Stationary bubble formation and Marangoni convection induced by CW laser heating of a single gold nanoparticle

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.

Directed Assembly of Nanomaterials for Making Nanoscale Devices and Structures: Mechanisms and Applications

Nanofabrication has been utilized to manufacture one-, two-, and three-dimensional functional nanostructures for applications such as electronics, sensors, and photonic devices. Although conventional silicon-based nanofabrication (top-down approach) has developed into a technique with extremely high precision and integration density, nanofabrication based on directed assembly (bottom-up approach) is attracting more interest recently owing to its low cost and the advantages of additive manufacturing. Directed assembly is a process that utilizes external fields to directly interact with nanoelements (nanoparticles, 2D nanomaterials, nanotubes, nanowires, etc.) and drive the nanoelements to site-selectively assemble in patterned areas on substrates to form functional structures. Directed assembly processes can be divided into four different categories depending on the external fields: electric field-directed assembly, fluidic flow-directed assembly, magnetic field-directed assembly, and optical field-directed assembly. In this review, we summarize recent progress utilizing these four processes and address how these directed assembly processes harness the external fields, the underlying mechanism of how the external fields interact with the nanoelements, and the advantages and drawbacks of utilizing each method. Finally, we discuss applications made using directed assembly and provide a perspective on the future developments and challenges.

Optical gradient force on chiral particles

When a chiral nanoparticle is optically trapped using a circularly polarized laser beam, a circular polarization (CP)–dependent gradient force can be induced on the particle. We investigated the CP-dependent gradient force exerted on three-dimensional chiral nanoparticles. The experimental results showed that the gradient force depended on the handedness of the CP of the trapping light and the particle chirality. The analysis revealed that the spectral features of the CP handedness–dependent gradient force are influenced not only by the real part of the refractive index but also by the electromagnetic field perturbed by the chiral particle resonant with the incident light. This is in sharp contrast to the well-known behavior of the gradient force, which is governed by the real part of the refractive index. The extended aspect of the chiral optical force obtained here can provide novel methodologies on chirality sensing, manipulation, separation, enantioselective biological reactions, and other fields.

Plasmonic nanoscale temperature shaping on a single titanium nitride nanostructure

Arbitrary shaping of temperature fields at the nanometre scale is an important goal in nanotechnology; however, this is challenging because of the diffusive nature of heat transfer. In the present work, we numerically demonstrated that spatial shaping of nanoscale temperature fields can be achieved by plasmonic heating of a single titanium nitride (TiN) nanostructure. A key feature of TiN is its low thermal conductivity (k_TiN = 29 [W m^-1 K^-1]) compared with ordinary plasmonic metals such as Au (k_Au = 314 [W m^-1 K^-1]). When the localised surface plasmon resonance of a metal nanostructure is excited, the light intensity is converted to heat power density in the nanostructure via the Joule heating effect. For a gold nanoparticle, non-uniform spatial distributions of the heat power density will disappear because of the high thermal conductivity of Au; the nanoparticle surface will be entirely isothermal. In contrast, the spatial distributions of the heat power density can be clearly transcribed into temperature fields on a TiN nanostructure because the heat dissipation is suppressed. In fact, we revealed that highly localised temperature distributions can be selectively controlled around the TiN nanostructure at a spatial resolution of several tens of nanometres depending on the excitation wavelength. The present results indicate that arbitrary temperature shaping at the nanometre scale can be achieved by designing the heat power density in TiN nanostructures for plasmonic heating, leading to unconventional thermofluidics and thermal chemical biology.

Formation of a Giant Anisotropically Ordered Assembled Structure of Inorganic Nanosheets through an Optically Induced Stream

Formation of a desirable submillimeter-scaled assembled structure of particles in the colloid is a difficult subject in colloidal chemistry. Herein, a submillimeter-scaled ordered assembled structure consisting of highly anisotropic two-dimensional plate-like particles, niobate nanosheets, was obtained through an optical manipulation technique that was assisted by a scattering-force-induced stream. A 532 nm continuous wave laser beam with a power of 400 mW was used to illuminate a liquid crystalline niobate nanosheet colloid from the bottom side of a sample cell, inducing the stream of oriented nanosheets toward the upper side of the sample cell. As a result, a 200 μm ordered assembled structure consisting of oriented nanosheets was formed. The assembled structure was also characterized by two-dimensional anisotropy, reflecting that the highly anisotropic morphologies of each nanosheet and the shape of that structure were dependent on the polarization of incident illumination. This study has revealed a new noncontact and on-demand way to obtain submillimeter-scaled ordered anisotropic colloidal assembled structures of nanosized particles such as nanosheets, contributing to fundamental materials science and expanding the utilities of nanosheets.

Interfacial Flows and Interfacial Shape Modulation Controlled by the Thermal Action of Light Energy

The review covers the research on thermocapillary convection caused by the thermal action of laser radiation in single-layer and bilayer liquid systems of capillary thickness. The advantages of using optical radiation are the instantaneous delivery of thermal energy to a place on demand (a bulk phase, interfaces); low radiation power required; concentrating heat flux on a spot of a few micrometers; the production of arbitrary spatial distributions of radiation intensity; and, as a result, corresponding thermal fields at a liquid interface and their fast reconfiguration. Thermocapillary stresses at the liquid interfaces lead to the transfer of the liquid and a change in the shape of the interface, in accordance with the distribution of the light-induced thermal field. Studies concerned with the methods of non-destructive testing of liquid media and solids, which are based on a photothermocapillary signal emitted by a laser-induced concave deformation of a thin layer, are considered. Features of thermocapillary deformation of a liquid–air interface caused by local heating of thin and thick (exceeding the capillary length) layers are demonstrated. A part of the review addresses the results of the study of thermocapillary rupture of films in the heating zone and the application of this effect in semiconductor electronics and high-resolution lithography. The works on the light-induced thermocapillary effect in bilayer (multilayer) liquid systems are analyzed, including early works on image recording liquid layer systems, liquid IR transducers, and nonlinear optical media.

A Molecular Dynamics Analysis on Interfacial Thermal Resistance between Particle and Medium in Light-Induced Heat Transfer of Plasmonic Nanofluid

Light-induced heat transfer process of plasmonic nanofluids is critical for many applications, but the energy conversion pathway still remains controversial. In this work, we develop a calculation model based on the combination of the electromagnetic theory and molecular dynamics (MD) simulation to investigate the impact of the localized surface plasmon resonance (LSPR) on the heat transfer between nanoparticles and the surrounding medium in gold and silver nanofluids. It is found that the LSPR-induced enhanced electric field (EEF) can obviously reduce the interfacial thermal resistance to promote the heat transfer process, especially in silver nanofluids. The results reveal that the movement of water molecules can be violently perturbed by the EEF to overcome the binding force of nanoparticles, and therefore the energy transfer process in water molecules can be obviously enhanced. The effect of EEF is significant, especially in the initial heating stages when the temperature of the nanoparticles is relatively low. When the silver nanoparticle temperature is 400 K, the relative reduction ratio of the interfacial thermal resistance can reach 19.0% under the effect of the LSPR-induced EEF. The results also indicate that two different energy conversion mechanisms: photothermal and photoexcited electric-field enhancement are likely to coexist and jointly impact the heat transfer process in plasmonic nanofluids, and the effect of the latter cannot be neglected. This work provides some new insights for a deeper understanding of the light-induced heat transfer process in plasmonic nanofluids.

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.

Contactless and spatially structured cooling by directing thermal radiation

In recent years, radiative cooling has become a topic of considerable interest for applications in the context of thermal building management and energy saving. The idea to direct thermal radiation in a controlled way to achieve contactless sample cooling for laboratory applications, however, is scarcely explored. Here, we present an approach to obtain spatially structured radiative cooling. By using an elliptical mirror, we are able to enhance the view factor of radiative heat transfer between a room temperature substrate and a cold temperature landscape by a factor of 92. A temperature pattern and confined thermal gradients with a slope of ~ 0.2 °C/mm are created. The experimental applicability of this spatially structured cooling approach is demonstrated by contactless supercooling of hexadecane in a home-built microfluidic sample. This novel concept for structured cooling yields numerous applications in science and engineering as it provides a means of controlled temperature manipulation with minimal physical disturbance.

Optical Force-Induced Chemistry at Solution Surfaces

When an intense 1,064-nm continuous-wave laser is tightly focused at solution surfaces, it exerts an optical force on molecules, polymers, and nanoparticles (NPs). Initially, molecules and NPs are gathered into a single assembly inside the focus, and the laser is scattered and propagated through the assembly. The expanded laser further traps them at the edge of the assembly, producing a single assembly much larger than the focus along the surface. Amino acids and inorganic ionic compounds undergo crystallization and crystal growth, polystyrene NPs form periodic arrays and disklike structures with concentric circles or hexagonal packing, and Au NPs demonstrate assembling and swarming, in which the NPs fluctuate like a group of bees. These phenomena that depend on laser polarization are called optically evolved assembling at solution surfaces, and their dynamics and mechanisms are elucidated in this review. As a promising application in materials science, the optical trapping assembly of lead halide perovskites, supramolecules, and aggregation-induced emission enhancement-active molecules is demonstrated and future directions for fundamental study are discussed.

Damage-free light-induced assembly of intestinal bacteria with a bubble-mimetic substrate

Rapid evaluation of functions in densely assembled bacteria is a crucial issue in the efficient study of symbiotic mechanisms. If the interaction between many living microbes can be controlled and accelerated via remote assembly, a cultivation process requiring a few days can be ommitted, thus leading to a reduction in the time needed to analyze the bacterial functions. Here, we show the rapid, damage-free, and extremely dense light-induced assembly of microbes over a submillimeter area with the "bubble-mimetic substrate (BMS)". In particular, we successfully assembled 104-105 cells of lactic acid bacteria (Lactobacillus casei), achieving a survival rate higher than 95% within a few minutes without cultivation process. This type of light-induced assembly on substrates like BMS, with the maintenance of the inherent functions of various biological samples, can pave the way for the development of innovative methods for rapid and highly efficient analysis of functions in a variety of microbes.

Harnessing bubble behaviors for developing new analytical strategies

Gas bubbles are easily accessible and offer many unique characteristic properties of a gas/liquid two-phase system for developing new analytical methods. In this minireview, we discuss the newly developed analytical strategies that harness the behaviors of bubbles. Recent advancements include the utilization of the gas/liquid interfacial activity of bubbles for detection and preconcentration of surface-active compounds; the employment of the gas phase properties of bubbles for acoustic imaging and detection, microfluidic analysis, electrochemical sensing, and emission spectroscopy; and the application of the mass transport behaviors at the gas/liquid interface in gas sensing, biosensing, and nanofluidics. These studies have demonstrated the versatility of gas bubbles as a platform for developing new analytical strategies.

Directed Concentrating of Micro-/Nanoparticles via Near-Infrared Laser Generated Plasmonic Microbubbles

Directed concentrating of micro- and nanoparticles via laser-generated plasmonic microbubbles in a liquid environment is an emerging technology. For effective heating, visible light has been primarily employed in existing demonstrations. In this paper, we demonstrate a new plasmonic platform based on nanoporous gold disk (NPGD) array. Thanks to the highly tunable localized surface plasmon resonance of the NPGD array, microbubbles of controlled size can be generated by near-infrared (NIR) light. Using NIR light provides several key advantages over visible light in less interference with standard microscopy and fluorescence imaging, preventing fluorescence photobleaching, less susceptible to absorption and scattering in turbid biological media, and much reduced photochemistry, phototoxicity, and so forth. The large surface-to-volume ratio of NPGD further facilitates the heat transfer from these gold nanoheaters to the surroundings. While the microbubble is formed, the surrounding liquid circulates and direct microparticles randomly dispersed in the liquid to the bottom NPGD surface, which can be made to yield a unique collection of 3D hollow dome microstructures with bubbles larger than 5 μm. Such capability can also be employed in concentrating suspended colloidal nanoparticles at desirable sites and with the preferred configuration enhancing the sensor performance. Specifically, the interaction among concentrated nanoparticles and their interactions with the underlying substrate have been investigated for the first time. These collections have been characterized using optical microscopy, scanning electron microscopy, hyperspectral localized surface plasmon resonance imaging, and hyperspectral Raman imaging. In addition to various micro- and nanoparticles, the plasmonic microbubbles are also shown to collect biological cells and extracellular nanovesicles such as exosomes. By using a spatial light modulator to project the laser in arbitrary patterns, parallel concentrating can be achieved to fabricate an array of clusters.

All-Optical Formation and Manipulation of Microbubbles on a Porous Gold Nanofilm

Microbubble generation and manipulation in aqueous environments are techniques that have attracted considerable attention for their microfluidic and biological applications. Ultrasonic and hydrodynamic methods are commonly used to form and manipulate microbubbles, but these methods are limited by the relatively low precision of the microbubble sizes and locations. Here, we report an all-optical method for generation and manipulation of microbubbles with ~100 nm precision by using “hot spots” on a porous gold nanofilm under the illumination of near-infrared focused laser beam. The microbubble diameter ranged from 700 nm to 100 μm, with a standard deviation of 100 nm. The microbubbles were patterned into two-dimensional arrays, with an average location deviation of 90 nm. By moving the laser beam, the microbubbles could be manipulated to a desired region. This work provides a controllable way to form and manipulate microbubbles with ~100 nm precision, which is expected to have applications in optofluidic and plasmonic devices.

Overcoming Diffusion-Limited Trapping in Nanoaperture Tweezers Using Opto-Thermal-Induced Flow

Nanoaperture-based plasmonic tweezers have shown tremendous potential in trapping, sensing, and spectroscopic analysis of nano-objects with single-molecule sensitivity. However, the trapping process is often diffusion-limited and therefore suffers from low-throughput. Here, we present bubble- and convection-assisted trapping techniques, which use opto-thermally generated Marangoni and Rayleigh-Bénard convection flow to rapidly deliver particles from large distances to the nanoaperture instead of relying on normal diffusion, enabling a reduction of 1-2 orders of magnitude in particle-trapping time (i.e., time before a particle is trapped). At a concentration of 2 × 107 particles/mL, average particle-trapping times in bubble- and convection-assisted trapping were 7 and 18 s, respectively, compared with more than 300 s in the diffusion-limited trapping. Trapping of a single particle at an ultralow concentration of 2 × 106 particles/mL was achieved within 2-3 min, which would otherwise take several hours in the diffusion-limited trapping. With their quick delivery and local concentrating of analytes at the functional surfaces, our convection- and bubble-assisted trapping could lead to enhanced sensitivity and throughput of nanoaperture-based plasmonic sensors.

Localized ZnO Growth on a Gold Nanoantenna by Plasmon-Assisted Hydrothermal Synthesis

The excitation of localized surface plasmon resonances (LSPRs) in metal nanostructures enables subwavelength photon localization and large electric field enhancement, which can be advantageously used to strongly enhance light-matter interactions at the nanoscale. For this purpose, efficient methods for deterministically handling and arranging nanomaterials at the exact position of the localized electric field are required. In this Letter, we propose a novel method based on a hydrothermal synthesis reaction to locally and selectively synthesize zinc oxide in a plasmonic nanoantenna. We first make evident the role of LSPR for achieving efficient heating of gold nanostructures. Then, by selectively addressing one of the LSPRs of a gold antenna, we demonstrate that localized zinc oxide formation at the targeted location of the antenna can be achieved due to the nanoscale confinement of the heat production.

Opto-thermophoretic separation and trapping of plasmonic nanoparticles

Optical tweezers are powerful tools to trap, transport, and analyse individual nano-objects at dilute concentrations. However, it is still challenging to manipulate isolated single nano-objects in dense target environments with various kinds of materials, such as in living cells and mixtures of nanocolloids. In the present work, we have succeeded in the selective trapping of a few gold nanoshells with specific sizes and sweeping others out completely, only by irradiating the dense colloidal suspension of gold nanoshells with a focused near infrared continuous-wave (CW) laser. This was achieved by an interplay between optical gradient- and thermophoretic forces: while the gradient force traps the targets at the focal spot, the thermophoretic force pushes others out from the focal spot. The distance between the trapped targets and the separated others was longer than 20 μm, allowing us to measure plasmonic scattering spectra of the trapped targets at a single-nanoparticle level. The present method paves a way for manipulating and analysing single nano-objects in dense mixtures of targets and various kinds of materials.

Optothermal microbubble assisted manufacturing of nanogap-rich structures for active chemical sensing

Guiding analytes to the sensing area is an indispensable step in a sensing system. Most of the sensing systems apply a passive sensing method, which waits for the analytes to diffuse towards the sensor. However, passive sensing methods limit the detection of analytes to a picomolar range on micro/nanosensors for a practical time scale. Therefore, active sensing methods need to be used to improve the detection limit in which the analytes are forced to concentrate on the sensors. In this article, we have demonstrated the manufacturing of nanogap-rich structures for active chemical sensing. Nanogap-rich structures are manufactured from metallic nanoparticles through an optothermally generated microbubble (OGMB) which is a laser-induced micron-sized bubble. The OGMB induces a strong convective flow that helps to deposit metallic nanoparticles to form nanogap-rich structures on a solid surface. In addition, the OGMB is used to guide and concentrate analytes towards the nanogap-rich structures for the active sensing of analytes. An active sensing method can improve the detection limit of chemical substances by an order of magnitude compared to a passive sensing method. The microbubble assisted manufacturing of nanogap-rich structures together with an active analyte sensing method paves a new way for advanced chemical and bio-sensing applications.

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.

Explosive, oscillatory, and Leidenfrost boiling at the nanoscale

We investigate the different boiling regimes around a single continuously laser-heated 80 nm gold nanoparticle and draw parallels to the classical picture of boiling. Initially, nanoscale boiling takes the form of transient, inertia-driven, unsustainable boiling events characteristic of a nanoscale boiling crisis. At higher heating power, nanoscale boiling is continuous, with a vapor film being sustained during heating for at least up to 20μs. Only at high heating powers does a substantial stable vapor nanobubble form. At intermediate heating powers, unstable boiling sometimes takes the form of remarkably stable nanobubble oscillations with frequencies between 40 MHz and 60 MHz, frequencies that are consistent with the relevant size scales according to the Rayleigh-Plesset model of bubble oscillation, though how applicable that model is to plasmonic vapor nanobubbles is not clear.

Plasmonic heating induced by Au nanoparticles for quasi-ballistic thermal transport in multi-walled carbon nanotubes

The plasmon resonances of nanostructures enable wide applications from highly sensitive sensing to high-resolution imaging, through the improvement of photogeneration rate stimulated by the local field enhancement. However, quantitative experimental studies on the localized heating and the thermal transport process in the vicinity of plasmonics are still lacking because of the diffraction limit in conventional optothermal methodologies. In this work, we demonstrate an approach based on Raman thermometry to probe the near-field heating caused by plasmonics. An array of Au nanoparticles (AuNPs) fabricated by the template-assisted method is used to generate the near field effect. Multi-walled carbon nanotubes (MWCNTs) dispersed on the AuNPs are employed to quantify the near-field heating from their Raman peak shifts. Results show that the temperature rise in MWCNTs on AuNPs is much higher than that in a control group under the same laser irradiation. Further analysis indicates that the enhanced photon absorption of MWCNTs attributed to plasmon resonances is partially responsible for the different heating effect. The nonuniform thermal hot spots at the nanoscale can result in the quasi-ballistic thermal transport of phonons in MWCNTs, which is another reason for the temperature rise. Our results can be used to understand plasmonic heating effects as well as to explore quasi-ballistic thermal transport in carbon-based low-dimensional materials by tailoring the geometry or size of plasmonic nanostructures.

Evolution of Cavitation Bubble in Tap Water by Continuous-Wave Laser Focused on a Metallic Surface

As an example of photon-matter interaction, we experimentally investigate the temporal evolution of a millimeter-sized cavitation bubble, induced by focusing a continuous-wave laser on a metallic plate in tap water. Our major interests are to understand the mechanism of bubble growth/shrinkage for a long time duration up to O(10²) seconds and to draw the time-dependency relation of a bubble size, depending on the incident laser power. With the time passed after the laser with different power is focused, it is found that the phase change and/or transport of dissolved gas into the bubble play a dominant role in determining the bubble growth and shrinkage. Thus, we propose two stages in terms of time and three regimes depending on the incident energy, in which the evolutions of cavitation bubble in short and long time durations are distinctively identified. In regime I (lower incident power), the water nearby the focal point undergoes a phase change, resulting in an initial rapid growth of a bubble (first stage), but the convection flow due to locally heated surface causes the bubble to shrink at later times (second stage). As the laser power increases (regime III), more dissolved gas in the surrounding water enters the growing bubble and prevents the water phase from being absorbed into the bubble. Thus, the bubble growth is dominated by the dissolved gas. Between regimes I and III, there is a transitional regime II in which both the phase change of water and the dissolved gas contribute to the bubble evolution. We further our understandings by developing the relations about the time-dependency of bubble size for each stage and regime, which agree well with the measured data. The scaling relations are also validated with different conditions of liquid such as degassed water and NaCl solution. While previous studies have mostly focused on the nano- and/or microsized bubble generation in a very short time (less than 1 s), we think that the present results will extend our knowledge on how to predict and control the size of laser-induced cavitation bubble for longer time duration.

Mesoscopic Motion of Optically Trapped Particle Synchronized with Photochromic Reactions of Diarylethene Derivatives

Not only the energy but also the momentum of photons transfers to material via photoabsorption; this momentum transfer, known as radiation pressure, can induce motions of small particles. It can therefore be expected to induce mechanical motions of mesoscopic materials synchronized with the reversible change of their absorption coefficient by external stimuli. We demonstrated quantitative photomechanical motions in mesoscopic regions by combining optical tweezer and photochromic reactions of diarylethene (DAE). A microparticle including DAE was optically trapped with 532 nm laser and the absorption band of the DAE was photoswitched with UV laser, resulting in the modulation of the radiation force through the change in the complex dielectric constant of the particle. In this process, mesoscopic mechanical motions were successfully induced by the photochromic reaction. The present approach is potentially applicable in a wide variety of nano/micromechanical devices and also paves the way for monitoring the absorption of photons by molecules via photomechanical response.

Investigation of transition from thermal- to solutal-Marangoni flow in dilute alcohol/water mixtures using nano-plasmonic heaters

We experimentally investigated Marangoni flows around a microbubble in diluted 1-butanol/water, 2-propanol/water, and ethanol/water mixtures using the thermoplasmonic effect of gold nanoisland film. A laser spot on the gold nanoisland film acted as a highly localized heat source that was utilized to generate stable air microbubbles with diameters of 32-48 μm in the fluid and to induce a steep temperature gradient on the bubble surface. The locally heated bubble has a flow along the bubble surface, with the flow direction showing a clear transition depending on the alcohol concentrations. The fluid is driven from the hot to cold regions when the alcohol concentration is lower than the transition concentration, whereas it is driven from the cold to hot regions when the concentration is higher than the transition concentration. In addition, the transition concentration increases as the carbon number of the alcohol decreases. The observed flow direction transition is explained by the balance of the thermal- and solutal-Marangoni forces that are cancelled out for the transition concentration. The selective evaporation of the alcohol at the locally heated surface allows us to generate stable and rapid thermoplasmonic solutal-Marangoni flows in the alcohol/water mixtures.

Nondestructive Approach for Additive Nanomanufacturing of Metallic Nanostructures in the Air

In this article, a mechanism for quick release and transfer of gold nanoparticles (GNPs) from a soft substrate to another substrate under laser illumination is investigated. The heating of GNPs on a soft substrate with a continuous-wave laser causes a rapid thermal expansion of the substrate, which can be used to selectively release and place GNPs onto another surface. In-plane and out-of-plane nanostructures are successfully fabricated using this method. This rapid release-and-place process can be used for additive nonmanufacturing of metallic nanostructures under ambient conditions, which paves a way for affordable nanomanufacturing and enables a wide variety of applications in nanophotonics, ultrasensitive sensing, and nonlinear plasmonics.

Fabricated nanogap-rich plasmonic nanostructures through an optothermal surface bubble in a droplet

A rapid and cost-effective method for the fabrication of nanogap-rich structures is demonstrated in this Letter. The method utilizes the Marangoni convection around an optothermal surface bubble inside a liquid droplet with a nanoliter volume. The liquid droplet containing metallic nanoparticles reduces the sample consumption and confines the liquid flow. The optothermal surface bubble creates a strong convective flow that allows for the rapid deposition of the metallic nanoparticles to form nanogap-rich structures on any substrate under ambient conditions. This method will enable a broad range of applications such as biosensing, environmental analysis, and nonlinear optics.

Understanding and Reducing Photothermal Forces for the Fabrication of Au Nanoparticle Dimers by Optical Printing

Optical printing holds great potential to enable the use of the vast variety of colloidal nanoparticles (NPs) in nano- and microdevices and circuits. By means of optical forces, it enables the direct assembly of NPs, one by one, onto specific positions of solid surfaces with great flexibility of pattern design and no need of previous surface patterning. However, for unclear causes it was not possible to print identical NPs closer to each other than 300 nm. Here, we show that the repulsion restricting the optical printing of close by NPs arises from light absorption by the printed NPs and subsequent local heating. By optimizing heat dissipation, it is possible to reduce the minimum separation between NPs. Using a reduced graphene oxide layer on a sapphire substrate, we demonstrate for the first time the optical printing of Au-Au NP dimers. Modeling the experiments considering optical, thermophoretic, and thermo-osmotic forces we obtain a detailed understanding and a clear pathway for the optical printing fabrication of complex nano structures and circuits based on connected colloidal NPs.

An optothermally generated surface bubble and its applications

Under laser illumination, a solid-state surface or nanostructure can turn into a micro/nano heating source with the so-called optothermal effect. This effect allows for non-invasive control of heat at the micro/nanoscale. In the presence of a liquid, a surface bubble can be generated on top of the solid surface or nanostructure at a temperature much higher than the boiling point of the liquid. The high temperature and the fluid flow associated with the optothermally generated surface bubble enable many intriguing applications, ranging from the micro/nano-manipulation of fluids, particles, cells, and light to the synthesis of micro/nano-structures under ambient conditions. In this review article, we present the fundamentals, recent developments, and future perspectives in this emerging field.

On the origin of the driving force in the Marangoni propelled gas bubble trapping mechanism

Particle trajectories around gas bubbles due to Marangoni induced flows of liquid.