Artificial Nucleation Sites Research Articles

Single bubble dynamics during subcooled nucleate boiling on a vertical heater surface: An experimental analysis of the effects of surface characteristics

In the present experimental study we investigated the effects of surface characteristics, such as wettability and roughness, on nucleate boiling in de-ionized water at a vertical heater. In the experiments, bubbles were generated from an artificial nucleation site on a stainless steel heater surface. High-resolution optical imaging has been used to capture the bubble life cycle, that is, departure, sliding, and lift-off. We found, that the lower wettability leads to larger departure diameter, longer sliding and larger lift-off diameter of bubbles. Also surface roughness effects have been analyzed and it was found that bubble departure and lift-off diameters are smaller and departure period is longer for a smooth surface. Bubble sliding velocity was found faster for a rough surface compared to a smooth surface. It was also found that the roughness is very influential to bubble growth and departure, which can be explained by considering its interaction with the microlayer underneath the bubble. An “optimal roughness”, which accelerates the bubble growth, was found. The knowledge gained from this study shall be particularly useful to improve nucleate boiling models for numerical simulations.

Gas Evolution at Porous Electrodes

Porous electrodes for PEM electrolyser and fuel cell technologies are optimized primarily to provide a high specific surface area for the desired electrochemical reactions. However, the increase in surface area comes at the cost of increased voltage losses due to the transport of reactant and product species through the porous medium. In gas evolving electrolyzer electrodes, the formation and transport of gas bubbles plays an important role: on the one hand, gas in the pores replaces the electrolyte phase, thus hindering ion transport and reducing the effective ionic conductivity in the porous electrode; on the other hand, gas bubbles cover and deactivate a portion of the internal surface area. The impacts of gas formation and removal must be accounted for in the design of electrolyzer electrodes. The focus of the presented work is on the fundamental understanding of the relationships between structure, properties and performance of porous gas-evolving electrodes. The approach combines a macro-scale performance model of the electrode with a micro-scale model of gas evolution. At the macro-scale, a classic porous electrode model describes the transport of ions, electrons and produced oxygen using effective medium theory and accounting for the gas phase volume and distribution. The micro-scale model describes the formation and growth of gas bubbles based on chemical energy considerations. It can explain the experimentally found high oversaturation that is necessary to nucleate bubbles [1]. Additionally, the size of the bubble nucleus in the experiment was estimated. The model was used to study the influence of structural parameters on the operation of porous electrodes and to establish guidelines for an enhanced electrode design. It was found that the transport regime of the dissolved gas, viz. diffusion control vs. transfer control at the liquid-gas interface, determines the bubble growth law. Applications of the model to gas evolving porous electrodes in electrolyzers with liquid electrolyte as well as polymer electrolyte electrolyzers (PEEC) will be discussed. [1] Chen, Q., Luo, L. and White, H. S. Electrochemical generation of a hydrogen bubble at a recessed platinum nanopore electrode. Langmuir 31, 4573–4581 (2015). Figure 1: Bubble growth on an artificial nucleation site at 20 mA/cm2. Comparison of model results with experimental data from C. Brussieux et al., Electrochimica Acta 56 (2011) 7194-7201. Deviation at large bubble radii due to deformation of the bubbles before detachment, which is neglected in the model. Figure 1

Performance Limits of the Self‐Aligned Nanowire Top‐Gated MoS2 Transistors

In order to realize the promising potential of MoS2 as the alternative channel material, it is essential to achieve high‐performance top‐gated MoS2 field‐effect transistors (FETs), especially since the back‐gated counterparts cannot control the device individually. Although uniform high‐k dielectric films, such as HfO2, can be obtained through the introduction of artificial nucleation sites on the MoS2 channel to fabricate top‐gated FETs, this would inevitably degrade their channel/dielectric interface quality, induce significant charged impurity scattering and lower carrier mobility. In this work, MoS2 FETs are fabricated using a self‐aligned nanowire top‐gate, which can effectively reduce the charged impurity scattering on the surface of MoS2. Specifically, the fabricated short‐channel devices exhibit impressive electrical performances, such as the high on/off current ratio, low interface trap density, and near‐ideal subthreshold slope at room temperature. In addition, the short channel effect is systematically analyzed, which indicates that the phonon scattering can be the dominant scattering mechanism in the devices when the amount of charged impurities is effectively reduced with the self‐aligned nanowire gate. All these provide an enhanced fabrication scheme to attain top‐gated short‐channel devices with the optimized interface and potentially to explore their corresponding performance limits.

Lateral coalescence of bubbles in the presence of a DC electric field

In this work, the influence of electrohydrodynamic forces on lateral bubble coalescence during nucleate pool boiling is investigated. An experimental pool boiling test facility was used with n-pentane as the working fluid. Boiling took place atop a polished copper surface on which two artificial nucleation sites were fabricated. The nucleation sites were 180μm in diameter and 500μm deep with a centre-to-centre spacing of 660μm. Two diametrically opposed windows allowed for illumination and high speed videography of the bubble growth process from the two nucleation sites. For the saturated boiling tests considered here, bubbles only formed at the two artificial nucleation sites allowing their coalescence behaviour to be scrutinized. A screen electrode above the boiling surface and a high voltage DC power supply facilitated the establishment of the electric field which was varied between 0 and 34.5kVcm−1. Observation of the high speed videos has revealed that bubble coalescence is influenced in such a way that it is delayed in the presence of the electric field to such an extent that, at the highest electric field strength tested, it is avoided all together. To help explain the observed results, a simple numerical model is solved showing that bubbles in close proximity to one another create an electric field distribution with high intensity between them. The overall result is net polarization forces that push the bubbles apart, and the closer they are together the larger this repulsive force becomes.

Reducing the nucleation barrier in magnetocaloric Heusler alloys by nanoindentation

Magnetocaloric materials are promising as solid state refrigerants for more efficient and environmentally friendly cooling devices. The highest effects have been observed in materials that exhibit a first-order phase transition. These transformations proceed by nucleation and growth which lead to a hysteresis. Such irreversible processes are undesired since they heat up the material and reduce the efficiency of any cooling application. In this article, we demonstrate an approach to decrease the hysteresis by locally changing the nucleation barrier. We created artificial nucleation sites and analyzed the nucleation and growth processes in their proximity. We use Ni-Mn-Ga, a shape memory alloy that exhibits a martensitic transformation. Epitaxial films serve as a model system, but their high surface-to-volume ratio also allows for a fast heat transfer which is beneficial for a magnetocaloric regenerator geometry. Nanoindentation is used to create a well-defined defect. We quantify the austenite phase fraction in its proximity as a function of temperature which allows us to determine the influence of the defect on the transformation.

Numerical investigation of nucleate boiling heat transfer on thin substrates

The objective of this paper is to define the guidelines for the design of new boiling test sections with a large number of artificial nucleation sites during nucleate boiling for thin substrates horizontally immersed in a saturated liquid with artificial cavities located on the upper surface. The findings of numerical simulations of pool boiling heat transfer for a single bubble and for a large number of nucleation sites based on the analysis of experimental cases were analysed. Dedicated test sections were used in experiments for the study of boiling mechanisms and interactions between active sites so that the numerical models representing the physics of the problem could be improved. The hybrid nature of the code used in this study, combining the complete solution of the three-dimensional time-dependent energy equation in the solid substrate with semi-empirical models representing the physical phenomena occurring in the liquid side, in a simplified way, allows a large number of simulations in a reasonable computational time.The present paper focuses in the first part on the capability of the model to reproduce the experimental results for various conditions, while in the second part, the results for a large number of nucleation sites are analysed. Regarding the single bubble growth, two series of simulations will be presented in this paper: the first one analyses the mechanisms of nucleate boiling on a silicon substrate immersed in the dielectric fluid FC-72. The second series studies the behaviour of bubbles on metallic substrates, platinum and titanium, in saturated water. In the last section, the effect of the position of a site during simulations of a large population of sites (of the order of 100) on the waiting time, growth time, type and occurrence of coalescence and the thermal characteristics is presented.

Integral momentum balance on a growing bubble

The integral momentum balance on a growing boiling bubble is investigated. All forces acting on the bubble are detailed, and the methods and assumptions used to calculate their integral resultants are discussed. The momentum balance computation is then performed using experimental data of bubbles growing on an artificial nucleation site in a controlled environment. The relative magnitude of each force component is compared, showing negligible dynamic forces, upwards forces composed mainly of the buoyancy and contact pressure components, and downwards forces being exclusively due to surface tension and adhesion. The difficulty encountered in measuring the apparent contact angle due to mirage effects has been highlighted; a new method, fitting numerically simulated bubble profile to the contour measurements has been proposed and used to correct the effects of refraction on the bubble profile determination. As all forces acting on the bubble were measured, it was possible to estimate the residuals of the momentum balance. Their small value validated both the expressions used for the forces and the methodology to evaluate their value.

Analysis of the Interface Curvature Evolution During Bubble Growth

This study presents an experimental investigation on the local curvature of the bubble interface and its relation to local bubble growth regimes and bubble detachment. An experimental facility has been built in order to produce bubbles by nucleating on a 180-m-diameter artificial nucleation site on a horizontal copper surface. High-speed videography was used to observe and capture the boiling phenomenon, and an image-processing code has been developed in order to determine the local curvature of the bubble interface through the two main radii of curvature. As the evolution of the curvature at the apex of the bubble reflects the evolution of the vapor pressure inside the bubble, it is shown that inertia plays a negligible role and that bubble growth is dictated by thermal diffusion considerations. The measured curvature profile along the bubble has been compared to the hydrostatic Young–Laplace curvature relation. It is observed that the curvature profile near the base diverges from the theoretical prediction as the bubble forms a neck during its growth. This observation can be related both to the successive stages or regimes of growth and to the mechanism of bubble detachment.

The Influence of System Pressure on Bubble Coalescence in Nucleate Boiling

Boiling is one of the most effective heat transfer mechanisms. In spite of a long time of research, the physical fundamentals are still not sufficiently understood. Pursuing the objective to predict heat transfer based on physical and geometrical properties, experimental and numerical investigations are conducted at the institute of the authors. The focus of the presented research is the coalescence of two single bubbles under varying pressure conditions. In the experiment a thin stainless-steel foil is used as a Joule heater. The experiments were performed in a pressure range of 300–1000 mbar using FC72 as working fluid. Two types of heaters with a distance between two artificial nucleation sites of 300 μm (type 3) and 500 μm (type 5) were used. The experimental results indicate a strong dependence of the occurrence of bubble coalescence on pressure. For the type 5 heater, a Gaussian distribution for the coalescence frequency when plotted over pressure is observed. Experimental results with the type 3 heater show a similar distribution of the frequency with a shifted maximum. Further, it is shown that during bubble coalescence a small droplet can remain inside the bubble and enhance the heat transfer, which is attributed to an additional thin film region. The formation of this remaining droplet is sensitive to system pressure. Numerical investigations of bubble coalescence were conducted with the computational fluid dynamics (CFD) software OpenFOAM. In OpenFOAM, dynamic mesh handling allows high spatial resolution at the phase boundary, which is captured with the volume-of fluid method. Evaporation and a subgrid microscale model were implemented in the flow solver to account for evaporation at the phase boundary and the three-phase contact line. The results show a strong dependence of bubble dynamics and coalescence on contact angle and bubble growth rate. Although it was possible to observe the creation of the residual droplet, more effort needs to be put into finding appropriate initial conditions.

Application of Two-Color LIF Thermometry to Nucleate Boiling

The laser-induced fluorescence (LIF) thermometry is applied to measure the temperature field surrounding a single vapor bubble growing at an artificial nucleation site. In order to correct measurement errors due to the non-uniformity of the incident laser intensity, the two-color LIF thermometry technique is used in this nucleate boiling experiment. This technique is based on the use of two fluorescent dyes: the temperature sensitive dye Rhodamine B and the temperature insensitive dye Sulforhodamine-101. The concentration of the dyes is optimized by analyzing the behavior of fluorescence intensities. The mapping between the two images is determined through a geometrical calibration procedure. This technique presents a success in correcting the non uniformities due to the reflection of the light at the bubble surface and to the temperature gradient. The obtained temperature fields show that the two-color LIF is a promising technique in the investigation of nucleate boiling.

The Effect of the Location of Nucleation Sites on the Thermal-Hydraulic Stability of a Short-Tube Natural Circulation Loop

The use of small two-phase natural circulation loops is an attractive option for efficient cooling applications and innovative steam generators. An experimental study was conducted to examine how the thermal-hydraulic stability and operating conditions of these devices are affected by nucleation sites. A very smooth glass tube with artificial nucleation sites was used as a boiling channel. The mass flow rate was obtained as a function of heat flux and nucleation site location. Nucleation sites have a strong impact on stability behavior, particularly for low heat flux levels. The observed flow instabilities were analyzed with regard to nonlinear effects and chaotic behavior.

Angular arrangements of triangular fins for controlling the magnetization processes in permalloy rings

The influences of triangular fins’ positions on controlling the magnetization processes are investigated. It is observed experimentally that when the included angle between the triangular fins and the field direction is 15° or 30°, the magnetization reversals are repeatable in the sweep-up and sweep-down processes, and the magnetization processes are controllable. In contrast, when the included angle between the triangular fins and the field direction is 45° or 60°, the magnetization reversals are unrepeatable and random in the sweep-up and sweep-down processes. Magnetization reversal can only be controlled when positions of the triangular fins are close to the original nucleation regions of the rings. When triangular fins are away from the original nucleation regions, two kinds of magnetization reversals randomly occur without being controlled by the artificial nucleation sites, and the triangular fins thus lose their control ability.

Inhibited and enhanced nucleation of gold nanoparticles at the water|1,2-dichloroethane interface

The deposition of gold at the interface between immiscible electrolyte solutions has been investigated using reduction of tetrachloroaurate or tetrabromoaurate in 1,2-dichloroethane, with aqueous phase hexacyanoferrate as reducing agent. In a clean environment without defects present at the interface, the Au(III) complex was reduced to the Au(I) complex, but no solid phase formation could be observed. A deposition process could only be observed through the addition of artificial nucleation sites in the form of palladium nanoparticles at the interface. This process could be associated with the reduction of the Au(I) halide complex to metallic gold, by determining the gold reduction potentials in 1,2-dichloroethane. XANES measurements indicate that tetrachloroaurate ion transfers intact into the organic phase, with the central Au atom retaining its oxidation state of +3 and the overall anion remaining charged at -1.

Single bubble dynamics on a superhydrophilic surface with artificial nucleation sites

The dynamics of single bubbles on a superhydrophilic surface with well-defined nucleation sites are studied. The superhydrophilic surfaces are prepared by forming CuO nanostructures on a silicon substrate with an isolated microcavity. The bubble departure diameter in water is observed to be 2.5 times smaller and the growth period 4 times shorter on the superhydrophilic surface than on a silicon substrate. A model balancing the buoyancy with the surface tension force captures the departure diameter well, confirming the importance of wettability on bubble dynamics. The present work helps develop new surfaces for controlled phase change heat transfer and bubble-based micro-devices.

Efficient Sonochemistry through Microbubbles Generated with Micromachined Surfaces

Sonochemical reactors are used in water treatment, the synthesis of fine chemicals, pharmaceutics and others. The low efficiency of sonoreactors have prevented its massive usage at industrial scales. Controlling the appearance of bubbles in place and time is the most limiting factor. A novel type of sonochemical reactor was designed making use of micro-fabrication techniques to control the nucleation sites of micro-bubbles. The efficiency was increased first by locating the nucleation sites in the most active region of a micro-chamber; additionally the desired chemical effect was significantly higher at the same powers than when not controlled. Silicon substrates were micromachined with "artificial nucleation sites" or pits, and placed at the bottom of the micro-chamber. The pits entrap gas which, upon ultrasonic excitation, sheds off a stream of microbubbles. The gas content of the pits is not depleted but is replenished by diffusion and the emission of microbubbles can continue for hours.

Convective boiling in a parallel microchannel heat sink with a diverging cross section and artificial nucleation sites

To develop a highly stable microchannel heat sink for boiling heat transfer, three types of diverging microchannels (Type 1, Type 2 and Type 3) were designed to experimentally investigate the effect of different distributions of artificial nucleation sites (ANS) on the enhancement of flow boiling heat transfer, in 10 parallel diverging microchannels with a mean hydraulic diameter of 120 μm. Water was used as the working fluid with mass flux, based on the mean cross section area, ranging from 99 to 297 kg/m 2 s. The Type-1 system did not contain any ANS; the Type-2 system contained ANS distributed uniformly along the downstream half of the channel; and the Type-3 system contained ANS distributed uniformly along the entire channel. The ANS are laser-etched pits on the bottom wall of the channel and have a mouth diameter of approximately 20–22 μm, as indicted by the heterogeneous nucleation theory. The results of the present study reveal that the presence of ANS for flow boiling in parallel diverging microchannels significantly reduces the wall superheat and enhances the boiling heat transfer performance. The Type-3 system shows the best boiling heat transfer performance.

Electric field effects during nucleate boiling from an artificial nucleation site

An experimental study of saturated pool boiling from a single artificial nucleation site on a polished copper surface has been performed. Isolated bubbles grow and depart from the artificial cavity and the bubble dynamics are recorded with a high speed camera. Experimental results are obtained for bubble growth, departure and vertical rise both with and without the application of an electric field between an upper electrode and the boiling surface. As detailed in a previous paper from the same research group the high spatial and temporal resolution of the video sequences facilitated the development of a baseline experimental bubble growth law which predicts the bubble volumetric growth characteristics for a range of surface superheats at atmospheric pressure. The presence of an electric field has been found to positively augment the convective heat transfer over that of buoyant natural convection. Further to this, for high electric field strengths, the bubble shape, volumetric growth characteristics and bubble rise are different from that of the baseline cases. These results provide compelling evidence that electric fields can be implemented to alter the bubble dynamics and subsequent heat transfer rates during boiling of dielectric liquids.

Field-driven creep motion of a composite domain wall in a Pt/Co/Pt/Co/Pt multilayer wire

We have studied the field-driven motion of a pair of coupled Bloch domain walls in a perpendicular magnetic anisotropy Pt/Co/Pt/Co/Pt multilayer Hall bar. The nucleation of an isolated but coincident pair of walls in the two Co layers, observed by Kerr microscopy, took place at an artificial nucleation site created by Ga + ion irradiation. The average velocity v of the wall motion was calculated from time-resolved magnetotransport measurements at fixed driving field H, where the influence of the extraordinary Hall effect leads to the observation of voltages at the longitudinal resistance probes. We observed a good fit to the scaling relation ln v ∝ H − 1 / 4 , consistent the motion of a single 1-dimensional wall moving in a 2-dimensional disordered medium in the creep regime: the two walls are coupled together into a 1-dimensional composite object.

Convective Boiling Between 2D Plates: Microgravity Influence on Bubble Growth and Detachment

The experiment detailed in this paper presents results obtained on the nucleation, growth and detachment of HFE-7100 confined vapour bubbles. Bubbles are created on an artificial nucleation site between two-dimensional plates under terrestrial and microgravity conditions. The experiments are performed by varying the shear flow by changing the convective mass flow rate, and varying the bubble nucleation rate by changing the heat flux supplied. The experiments are performed under normal (1 g) and reduced gravity (μg). The distance between the plates is equal to 1 mm. The results of these experiments are related to the detachment diameters of bubbles on the single artificial nucleation site and to the associated effects on the heat transfer by the confinement influence. The experimental device allows the observation of the flow using both visible video camera and infrared video camera. Here, we present the results obtained concerning the influence of gravity on the bubble detachment diameter and the images of 2D bubbles obtained in microgravity by means of an infrared camera. The following parameters: nucleation site surface temperature, bubble detachment diameter and bubble nucleation frequency evidence modifications due to microgravity.

A highly stable microchannel heat sink for convective boiling

To develop a highly stable two-phase microchannel heat sink, we experimented with convective boiling in diverging, parallel microchannels with different distributions of laser-etched artificial nucleation sites. Each microchannel had a mean hydraulic diameter of 120 µm. The two-phase flow visualization and the magnitudes of pressure drop and inlet temperature oscillations under boiling conditions demonstrated clearly the merits of using artificial nucleation sites to further stabilize the flow boiling in diverging, parallel microchannels. The stability map showed the plane of subcooling number versus phase change number. It illustrated that diverging, parallel microchannels with artificial nucleation cavities have a much wider stable region than parallel microchannels with uniform cross-sections or diverging, parallel microchannels without artificial nucleation cavities. In addition, the results revealed that the design with cavities distributed uniformly along the downstream half of the channel presented the best stability performance among the three distributions of nucleation sites. This particular design can be regarded as a highly stable microchannel heat sink for convective boiling.