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In recent years, agricultural non-point source pollution (ANPSP) has become increasingly prominent, and nitrogen plays an important role in ANPSP. Therefore, we carried out traditional flooded irrigation (TFI) experiments in the paddy field, and applied HYDRUS-2D model to simulate the nitrogen transport in this study. Three observation points A1, A2, and A3 were arranged on the diagonal of the paddy field. We observed ponding water depth on soil surface and nitrogen concentrations in ponding water and soil water at 0.1 m, 0.2 m, and 0.3 m below soil surface. HYDRUS-2D model was proved to be effective in simulating the ponding water depth with root mean squared error (RMSE) = 0.717 cm and Nash-Sutcliffe coefficient (NSE) = 0.805 for the simulated and measured ponding water depth. The simulated and measured NH

X-ray free-electron lasers (XFELs) open a new era of X-ray based research by generating extremely intense X-ray flashes. To further improve the spectrum brightness, a self-seeding FEL scheme has been developed and demonstrated experimentally. As the next step, new-generation FELs with high repetition rates are being designed, built and commissioned around the world. A high repetition rate would significantly speed up the scientific research; however, alongside this improvement comes new challenges surrounding thermal management of the self-seeding monochromator. In this paper, a new configuration for self-seeding FELs is proposed, operated under a high repetition rate which can strongly suppress the thermal effects on the monochromator and provides a narrow-bandwidth FEL pulse. Three-dimension time-dependent simulations have been performed to demonstrate this idea. With this proposed configuration, high-repetition-rate XFEL facilities are able to generate narrow-bandwidth X-ray pulses without obvious thermal concern on the monochromators.

The electron beam (e-beam) in the scanning electron microscopy (SEM) provides an appealing mobile heating source for thermal metrology with spatial resolution of ∼1 nm, but the lack of systematic quantification of the e-beam heating power limits such application development. Here, we systemically study e-beam heating in LPCVD silicon nitride (SiNx) thin-films with thickness ranging from 200 to 500 nm from both experiments and complementary Monte Carlo simulations using the CASINO software package. There is good agreement about the thickness-dependent e-beam energy absorption of thin-film between modeling predictions and experiments. Using the absorption results, we then demonstrate adapting the e-beam as a quantitative heating source by measuring the thickness-dependent thermal conductivity of SiNx thin-films, with the results validated to within 7% by a separate Joule heating experiment. The results described here will open a new avenue for using SEM e-beams as a mobile heating source for advanced nanoscale thermal metrology development.

We present a new mounting design for thin crystal optics with cryogenic cooling compatibility. We design a crystal geometry with two symmetric strain-relief cuts to mitigate the distortion from mounting. We propose to sputter gold onto the crystal and the holder to ensure excellent thermal contact and sufficient mechanical bonding. The system is analyzed and verified by finite element analysis to have an acceptable level of strain due to mounting. The thermal performance of this mounting scheme is validated in an example cryogenic cooling system and the results indicate a tolerance of power density up to ∼1 kW/mm2.

Minichannel technology has been shown to be a promising solution for the battery thermal management system (BTMS). Since the design space of the BTMS using minichannel technology consists a large number of parameters, performing parametric study and optimization of such a system is challenging. To overcome this challenge, a simplified numerical model is developed. In this model, effective convective heat transfer (heff) boundary condition is used at the cell-minichannel interface to circumvent simulating conjugate heat transfer and fluid flow in the minichannel. After validation with past work based on a three-dimensional conjugate heat transfer model, the simplified model is used to conduct a parametric study to determine the effect of various design and operation parameters on the performance of the BTMS. For the proposed BTMS, it is found that cooling half of the cell surface on single side of the cell is enough to keep the maximum temperature difference in the pack to be less than 3 °C at 2C discharge rate. At module level, it has been found that minichannel cooling on only one side of the cell or on every other cell can significantly increase the system level energy density. The results from the parametric study would be useful in designing a robust, effective, and economical minichannel-based BTMS for Electrical Vehicles.

In the field of beam physics, two frontier topics have taken center stage due to their potential to enable new approaches to discovery in a wide swath of science. These areas are: advanced, high gradient acceleration techniques, and x-ray free electron lasers (XFELs). Further, there is intense interest in the marriage of these two fields, with the goal of producing a very compact XFEL. In this context, recent advances in high gradient radio-frequency cryogenic copper structure research have opened the door to the use of surface electric fields between 250 and 500 MV/m. Such an approach is foreseen to enable a new generation of photoinjectors with six-dimensional beam brightness beyond the current state-of-the-art by well over an order of magnitude. This advance is an essential ingredient enabling an ultra-compact XFEL (UC-XFEL). In addition, one may accelerate these bright beams to GeV scale in less than 10 meters. Such an injector, when combined with inverse free electron laser-based bunching techniques can produce multi-kA beams with unprecedented beam quality, quantified by ~50 nm-rad normalized emittances. These beams, when injected into innovative, short-period (1-10 mm) undulators uniquely enable UC-XFELs having footprints consistent with university-scale laboratories. We describe the architecture and predicted performance of this novel light source, which promises photon production per pulse of a few percent of existing XFEL sources. We review implementation issues including collective beam effects, compact x-ray optics systems, and other relevant technical challenges. To illustrate the potential of such a light source to fundamentally change the current paradigm of XFELs with their limited access, we examine possible applications in biology, chemistry, materials, atomic physics, industry, and medicine which may profit from this new model of performing XFEL science., 80 pages, 24 figures

With dramatically improved brightness and repetition rate, the thermal load for crystal optics in x-ray free-electron laser applications has also significantly increased. To mitigate the thermal load, one effective method is cryogenic cooling. However, the emerging nondiffusive heat transfer phenomenon at cryogenic temperature may cause design failure if overlooked. To evaluate the optical performance of thin crystal optics under thermal load at cryogenic temperature, an integrated numerical tool is presented and applied to characterize the thermal load on thin crystals with nondiffusive phenomena accounted. Significant thermally induced distortion of the rocking curve is observed from numerical simulation, leading to potential seed power reduction in hard x-ray self-seeding application. Cryogenic cooling is confirmed necessary by simulation to handle the thermal load at high repetition rate operation.

Optical trapping and manipulation have emerged as a powerful tool in the biological and physical sciences. In this work, we present a miniature optical tweezers device based on multifocal optical vortex metalens (MOVM). The MOVM is capable of generating multiple focal fields with specific orbital angular momentum at arbitrary position. The optical force of the vortex field exerted on both high-refractive-index particle and low-refractive-index particle are analyzed. The simulation results show that the two kinds of dielectric particles can be trapped simultaneously. Besides, it is also feasible to manipulate plasmonic nanoparticles even under the resonant condition, which is realized by constructing a 4Pi focusing system with metalenses. Moreover, the metalens can be made into an array format that is suitable for trapping and manipulating various nanoparticles with diverse motion behaviors. The work illustrates the potential of such optical tweezers for further development in lab-on-a-chip devices, and may open up new avenues for optical manipulation and their applications in extensive scientific fields.

Thermal load has been a haunting factor that undermines the brightness and coherence of the self-seeded X-ray free-electron laser. Different from uniformly pulsed mode, in pulse train mode a thermal quasi-steady state of the crystal monochromator may not be reached. This leads to a dynamic thermal distortion of the spectral transmission curves and seed quality degradation. In this paper, the pulse-to-pulse thermal load effects on the spectral transmission curves and seed quality are shown, and some instructive information for the tuning process is provided.

Water bridge dynamics between an atomic force microscopy (AFM) tip and a flat substrate is studied by using a multibody dissipative particle dynamics (MDPD) model. First, the numerical model is validated by comparing the present results of droplet contact angles and liquid bridges with those reported in the literature. Then, the ability of MDPD to capture the meniscus shape and behavior for different operating conditions and geometric parameters is examined for both static and dynamic cases. Hence, several parametric studies and analyses of the AFM tip configuration and its operating conditions are reported. It is found that a critical capillary number of about 0.001 is calculated based on 5% change on the force measurements between the static and dynamic results. It is also demonstrated that the hysteresis behavior in the capillary force exerted on the AFM tip can be successfully predicted by using the MDPD model when the tip approaches or retracts from the substrate. Moreover, there is an excellent agreement in the results of breakup distance for different water bridge volumes between the predictions of the MDPD model and the theory. Also, the hysteresis of capillary force exerted on an AFM tip composed of multibody design is studied. The prediction on the transition of the capillary force vs distance between the AFM tip and the substrate is in good agreement with the experimental results. Therefore, we demonstrate a validated MDPD model which can successfully capture liquid bridge dynamics. This model can be used as a powerful design tool for meniscus manipulation technology, such as dip-pen nanolithography, as well as for studying dynamic, e.g., tapping mode AFM tip, interactions with a liquid bridge.

In this paper, we propose a microscale liquid oscillator using electrowetting-on-dielectric (EWOD). Specifically, a mesoscale liquid bridge (LB) between two horizontal surfaces with EWOD is considered. When EWOD is applied, the solid surface becomes more hydrophilic, and hence the contact angle (CA) is reduced. Following the activation of EWOD, the LB can remain connected or it can break into either symmetric or asymmetric shapes depending on the initial liquid volume and wettability of the two surfaces. The LB dynamics activated by EWOD is studied using the multibody dissipative particle dynamics (MDPD) method. Our numerical results show that the behavior of an LB under EWOD can be interpreted via three modes. In the first mode, the LB does not break after applying EWOD. In the second mode, the LB breaks and does not reform. The third mode happens when, depending on the interplay of the volume of the liquid and CA manipulation, the LB continuously breaks, recoils, and reforms. For asymmetric cases, it was observed that the LB may completely detach from one surface and may not reform. It was also observed that decreasing the wettability of a surface, for cases with a continuous breaking and reformation behavior, increases the connecting time interval and decreases the breaking time interval in one break-reform cycle. The results provided in this investigation facilitate fundamental understanding of LB dynamics and their application for the design of microscale liquid oscillators using EWOD.

California has some of the key agricultural regions in the United States. One of these key regions, San Joaquin Valley, frequently experiences severe droughts leading to shortage of irrigation water. This has a significant impact on the agricultural based economy of the region. It is imperative to develop new strategies to reduce overall water consumption in agriculture without affecting crop yield. A large fraction of irrigation water is lost due to the evapotranspiration (ET) process in the crops and the soil. The classical Penman-Monteith model has been used in the present work to analyze the effect of different environmental variables and water saving strategies on the ET. Some of the scenarios considered show potential for significant water savings without much reduction in the amount of sunlight available to facilitate crop growth. The central idea considered in this study is the use of canopy shading to cover the crop field resulting in reduction in the ET. Among the strategies considered, the most promising strategy is to partially cover the crop field for a certain part of the day by employing a partially covering retractable canopy. Based on numerical calculations, total reduction in ET is calculated to be 37% from June to August for the partially covering retractable canopy.

Thermal management on lithium-ion batteries is a crucial problem for the performance, lifetime, and safety of electric vehicles (EVs) and hybrid electric vehicles (HEVs). Fire and explosions can be triggered by thermal runaway if the temperature of the lithium-ion batteries is not maintained properly. This work describes a minichannel cooling system designed at the battery module level and the investigation on its efficacy on the mitigation of thermal runaway. Nail penetration was employed to simulate the internal short circuits, which in reality may be caused by vehicle collisions and/or manufacturing defects. Two integrated models were utilized to study thermal runaway: the conjugate heat transfer model and the reaction kinetics model. Numerical simulations were conducted to understand the thermal runaway process and the effects of flow rate, thermal abuse reactions, nail penetration depth, and nail diameter. It is concluded that minichannel cooling at cell level cannot cease thermal runaway in a single cell, but it can prevent battery fratricide due to thermal runaway propagation between cells.

The appearance of a ground surface can play an important role in the flow structures for the flows past a flat plate. We conduct two-dimensional numerical simulations on viscous flows past a flat plate inclined at an angle of attack of $$20^\circ $$ with ground effects using a finite-volume method. Results show that the effects on the separated flow from the ground are highly dependent on the gap (G) between the plate and the ground. As the gap decreases, the strength of vortices generated from the trailing edge is restrained, which is consistent with experimental observations. Further decrease in the gap even eliminates the vortex shedding and yields a steady flow. It is also found that the flow between the gap can either be accelerated at large gap ratios ( $${G/L >1}$$ , G is the gap, L is the plate length), or be decelerated at small gap ratios ( $${G/L

Lithium-ion batteries are widely used for battery electric (all-electric) vehicles (BEV) and hybrid electric vehicles (HEV) due to their high energy and power density. An battery thermal management system (BTMS) is crucial for the performance, lifetime, and safety of lithium-ion batteries. In this paper, a novel design of BTMS based on aluminum minichannel tubes is developed and applied on a single prismatic Li-ion cell under different discharge rates. Parametric studies are conducted to investigate the performance of the BTMS using different flow rates and configurations. With minichannel cooling, the maximum cell temperature at a discharge rate of 1C is less than 27.8 °C, and the temperature difference across the cell is less than 0.80 °C using flow rate at 0.20 L/min, at the expense of 8.69e-6 W pumping power. At higher discharge rates, e.g., 1.5C and 2C, higher flow rates are required to maintain the same temperature rise and temperature difference. The flow rate needed is 0.8 L/min for 1.5C and 2.0 L/min for 2C, while the required pumping power is 4.23e-4 W and 5.27e-3 W, respectively. The uniform temperature distribution (

Many studies have revealed that Fourier’s law becomes invalid in the study of heat transfer in micro/nanoscale systems due to phonon-mediated nondiffusive heat transport. It is necessary to develop high-fidelity non-Fourier models and numerical methods to study nondiffusive heat transfer for both experimental data analysis in nanothermometry and thermal design in micro-/nano-systems. Starting from the phonon Boltzmann transport equation (BTE), we develop an enhanced Gray (EG) model by considering the second-order terms in Taylor expansion. In the proposed enhanced Gray BTE (EG-BTE), two parameters associated with inherent material properties, i.e., the diffusive relaxation time and the ballistic mean free time, are used to characterize the diffusive heat conduction and nondiffusive heat conduction, respectively. While the diffusive relaxation time is calculated directly from the thermal conductivity, the ballistic mean free time of nonmetallic materials can be determined from transient grating experiments. An implicit lattice Boltzmann method (ILBM) is developed to solve the EG-BTE, which is unconditionally stable. The EG-BTE model is validated by comparing predictions of thermal decay processes with transient grating experiments. After being validated, the EG-BTE model is applied to study heat conduction across thin films from ballistic to diffusive regime at room temperature as well as thermal wave propagation in crystal at low temperatures. The numerical results demonstrate that the EG-BTE provides a unified model to describe multiscale heat conduction covering both nondiffusive and diffusive regimes.

High-speed cold spraying of micro-particles, at temperatures well below their melting point, impinging on desired substrates is a useful emerging additive process. One principal element believed to dramatically influence the bonding strength and final coating quality of many metallic cold-spray deposits is the native oxide layer on the surface of the impinging particles and substrate. Here, smoothed-particle hydrodynamics (SPH), a mesh-free, particle-based method, is used to simulate cold-spraying, including the action of oxide layers in the deposition and bonding processes. The effect of increasing particle impact velocity on oxide breakup and subsequent bonding is studied. Particle/substrate bonding is quantified via metallic interaction at the interface and found to continuously improve with increasing velocity well beyond the critical velocity. The coefficient of restitution is also calculated for different oxide layer thicknesses on the particle and the substrate. The effects of particle diameter and oxygen content on the impact process are reported and compared to results from the literature. The particle-based simulation approach is found to reproduce experimentally characterized trends in oxide content effect and provide insight into the mechanisms of oxide breakup during bonding.

Non-uniform thermal load causes performance degradation of crystal X-ray optics. With the development of high-brightness X-ray free-electron lasers, the thermal load on X-ray optics becomes even more severe. To mitigate the thermal load, a quantitative understanding of thermal effects on the optical performance is necessary. We derived an analytical model for monochromator performance under a non-uniform thermal load. This analytical model quantitatively describes the distortion of the rocking curve and attributes different contributions to different factors of thermal load. It provides not only monochromator design insights and considerations, but also a quick estimation of the rocking curve distortion due to thermal load for practical situations such as pump-probe experiments.

One approach to achieve longitudinally coherent hard X-ray free electron laser (XFEL) radiation is to introduce a single-crystal monochromator in the beam path to generate a co-propagating, time-synchronized self-seeding radiation pulse. For modern high-repetition rate XFELs, however, thermal loading on the crystal can degrade the quality of the seed pulse. To assess limitations on the maximum XFEL pulse repetition rate imposed by the seeding crystal, a comprehensive study of thermomechanical, and X-ray diffraction effects within the crystal is presented for two consecutive self-amplified spontaneous emission pulses incident on both reflective and transmissive crystal monochromators.

Li-ion based energy storage devices have highly temperature dependent characteristics such as performance, life-cycle, efficiency and safety. Large temperature gradient within a cell results in thermal stresses and nonuniform current density leading to accelerated degradation. This adversely affects the life cycle of the cell due to capacity and power fade. There are similar issues due to large temperature variation within a battery pack. Operation of Li-ion cell outside the desirable temperature range also leads to lower efficiency, degradation and safety related issues. Different thermal management approaches have been proposed and demonstrated in past. The present work focuses specifically on minichannel based liquid cooling for conducting a parametric study. Minichannels have been found effective in various thermal management applications due to their simple construction and high convective heat transfer. In past, minichannels have been proposed and used in battery thermal management. However, designing of such systems has been somewhat arbitrary without considering various factors and trade-offs involved. There is a lack of rigorous studies for determining various parameters related to thermal management system that would result in adequate thermal management in a cost-effective manner. In the present work, a comprehensive parametric study has been carried out on the minichannel based liquid cooling for thermal management of Li-ion battery pack. A simplified computationally efficient numerical simulation-based approach has been used to conduct parametric study for optimizing the design and operating parameters of the thermal management system.

Optical trapping and manipulation have emerged as a powerful tool in the biological and physical sciences. In this work, we present a miniature optical tweezers device based on multifocal optical vortex metalens (MOVM). The MOVM is capable of generating multiple focal fields with specific orbital angular momentum at arbitrary position. The optical force of the vortex field exerted on both high-refractive-index particle and low-refractive-index particle are analyzed. The simulation results show that the two kinds of dielectric particles can be trapped simultaneously. Besides, it is also feasible to manipulate plasmonic nanoparticles even under the resonant condition, which is realized by constructing a 4Pi focusing system with metalenses. Moreover, the metalens can be made into an array format that is suitable for trapping and manipulating various nanoparticles with diverse motion behaviors. The work illustrates the potential of such optical tweezers for further development in lab-on-a-chip devices, and may open up new avenues for optical manipulation and their applications in extensive scientific fields.

The detachment of droplets from cylindrical fibers is of fundamental importance for both scientific research and engineering applications. Due to the challenges to determine dynamic contact angles on the fiber surface, the process of the droplet detachment from a fiber is not well understood. In this paper, a multibody dissipative particle dynamics (MDPD) method, a particle-based mesh-free method that can automatically capture the dynamic contact angles through direct modeling of liquid-solid particle interactions, was applied to study the detachment process of a liquid microdroplet from a cylindrical solid fiber pulled by an atomic force microscopy (AFM) tip under a constant velocity. After the validation of the numerical results through comparison with experiments in a benchmark case, the same numerical tool was applied to analyze the droplet detachment mechanisms. Based on the slope of the time history curve for the displacement of the droplet mass center, the detachment process can be divided into six stages. The change of slope in each stage can be explained from the change of surface energy. The results can greatly advance the fundamental understanding of the detachment process of microdroplets from cylindrical fibers.

Manipulation of nanoparticles in solution is of great importance for a wide range of applications in biomedical, environmental, and material sciences. In this work, we present a novel plasmonic tweezers based on metahologram. We show that various kinds of nanoparticles can be stably trapped in a surface plasmon (SP) standing wave generated by the constructive interference between two coherent focusing SPs. The absence of the axial scattering force and the enhanced gradient force enable to avoid overheating effect while maintaining mechanical stability even under the resonant condition of the metallic nanoparticle. The work illustrates the potential of such plasmonic tweezers for further development in lab-on-a-chip devices.

As reported by many studies, Fourier’s law breaks down in micro/nanoscale due to the nondiffusive heat transport. To account for the nondiffusive heat transport, high-fidelity nondiffusive models with good efficiency for the experimental data analysis in nanothermometry are necessary but unfortunately missing. In this paper, based on a validated enhance Gray Boltzmann transport equation, we offer the analytical solutions for two important nanothermometry techniques, namely the transient thermal gratings (TTG) and time-domain thermoreflectance (TDTR) experiments. The analytical solutions obtained by inverse Fourier transform are compared to the experimental signals in both TTG and TDTR cases. The excellent agreements between the analytical solutions and the experiments demonstrate the applicability of the EG-BTE in experimental data analysis as an efficient replacement of Fourier’s law.

A two-dimensional model is developed to study the flutter instability of a flag immersed in an inviscid flow. Two dimensionless parameters governing the system are the structure-to-fluid mass ratio M⁎ and the dimensionless incoming flow velocity U⁎. A transition from a static steady state to a chaotic state is investigated at a fixed M⁎=1 with increasing U⁎. Five single-frequency periodic flapping states are identified along the route, including four symmetrical oscillation states and one asymmetrical oscillation state. For the symmetrical states, the oscillation frequency increases with the increase of U⁎, and the drag force on the flag changes linearly with the Strouhal number. Chaotic states are observed when U⁎ is relatively large. Three chaotic windows are observed along the route. In addition, the system transitions from one periodic state to another through either period-doubling bifurcations or quasi-periodic bifurcations, and it transitions from a periodic state to a chaotic state through quasi-periodic bifurcations.

Significant influence from coking (aggregation and formation of a non-desirable carbonaceous porous layer on a solid metal surface) on the heat transfer in a cooling system of hypersonic aircraft with hydrocarbon fuel as the coolant is observed. In the present work, an explanation was proposed to understand the influence mechanism of coking on the heat transfer in cooling channel. In addition, a numerical model was further established to verify the explanation by comparing the heat transfer in cooling channel with different thicknesses of coke layer. The results indicated that critical thicknesses of coke layer with respect to effective thermal conductivity ( δ cl , λ c ) and heat transfer ( δ cl , h c ) existed. More specifically, the heat transfer was noticeably enhanced when δcl δ cl , h c , while the heat transfer is deteriorated as δcl > δ cl , h c . A transition thickness with length of ( δ cl , h c – δ cl , λ c ) existed between δ cl , λ c and δ cl , h c under the combined influences of convection of core flow and coke layer. By comparing the heat transfer in cooling channel after coke deposit at different inlet Reynolds number, it was further found that δ cl , λ c and δ cl , h c decrease with increasing the turbulent intensity. This implied that the influence of inlet Reynolds number on the coke deposition rate and turbulent heat transfer were dissimilar. This study is expected to provide further insight into the optimization of cooling channel for the purpose of reducing the heat transfer deterioration caused by coke.

Thermal runaway is the top safety concern for Li-ion electrochemical energy storage systems. There are multiple abuse conditions that may cause thermal runaway in Li-ion cells. Although thermal runaway has been extensively studied, the runaway mechanisms due to external short circuit and ultra-high discharge rates (>10C) are relatively less studied. In the present work, an analytical thermal runaway model is developed to predict thermal runaway in prismatic and pouch Li-ion cells due to either external short circuit or ultra-high discharge rates. The heat generation data corresponding to the ultra-high discharge rates considered in the thermal runaway model have been obtained separately using an electrochemical-thermal coupled model. The analytical thermal runaway model is validated against experiments as well as COMSOL, a finite element-based commericial solver. Using the analytical model, the effects of key parameters on cell safety have been analyzed. Finally, the analytical model has been used to evaluate effectiveness of a thermal runaway prevention strategy based on boiling in minichannels of a water-cooled minichannel based battery thermal management system.

A hybrid phonon gas model was developed to describe heat pulse propagation in dielectric crystals. In this model, heat energy is described as a mixture of longitudinal and transverse phonon gases. The novel aspects of this model include: (1) the distinction between longitudinal and transverse phonon excitations is taken into account; (2) transitions between ballistic and second sound transport, ballistic and diffusive transport, and second sound and diffusive transport are successfully captured. For the first time, benchmark cases of heat-pulse experiments in NaF at low temperatures in ballistic, ballistic–diffusive, and diffusive regions have been completely reconstructed in numerical simulations, which demonstrate the validity of the new model. It was elucidated how heat pulses are transmitted by longitudinal, ballistic transverse and dispersive transverse phonons at low temperatures. The numerical results not only yield new insight in physics of transient ballistic–diffusive heat conduction with effects from different phonon scattering processes, but also provide numerical tools to study similar transient ballistic–diffusive heat conduction in nanoelectronics and modern optoelectronics.

While there are intensive studies on the coalescence of sessile macroscale droplets, there is little study on the coalescence of sessile microdroplets. In this paper, the coalescence process of two sessile microdroplets is studied by using a many-body dissipative particle dynamics numerical method. A comprehensive parametric study is conducted to investigate the effects on the coalescence process from the wettability gradient, hydrophilicity of the solid surface, and symmetric or asymmetric configurations. A water bridge is formed after two microdroplets contact. The temporal evolution of the coalescence process is characterized by the water bridge's radii parallel to the solid surface $({W}_{m})$ and perpendicular to the solid surface $({H}_{m})$. It is found that the changes of both ${H}_{m}$ and ${W}_{m}$ with time follow a power law; i.e., ${H}_{m}={\ensuremath{\beta}}_{1}{\ensuremath{\tau}}^{\ensuremath{\beta}}$ and ${W}_{m}={\ensuremath{\alpha}}_{1}{\ensuremath{\tau}}^{\ensuremath{\alpha}}$. The growth of ${H}_{m}$ and ${W}_{m}$ depends on the hydrophilicity of the substrate. ${W}_{m}$ grows faster than ${H}_{m}$ on a hydrophilic surface, and ${H}_{m}$ grows faster than ${W}_{m}$ on a hydrophobic surface. This is due to the strong competition between capillary forces induced by the water-bridge curvature and the solid substrate hydrophobicity. Also, flow structure analysis shows that regardless of the coalescence type once the liquid bridge is formed the liquid flow direction inside the capillary bridge is to expand the bridge radius. Finally, we do not observe oscillation of the merged droplet during the coalescence process, possibly due to the significant effects of the viscous forces.