Bubble Center Research Articles

Experimental and numerical research on jet dynamics of cavitation bubble near dual particles

The current paper delves into the jet dynamics arising from a cavitation bubble in proximity to a dual-particle system, employing both experimental methodology and numerical simulation. The morphological development of a laser-induced bubble as well as the production of jets are captured by utilizing high-speed photography. The principles of bubble morphology evolution and jet formation are revealed by a OpenFOAM solver, which takes into account the effects of two-phase fluid compressibility, phase changes, heat transfer, and surface tension. Fluid temperature variations induced by bubble oscillations are discussed. The results indicate that the jet dynamics can be categorized into three cases, i.e. bubble-splitting double jets, impacting single jet, non-impacting double jets. For bubble-splitting double jets, bubble splitting is induced by an annular pressure gradient towards the bubble axis. This resulted in the production of two unequal-sized sub-bubbles, which subsequently produced double jets in opposite directions. The fluid temperature close to the bubble interface is low, while the bubble center is high. For impacting single jet, it is induced by a conical pressure gradient towards the nearest particle and the jet impacts the particle. The fluid temperature is low near the jet and high near the particle. When the jet penetrates the bubble interface, the temperature inside the bubble reaches its peak. For non-impacting double jets, they are induced by pressure gradients facing each other and they do not impact particles. The temperature inside the bubble increases with the proximity of the two jets.

Mesoscopic modeling of interaction dynamics for two bubbles in the near-wall region

A nonorthogonal hybrid thermal lattice Boltzmann cavitation model is proposed in this study. The flow and density field are described by the pseudopotential model, while the temperature is realized by solving the macroscopic temperature function. The model is verified by simulating the Laplace law and a bubble evolves in the unbounded region. Furthermore, the interaction dynamics between two bubbles near the wall are investigated. The bubble collapse intensity is dominated by the initial bubble–bubble distance and bubble–wall distance. A pair of inclined reentrant jets are reported for the strong interaction modes, and splashing resulting from the jet collisions may lead to the bubble center shifting away from the wall or the axis of symmetry. For the weak interaction mode with dimensionless bubble-wall distance γ between 1 and 3.5, the maximum collapse pressure increases linearly. The maximum collapse pressure Pc_max decreases with increasing bubble spacing, and Pc_max for distance between two bubbles with d2=40lu increases by 9 % to 16.2 % compared to that of d2=30lu. The variation of the liquid film thickness h between adjacent cavitation bubbles is primarily governed by the expansion velocity of the bubble wall ḣ, and similarly, the thickness of the liquid film between the bubble wall follows the same principle with h∼ḣ−2. The bubble radius in the growth stage aligns closely with the theoretical analysis for the weak interaction mode, while discrepancies appear between the simulation and prediction due to the bubble losing its roundness in the collapse stage.

Uncertainty study of two-phase flow distribution characteristics measurement based on bubble trajectory tracking method

Bubbles undergo complex motions such as nonlinear trajectories in two-phase flow, where the position, velocity magnitude, and direction of bubble centers continuously change as they evolve under the influence of forces. The complexity of bubble trajectory increases the difficulty in measuring the flow field distribution characteristics, which has been understudied in the previous. The bubble trajectory tracking model based on the Monte Carlo method is constructed to study the uncertainty of two-phase flow distribution characteristics measurement by the conductivity probe. The Monte Carlo method is adopted to randomly generate the bubble velocity at the initial moment and the bubble force characteristics are embedded to simulate the three-dimensional motion of the bubble. The influence of various factors on the distribution of effective bubble numbers and velocity relative error at the local point is quantitatively analyzed. It is found that the lateral spacing of the sensors has the greatest influence on the measurement results of small-sized bubbles, the accuracy is greatly reduced when the space is larger than the radius of the measured bubbles. Moreover, the measurement accuracy of the probe for medium-sized bubbles is high, and the effect of bubble lateral velocity and aspect ratio on the effective bubble ratio and velocity relative error is negligible. In addition, the maximum measurement range of the probe at local points is investigated which is found to be 4 mm in the lateral direction and 2 mm in the longitudinal direction. Finally, uniformly distributed bubbles are generated in a 20 mm × 10 mm transverse plane and the distribution of the effective bubble ratio of the probe is found to be consistent at different measurement points, which verifies the reliability of the probe measurement.

Dynamics of a single cavitation bubble near an oscillating boundary

Cavitation and its effects are well investigated, especially single bubble cavitation and its collapse near rigid and elastic boundaries. In our current article, we investigated novel experiments of a single cavitation bubble near an oscillatory boundary. We generated the cavitation bubble by laser focusing in water. A flat glass plate was fixed to the shaft of the magnetostriction oscillator coil. We investigated the dynamics of bubbles at two relative wall distances (ratio of the distance between the bubble center and plate surface to the maximum radius of the bubble) of the bubble from the glass plate in combination with four modes of oscillation. Each mode has specific frequency and amplitude of oscillation. The high-speed camera captured the dynamics of the bubble using the back-illumination method with a framing rate of 120Kfps and simultaneously we used an optical CMOS sensor to measure the oscillation of the glass plate. We presented a clear comparison among the bubble dynamics near stationary and oscillating plates with parameters such as oscillating modes and direction. We correlated the dynamics of the bubble with the motion of the plate. In addition, we highlighted the differences including the characteristics of bubble shape and jetting that occurred during the collapse phase. The comparison of the time histories of the bubble’s equivalent size postulated that the bubble’s collapse times vary significantly in some cases compared to the bubble’s dynamics near the stationary plate. In all cases, we noticed the shortening of the bubble’s collapsing time, i.e. accelerated collapses. In our findings, we noticed a collapse times reduction of about 4–15%. Our finding signifies the importance of introducing the oscillation of the boundaries to obtain effective energy concentration over the time during the collapse. Our study also suggests that forced oscillation of boundaries is undesirable for destructive cavitation effects. The method we suggested for the manipulation of bubble dynamics holds potential for enhancing the efficiency of applications such as lithotripsy in biomedical devices, actuation and micro pumping in microfluidic devices, and effective semiconductor surface cleaning. Not but least, obtained results can be used as benchmark in future for validating numerical methods.

Experimental study on attenuation effect of liquid viscosity on shockwaves of cavitation bubbles collapse

How to precisely control and efficiently utilize the physical processes such as high temperature, high pressure, and shockwaves during the collapse of cavitation bubbles is a focal concern in the field of cavitation applications. The viscosity change of the liquid will affect the bubble dynamics in turn, and further affect the precise control of intensity of cavitation field. This study used high-speed photography technology and schlieren optical path system to observe the spatiotemporal evolution of shockwaves in liquid with different viscosities. It was found that as the viscosity of the liquid increased, the wave front of the collapse shockwave of the cavitation bubble gradually thickened. Furthermore, a high-frequency pressure testing system was used to quantitatively analyze the influence of viscosity on the intensity of the shockwave. It was found that the pressure peak of the shockwave in different viscous liquid was proportional to Lb (L represented the distance between the center of bubble and the sensor measuring point), and the larger the viscosity was, the smaller the value of b was. Through in-depth analysis, it was found that as the viscosity of the liquid increased, the proportion of the shockwave energy of first bubble collapse to the maximal mechanical energy of bubble gradually decreased. The proportion of the mechanical energy of rebounding bubble to the maximal mechanical energy of bubble gradually increased. These new findings have an important theoretical significance for the efficient utilization of ultrasonic cavitation.

On the transient dynamics of laser-induced cavitation bubbles near the end of a slender cylinder

In this work, we perform high-speed imaging and numerical simulation to investigate the transient dynamics of cavitation bubbles near the end of a slender cylinder. The bubble dynamics can be categorized into four distinct regimes in terms of the types of bubble collapse, corresponding to the regular jet, needle jet, in-phase double jets, and anti-phase double jets, respectively, depending on two dimensionless parameters, the normalized cylinder radius η (=rc/Rmax, where rc is the cylinder radius and Rmax is the spherical bubble radius at maximum expansion), and the dimensionless standoff distance γ (=SD/Rmax, where SD is the standoff distance between the end surface of the cylinder and bubble center). The peak velocity of the liquid jet could easily reach a supersonic state in the regime of the needle jet when the cavitation bubble collapses near a slender cylinder, and the maximum jet velocity can reach up to 635 m/s. Quantitative analysis of the evolution of pressure distribution also indicates that the end surface of the cylinder will have strong hydrodynamic pressure loading, particularly for the case of η=0.3 and γ ranging from 0.5 to 0.83. Additionally, we find that the collapse time of the cavitation bubble near a slender cylinder is close to the Rayleigh collapse time. We believe that our findings can be valuable in mitigating or utilizing cavitation near solid cylinders.

The velocity fields around aspherical bubbles

Abstract This paper studies the simplest system, which can possess the left-right symmetrical and asymmetrical surroundings, three bubbles in a line. Assuming that the deformations are small, the surfaces of bubbles are described by a combination of the first three Legendre polynomials, that is, spherical symmetrical mode P0, antisymmetrical mode P1 and symmetrical mode P2. Based on the dynamics of three-bubble system, this paper further studies the velocity fields distribution around them. It can be seen from the contour distribution of the velocity field that the velocity component always decreases with the increase of the distance r. When three identical bubbles are separated uniformly, the velocity field around the central bubble is always in a symmetric, while there are asymmetric velocity fields around two side bubbles.

Interaction of ultrasonically driven bubble with a soft tissue-like boundary

This study investigates the size-dependent dynamics of bubbles and their interaction with soft boundaries under various ultrasound (US) conditions. We found that bubble behavior is influenced by size, with smaller bubbles displaying reduced inertial motion in similar ultrasound environments. Detailed analyses of three bubble sizes (1.5 µm, 15 µm, and 150 µm) next to a soft 1 kPa boundary revealed distinct patterns in radial oscillation, bubble center displacement, and boundary deflection for different ultrasound frequencies (5 kHz − 4 MHz). The smallest bubble maintained a spherical shape, while the largest experienced significant shape changes, indicative of impending jet formation. Investigating interactions at various frequencies highlighted the collapse tendency of the larger bubbles, showcasing maximum radial amplitude, displacement, and bubble wall velocity around its natural frequency. The presence of a soft boundary minimally affected radial amplitude and velocity, while the bubble displacement was contingent on the soft boundary modulus. Furthermore, boundary responses demonstrated that softer boundaries experienced less stress during bubble oscillations, exhibiting sharper peaks at resonance frequencies for larger bubbles. These findings provide valuable insights into optimizing ultrasound conditions for a variety of applications, highlighting the influence of bubble size and boundary properties on outcomes.

Experimental study on radial suction flow and its effect in water vortex unit

A water vortex unit is a non-contact adsorption unit that utilizes water as a flowing medium. Negative pressure is generated on the adsorbed surface through the high-velocity rotation of the fluid, resulting in suction force. In comparison to the air vortex unit, the water vortex unit exhibits a substantial increase in suction force and has the capability to deliver greater power. However, the presence of the central bubble in the water vortex unit weakens the suction force. To address the impact of the central bubble, a suction hole was introduced on the vortex unit to remove them. Experimental results indicate that solely removing the central bubble has almost no effect on the radial pressure gradients in the gap flow path and chamber center of the water vortex unit. However, in the cylindrical neighborhood, the radial pressure gradient increases, leading to a certain degree of improvement in suction force. Further increasing the suction flow rate induces inward radial flow within the vortex chamber. This flow field further enhances the suction force. To analyze the reasons for this pressure distribution, we indirectly obtained the radial velocity distribution by examining the relationship between the pressure gradient and tangential velocity. The results of the tangential velocity distribution indicate that the radially inward suction flow significantly increases the rotational velocity of the fluid in the chamber center, enhancing the centrifugal inertial effect. This further reduces the negative pressure, resulting in a significant improvement in the suction of the vortex unit.

Jetting enhancement from wall-proximal cavitation bubbles by a distant wall

An additional distant wall is known to highly alter the jetting scenarios of wall-proximal bubbles. Here, we combine high-speed photography and axisymmetric volume of fluid (VoF) simulations to quantitatively describe its role in enhancing the micro-jet dynamics within the directed jet regime (Zeng et al., J. Fluid Mech., vol. 896, 2020, A28). Upon a favourable agreement on the bubble and micro-jet dynamics, both experimental and simulation results indicate that the micro-jet velocity increases dramatically as $\eta$ decreases, where $\eta =H/R_{max}$ is the distance between two walls $H$ normalized by the maximum bubble radius $R_{max}$ . The mechanism is related to the collapsing flow, which is constrained by the distant wall into a reverse stagnation-point flow that builds up pressure near the bubble's top surface and accelerates it into micro-jets. We further derive an equation expressing the micro-jet velocity $U_{jet}=87.94\gamma ^{0.5}(1+(1/3)(\eta -\lambda ^{1.2})^{-2})$ , where ${\gamma =d/R_{max}}$ is the stand-off distance to the proximal wall with $d$ the distance between the initial bubble centre and the wall, $\lambda =R_{y,m}/R_{max}$ with $R_{y,m}$ the distance between the top surface and the proximal wall at the bubble's maximum expansion. Viscosity has a minimal impact on the jet velocity for small $\gamma$ , where the pressure buildup is predominantly influenced by geometry.

Simulation and analysis of the over-expanded flow field in asymmetric nozzles with lateral expansion

The study presented a numerical simulation of three-dimensional asymmetric nozzles featuring lateral expansion to investigate the flow field characteristics under over-expanded conditions and assess the influence of the lateral expansion angle. The research findings elucidate the distinct shape of the separation zones within the nozzle resulting from varying lateral expansion angles. In the restricted shock separation with separation bubbles forming on the flap pattern, the central separation bubble and corner separation bubbles exhibit independent characteristics. With an increasing lateral expansion angle under the same nozzle pressure ratio, the corner separation shock wave significantly impacts the shape of the central separation bubble.

Dynamics of bubble collapse near an armored free surface

Armored surfaces refer to a liquid–air interface covered by a layer of floating particles. This paper examines the collapse dynamics of a bubble generated by an electric spark near such an armored surface. The collapse of the bubble near the armored surface is similar to that near a free surface, with the formation of spraying liquid film, upward liquid jet in the forms of water dome, water spike, and water skirt with the change of the vertical distance (l) from the bubble center to the liquid surface. However, on the armored surface, we also observe particle splash and solid–liquid mixture splash. We confirm that the particle splash occurs due to the transfer of shock wave energy during bubble expansion and collapse. The splashing velocity (vp) and the distance (d0) from the bubble center to the particle follow the scaling law of vp ∼ l/d02. Additionally, we discuss the motion of the upward liquid jet and downward vortex ring. We find that the armored surface only affects the initial velocity of the jet without affecting its acceleration. This can be attributed to the additional energy dissipation caused by the splash formed on the armored surface. In contrast, the trajectory of the vortex ring remains unaffected by the armored surface. This study provides valuable insights into the dynamics of bubble collapse near an armored surface and highlights the role of floating particles in altering the behavior of liquid jets.

Improving sub-pixel accuracy in ultrasound localization microscopy using supervised and self-supervised deep learning

With a spatial resolution of tens of microns, ultrasound localization microscopy (ULM) reconstructs microvascular structures and measures intravascular flows by tracking microbubbles (1–5 μm) in contrast enhanced ultrasound (CEUS) images. Since the size of CEUS bubble traces, e.g. 0.5–1 mm for ultrasound with a wavelength λ = 280 μm, is typically two orders of magnitude larger than the bubble diameter, accurately localizing microbubbles in noisy CEUS data is vital to the fidelity of the ULM results. In this paper, we introduce a residual learning based supervised super-resolution blind deconvolution network (SupBD-net), and a new loss function for a self-supervised blind deconvolution network (SelfBD-net), for detecting bubble centers at a spatial resolution finer than λ/10. Our ultimate purpose is to improve the ability to distinguish closely located microvessels and the accuracy of the velocity profile measurements in macrovessels. Using realistic synthetic data, the performance of these methods is calibrated and compared against several recently introduced deep learning and blind deconvolution techniques. For bubble detection, errors in bubble center location increase with the trace size, noise level, and bubble concentration. For all cases, SupBD-net yields the least error, keeping it below 0.1 λ. For unknown bubble trace morphology, where all the supervised learning methods fail, SelfBD-net can still maintain an error of less than 0.15 λ. SupBD-net also outperforms the other methods in separating closely located bubbles and parallel microvessels. In macrovessels, SupBD-net maintains the least errors in the vessel radius and velocity profile after introducing a procedure that corrects for terminated tracks caused by overlapping traces. Application of these methods is demonstrated by mapping the cerebral microvasculature of a neonatal pig, where neighboring microvessels separated by 0.15 λ can be readily distinguished by SupBD-net and SelfBD-net, but not by the other techniques. Hence, the newly proposed residual learning based methods improve the spatial resolution and accuracy of ULM in micro- and macro-vessels.

Analysis of nanobubble collapse process by molecular simulation method

This study employs molecular dynamics simulations to investigate the process of nanobubble gradual indentation and eventual collapse. The research primarily focuses on the mechanisms by which impact velocity and bubble size influence the dynamic characteristics of nanobubble collapse. The results indicate that nanobubble collapse generally proceeds through three stages. Initially, there is a compression phase of water molecules surrounding the bubble, followed by a phase where the shock wave disrupts the stable structure of the liquid film, and finally, the complete collapse of the bubble. At higher impact velocities, smaller bubbles collapse more rapidly due to stronger shock effects. Post-collapse, a high-speed jet forms a protrusion on the right end of the velocity contour. The degree of protrusion increases with bubble size and impact velocity. Water molecules converge towards the bubble center, forming vortex structures above and below the bubble, effectively enhancing internal mass transfer. As bubble size and impact velocity increase, the density around the bubble gradually rises, reaching approximately 1.5 g/cm³ in localized areas upon complete collapse. When the bubble system decays to half its original size, a water hammer effect occurs. This effect becomes more pronounced with increasing bubble size and impact velocity. For a nanobubble structure with <i>u</i><sub>p</sub> = 3.0 km/s and <i>D</i> = 10 nm, the local pressure formed by the water hammer impact of the jet after collapse can reach 30 GPa.

Numerical Simulation of Cavitation Bubble Collapse inside an Inclined V-Shape Corner by Thermal Lattice Boltzmann Method

Cavitation happening inside an inclined V-shaped corner is a common and important phenomenon in practical engineering. In the present study, the lattice Boltzmann models coupling velocity and temperature fields are adopted to investigate this complex collapse process. Based on a series of simulations, the fields of density, pressure, velocity and temperature are obtained simultaneously. Overall, the simulation results agree with the experiments, and they prove that the coupled lattice Boltzmann models are effective to study cavitation bubble collapse. It was found that the maximum temperature of bubble collapse increases approximately linearly with the rise of the distance between the single bubble center and the corner. Meanwhile, the velocity of the micro-jet increases and the pressure peak at the corner decreases correspondingly. Moreover, the effect of angle of the V-shaped wall on the collapse process of bubbles is similar to the effect of distance between the single bubble center and the corner. Moreover, with the increase in bubble radius, the maximum temperature of bubble collapse increases proportionally, the starting and ending of the micro-jet are delayed and the pressure peak at the corner becomes larger and also is delayed. In the double bubble collapse, the effect of distance between two bubble centers on the collapse process of bubbles is discussed in detail. Based on the present study, appropriate measures can be proposed to prevent or utilize cavitation in practical engineering.

The mechanisms of jetting, vortex sheet, and vortex ring development in asymmetric bubble dynamics

Bubble dynamics near a rigid boundary at Reynolds numbers of O(10–100) exhibit significant viscous effect, associated with ultrasonic cavitation and cavitation damage. We study this phenomenon experimentally using high-speed photography of spark-generated bubble oscillation in silicone oils, whose viscosity is about three orders larger than water. Comparing to bubbles in water, bubble surfaces in silicone oil are more stable and thus more cycles of oscillations may be observed and studied. Additionally, we investigate this phenomenon numerically using the volume of fluid method. We propose a non-reflective boundary condition, reducing the computational domain's dimensions tenfold based on the far-field asymptotic behavior. This paper pays particular attention in the mechanism for the bubble jetting, the vortex sheet, and the vortex ring development. Initially, a stagnation point at the bubble center moves away from the wall owing to asymmetric bubble expansion, leaving the bubble around the moment the bubble reaches its maximum volume. During this process, a vortex sheet forms inside the bubble. As the vortex sheet approaches the bubble interface, it transfers momentum to the gas–liquid interface, influencing the flow near the bubble wall. The high-pressure zone at the stagnation point drives the distal bubble surface to collapse first and fastest subsequently. This asymmetric collapse generates circulation around the bubble's side cross section, leading to the development of a vortex ring within the bubble gas at the outer rim of the decaying vortex sheet. The vortex ring, with its core inside the bubble gas, functions like a bearing system in accelerating the jet.

Manipulation of bubble collapse patterns near the wall of an adherent gas layer

This paper aims to apply experimental methods to investigate the effect of the thickness of gas layers on the wall on the collapse direction of spark-induced bubbles. In the experiment, two high-speed cameras synchronously record the time evolution of the bubbles and the corresponding parameters such as the normalized collapse position and bubble collapse time. Experiments yielded results for individual bubbles over a range of normalized distances from 0 to 4.0 for different air layer thicknesses. Based on the morphology of the bubbles, the experimental jets were visualized into six different modes, namely, forward jet (FJ), merging jet (MJ), bidirectional jet (BJ), reversing jet (RJ), forward followed by reversing jet (FRJ), and non-directional jet (NDJ). The height of the air layer on the wall is affected by the fluctuation of the bubble volume and shows the opposite trend to the change of the bubble volume. The air film reaches its maximum height when the bubble collapses, which affects the final jet pattern. In addition, as the thickness of the air layer increases, the center of the bubble gradually migrates away from the wall. The different collapse modes and the migration of the bubble centers have positive significance for reducing cavitation erosion in engineering.

А particle model of interaction between weakly non-spherical bubbles

A mathematical model of interaction of weakly non-spherical gas bubbles in liquid is proposed. It is a system оf the second order ODEs in the radii of bubbles, the position vectors of their centers and the amplitudes of their small deformations in the form of spherical harmonics. Compared to the available analogs, the proposed model equations are more accurate in terms of the ratio of the radii of bubbles to the distance between their centers. This allows one to study bubble-bubble interaction at smaller distances between the bubbles. The model equations are quite compact, which simplifies their theoretical analysis and construction of algorithms of their solution. Five problems (dynamics of a single bubble under harmonic excitation, bubble collapse near a plane rigid wall, interaction of three synchronized and nonsynchronized bubbles located in line, and interaction between sixteen bubbles with the centers located on a plane surface) are considered for validation. It is shown that in all cases the proposed model results well agree with the corresponding available experimental data and numerical solutions. For demonstration of the applicability, the proposed model is used to analyze radial oscillations, spatial movements and surface deformations of the bubbles in rectilinear, planar and spatial clusters under harmonic forcing. Initially 125, 121 and 125 bubbles of the clusters are located at the nodes of 1D, 2D and 3D uniform grids, respectively. It is found that the monotonically decaying radial oscillations of the 1D cluster bubbles during one period of forcing are quite similar to those of a single bubble. Unlike this, the attenuation of the radial oscillations of the central bubbles in the 2D and 3D clusters is essentially non-monotonic. The maximum pressures in the central bubbles of the 2D and 3D clusters are significantly higher than in the single and 1D cluster bubbles. In all clusters the maximum deformations of bubbles are in the form of the second spherical harmonics, except for the central bubble of the 3D cluster, the maximum deformations of which are in the form of the fourth harmonics.

Translation of interacting cavitation bubble in a plane acoustic field

In a plane acoustic field, a model involving the radial and translational motions of bubble is derived, which is used to numerically investigate the translation of two interacting spherical cavitation bubbles. For the smaller initial distance between bubble centers, we investigate the dynamics of two bubbles, and find that they could come into contact but the velocities of closing to each other are different when the equilibrium radius is different. The results show that when the wavelength of plane acoustic field is much larger than the initial distance, the bubbles are approximately pulsating in phase. Moreover, the velocity of contact process of two bubbles can be changed by adjusting the parameters of plane acoustic field. For increasing in the initial distance, the bubbles would present two translation motions: close to each other for the pulsating in phase and away from each other for the pulsating out of phase, which could be verified by calculating the secondary Bjerknes force. Meanwhile, the larger the initial distance, the smaller the secondary Bjerknes force value is. Understanding the translation of bubble is of great significance for helpful explaining formation of streamer structures in ultrasonic cavitation.

Droplet boiling on micro-pillar array surface – Nucleate boiling regime

Despite spray cooling in the form of droplet boiling on a textured surface being a very promising phase-change heat dissipating method, the understanding of droplet/bubble two-phase dynamics in the nucleate boiling is extraordinarily limited. In this study, we report sessile droplet boiling on micro-pillar array surface in the nucleate boiling regime using a three-dimensional lattice Boltzmann model comprehensively. Effects of micro-pillar size on bubble behaviors inside droplet are discussed in detail, covering bubble nucleation, growth, coalescence, and rupture. For the micro-pillars with large side length or small spacing, nucleation sites are activated around micro-pillar top surface. The preferential activation location of nucleation sites is determined by temperature, confined space and fluid flow. In bubble growth stage, the variation of bubble radius with time follows the square root law, being consistent with previous experiments. Bubbles merge into a large central bubble beneath droplet for the short micro-pillars while into a vapor layer for the long micro-pillars. Emergence of large central bubble prolongs droplet lifetime but deteriorates heat transfer. In addition, increasing micro-pillar side length or decreasing micro-pillar height can delay activation of nucleation sites.