting. Furthermore, it demonstrates that rising bubbles can be pulled back by laser irradiation induced thermal Marangoni flow, which is verified by particle image velocimetry measurements and numerical simulations. This study provides novel insights into flexible bubble manipulation and integration in microfluidics, with significant implications for various applications including mixing, drug delivery, and the development of soft actuators.

Transition in the growth mode of plasmonic bubbles in binary liquids

Multi-component fluids with phase transitions show a plethora of fascinating phenomena with rich physics. Here we report on a transition in the growth mode of plasmonic bubbles in binary liquids. By employing high-speed imaging we reveal that the transition is from slow evaporative to fast convective growth and accompanied by a sudden increase in radius. The transition occurs as the three-phase contact line reaches the spinodal temperature of the more volatile component leading to massive, selective evaporation. This creates a strong solutal Marangoni flow along the bubble which marks the beginning of convective growth. We support this interpretation by simulations. After the transition the bubble starts to oscillate in position and in shape. Though different in magnitude the frequencies of both oscillations follow the same power law , which is characteristic of bubble shape oscillations, with the surface tension σ as the restoring force and the bubble's added mass as inertia. The transitions and the oscillations both induce a strong motion in the surrounding liquid, opening doors for various applications where local mixing is beneficial.

Thermally Assisted Heterogeneous Cavitation through Gas Supersaturation

We demonstrate that besides gaseous pockets also a gas supersaturated spot on a substrate can be a nucleus for cavitation. The supersaturation is achieved by either a formerly dissolved bubble or by heating locally the surface below the boiling temperature. The experiments are conducted in a thin film of water; one side of the water film is in contact with a gold coated substrate that is heated by a continuous laser through plasmonic heating. For nucleation of a bubble, the pressure at the heated spot is reduced by a transient rarefaction wave. The experimental findings suggest that the local gas supersaturation is responsible to nucleate cavitation and thus connects the phase transitions of cavitation and boiling. Additionally, the pressure waves in the liquid gap are studied numerically.

The effect of preparation time and aeration rate on the properties of bulk micro-nanobubble water using hydrodynamic cavitation

Fundamental research on bulk micro-nanobubbles (BMNBs) has grown rapidly due to the demand for their industrial applications and potential role in interfacial sciences. This work focuses on examining properties of such bubbles, including the number, concentration, zeta potential, and surface tension in water. For this purpose, BMNBs were generated by the hydrodynamic cavitation (HC) mechanism. Distilled water and air in the experiments were the liquid and gas phases, respectively. The characterization of bulk microbubbles (BMBs) and bulk nanobubbles (BNBs) were performed through focused beam reflectance measurement (FBRM) and nanoparticle tracking analysis (NTA) techniques, respectively. Zeta potential and surface tension of aqueous solutions were measured at different time and aeration rates. The results showed that aeration rate and preparation time had an important role in the properties of BNBs (concentration, bubble size, and surface charge) and BMBs (number, and bubble size). The instability of BMBs led to the rapid changes in the dissolved oxygen (DO) content in the water. The number of BMBs decreased when preparation time and aeration rate increased, but their size remained constant. By enhancing the preparation time and aeration rate, the concentration of BNBs improved first and then reduced. Additionally, the surface tension of an aqueous solution containing BNBs was significantly lower than that of pure water.

Heat-Mediated Optical Manipulation

Progress in optical manipulation has stimulated remarkable advances in a wide range of fields, including materials science, robotics, medical engineering, and nanotechnology. This Review focuses on an emerging class of optical manipulation techniques, termed heat-mediated optical manipulation. In comparison to conventional optical tweezers that rely on a tightly focused laser beam to trap objects, heat-mediated optical manipulation techniques exploit tailorable optothermo-matter interactions and rich mass transport dynamics to enable versatile control of matter of various compositions, shapes, and sizes. In addition to conventional tweezing, more distinct manipulation modes, including optothermal pulling, nudging, rotating, swimming, oscillating, and walking, have been demonstrated to enhance the functionalities using simple and low-power optics. We start with an introduction to basic physics involved in heat-mediated optical manipulation, highlighting major working mechanisms underpinning a variety of manipulation techniques. Next, we categorize the heat-mediated optical manipulation techniques based on different working mechanisms and discuss working modes, capabilities, and applications for each technique. We conclude this Review with our outlook on current challenges and future opportunities in this rapidly evolving field of heat-mediated optical manipulation.

Droplet plume emission during plasmonic bubble growth in ternary liquids

Plasmonic bubbles are of great relevance in numerous applications, including catalytic reactions, micro/nanomanipulation of molecules or particles dispersed in liquids, and cancer therapeutics. So far, studies have been focused on bubble nucleation in pure liquids. Here we investigate plasmonic bubble nucleation in ternary liquids consisting of ethanol, water, and trans-anethole oil, which can show the so-called ouzo effect. We find that oil (trans-anethole) droplet plumes are produced around the growing plasmonic bubbles. The nucleation of the microdroplets and their organization in droplet plumes is due to the symmetry breaking of the ethanol concentration field during the selective evaporation of ethanol from the surrounding ternary liquids into the growing plasmonic bubbles. Numerical simulations show the existence of a critical Marangoni number Ma (the ratio between solutal advection rate and the diffusion rate), above which the symmetry breaking of the ethanol concentration field occurs, leading to the emission of the droplet plumes. The numerical results agree with the experimental observation that more plumes are emitted with increasing ethanol-water relative weight ratios and hence Ma. Our findings on the droplet plume formation reveal the rich phenomena of plasmonic bubble nucleation in multicomponent liquids and help to pave the way to achieve enhanced mixing in multicomponent liquids in chemical, pharmaceutical, and cosmetic industries.

Optically Driven Gold Nanoparticles Seed Surface Bubble Nucleation in Plasmonic Suspension

Photothermal surface bubbles play important roles in applications like microfluidics and biosensing, but their formation on transparent substrates immersed in a plasmonic nanoparticle (NP) suspension has an unknown origin. Here, we reveal NPs deposited on the transparent substrate by optical forces are responsible for the nucleation of such photothermal surface bubbles. We show the surface bubble formation is always preceded by the optically driven NPs moving toward and deposited to the surface. Interestingly, such optically driven motion can happen both along and against the photon stream. The laser power density thresholds to form a surface bubble drastically differ depending on if the surface is forward- or backward-facing the light propagation direction. We attributed this to different optical power densities needed to enable optical pulling and pushing of NPs in the suspension, as optical pulling requires higher light intensity to excite supercavitation around NPs to enable proper optical configuration.

Periodic bouncing of a plasmonic bubble in a binary liquid by competing solutal and thermal Marangoni forces

The physicochemical hydrodynamics of bubbles and droplets out of equilibrium, in particular with phase transitions, display surprisingly rich and often counterintuitive phenomena. Here we experimentally and theoretically study the nucleation and early evolution of plasmonic bubbles in a binary liquid consisting of water and ethanol. Remarkably, the submillimeter plasmonic bubble is found to be periodically attracted to and repelled from the nanoparticle-decorated substrate, with frequencies of around a few kilohertz. We identify the competition between solutal and thermal Marangoni forces as the origin of the periodic bouncing. The former arises due to the selective vaporization of ethanol at the substrate's side of the bubble, leading to a solutal Marangoni flow toward the hot substrate, which pushes the bubble away. The latter arises due to the temperature gradient across the bubble, leading to a thermal Marangoni flow away from the substrate, which sucks the bubble toward it. We study the dependence of the frequency of the bouncing phenomenon from the control parameters of the system, namely the ethanol fraction and the laser power for the plasmonic heating. Our findings can be generalized to boiling and electrolytically or catalytically generated bubbles in multicomponent liquids.

Laser photonic nanojets triggered thermoplasmonic micro/nanofabrication of polymer materials for enhanced resolution

Micro/nanofabrication of polymer materials is of interest for micro/nanofluidic systems. Due to the optical diffraction limit, it remains a challenge to achieve nanoscale resolution fabrication using an ordinary continuous-wave laser system. In this study, we therefore propose a laser photonic nanojet-based micro/nanofabrication method for polymer materials using a low-power and low-cost continuous-wave laser. The photonic nanojets were produced using glass microspheres. Moreover, a thermoplasmonic effect was employed by depositing a gold layer beneath the polymer films. By applying the photonic nanojet triggered thermoplasmonics, sub-micrometer surface structures, as well as their arrays, were fabricated with a laser power threshold value down to 10 mW. The influences of the microsphere diameters, and thicknesses of gold layers and polymer films on the fabricated microstructures were systematically investigated, which aligns well with the finite-difference time-domain simulation results.

Plasmonic nano-antimicrobials: properties, mechanisms and applications in microbe inactivation and sensing

Bacterial, viral and fungal infections pose serious threats to human health and well-being. The continuous emergence of acute infectious diseases caused by pathogenic microbes and the rapid development of resistances against conventional antimicrobial drugs necessitates the development of new and effective strategies for the safe elimination of microbes in water, food or on surfaces, as well as for the inactivation of pathogenic microbes in human hosts. The need for new antimicrobials has triggered the development of plasmonic nano-antimicrobials that facilitate both light-dependent and -independent microbe inactivation mechanisms. This review introduces the relevant photophysical mechanisms underlying these plasmonic nano-antimicrobials, and provides an overview of how the photoresponses and materials properties of plasmonic nanostructures can be applied in microbial pathogen inactivation and sensing applications. Through a systematic analysis of the inactivation efficacies of different plasmonic nanostructures, this review outlines the current state-of-the-art in plasmonic nano-antimicrobials and defines the application space for different microbial inactivation strategies. The advantageous optical properties of plasmonic nano-antimicrobials also enhance microbial detection and sensing modalities and thus help to avoid exposure to microbial pathogens. Sensitive and fast plasmonic microbial sensing modalities and their theranostic and targeted therapeutic applications are discussed.

Strong Transient Flows Generated by Thermoplasmonic Bubble Nucleation

The challenge of inducing and controlling localized fluid flows for generic force actuation and for achieving efficient mass transport in microfluidics is key to the development of next-generation miniaturized systems for chemistry and life sciences. Here we demonstrate a methodology for the robust generation and precise quantification of extremely strong flow transients driven by vapor bubble nucleation on spatially isolated plasmonic nanoantennas excited by light. The system is capable of producing peak flow speeds of the order mm/s at modulation rates up to ∼100 Hz in water, thus allowing for a variety of high-throughput applications. Analysis of flow dynamics and fluid viscosity dependence indicates that the transient originates in the rapid bubble expansion that follows nucleation rather than being strictly thermocapillary in nature.

Giant plasmonic bubbles nucleation under different ambient pressures

Water-immersed gold nanoparticles irradiated by a laser can trigger the nucleation of plasmonic bubbles after a delay time of a few microseconds [Wang et al., Proc. Natl. Acad. Sci. USA 122, 9253 (2018)]. Here we systematically investigated the light-vapor conversion efficiency, η, of these plasmonic bubbles as a function of the ambient pressure. The efficiency of the formation of these initial-phase and mainly water-vapor containing bubbles, which is defined as the ratio of the energy that is required to form the vapor bubbles and the total energy dumped in the gold nanoparticles before nucleation of the bubble by the laser, can be as high as 25%. The amount of vaporized water first scales linearly with the total laser energy dumped in the gold nanoparticles before nucleation, but for larger energies the amount of vaporized water levels off. The efficiency η decreases with increasing ambient pressure. The experimental observations can be quantitatively understood within a theoretical framework based on the thermal diffusion equation and the thermal dynamics of the phase transition.

Plasmonic Microbubble Dynamics in Binary Liquids

The growth of surface plasmonic microbubbles in binary water/ethanol solutions is experimentally studied. The microbubbles are generated by illuminating a gold nanoparticle array with a continuous wave laser. Plasmonic bubbles exhibit ethanol concentration-dependent behaviors. For low ethanol concentrations (f_e) of ≲67.5%, bubbles do not exist at the solid-liquid interface. For high f_e values of ≳80%, the bubbles behave as in pure ethanol. Only in an intermediate window of 67.5% ≲ f_e ≲ 80% do we find sessile plasmonic bubbles with a highly nontrivial temporal evolution, in which as a function of time three phases can be discerned. (1) In the first phase, the microbubbles grow, while wiggling. (2) As soon as the wiggling stops, the microbubbles enter the second phase in which they suddenly shrink, followed by (3) a steady reentrant growth phase. Our experiments reveal that the sudden shrinkage of the microbubbles in the second regime is caused by a depinning event of the three-phase contact line. We systematically vary the ethanol concentration, laser power, and laser spot size to unravel water recondensation as the underlying mechanism of the sudden bubble shrinkage in phase 2.

Generation and Evolution of Nanobubbles on Heated Nanoparticles: A Molecular Dynamics Study

Molecular dynamics simulations were conducted to investigate the generation and evolution of nanobubbles on heated gold-like nanoparticles (GNPs). The effects of surface wettability (β) and heating intensity (Q) of the GNPs are studied. We found that nanobubbles are generated faster on the superhydrophobic GNP than on the superhydrophilic GNP where nanobubble formation appears after a delay. In the case of the superhydrophilic GNP, the nanobubble is observed to grow explosively because it is initially generated at a distance from the GNP surface instead of on its surface. In the case of the superhydrophobic GNP, the faster generation of the nanobubble is promoted by the larger temperature difference between the GNP and the surrounding fluid and an ultrathin low-density layer that exists before the GNP is heated. For a given β, faster generation and growth of nanobubbles are observed with increasing Q. Furthermore, the maximum radius of the nanobubble is found to be dependent on β and not Q. The mechanism is elaborated based on the thermal resistance analysis at the melting point of GNPs. Additionally, it was found that there exists a threshold Q for nanobubble generation and the threshold value for the case of the superhydrophobic GNP is lower than that for the case of the superhydrophilic GNP. The present results have demonstrated that the superhydrophobic GNP is favorable for fast and energy-saving nanobubble generation. Our work provides further understanding in the generation and evolution of nanobubbles and potentially offers a new insight for nanobubble manipulation.

Ultrafast Modulation of Thermoplasmonic Nanobubbles in Water

Thermo-optically generated bubbles in water provide a powerful means for active matter control in microfluidic environments. These bubbles are often formed via continuous-wave illumination of an absorbing medium resulting in bubble nucleation via vaporization of water and subsequent bubble growth from the inward diffusion of gas molecules. However, to date, such bubbles tend to be several microns in diameter, resulting in slow dissipation. This limits the dynamic rate, spatial precision, and throughput of operation in any application. Here we show that isolated plasmonic structures can be utilized as highly localized heating elements to generate thermoplasmonic nanobubbles that can be modulated at frequencies up to several kilohertz in water, orders of magnitude faster than previously demonstrated for microbubbles. The nanobubbles are envisioned as advantageous localized active manipulation elements for high throughput microfluidic applications.