Exploring mounting solutions for cryogenically cooled thin crystal optics in high power density x-ray free electron lasers.

This study investigates three mounting methods-clamping, soldering, and a hybrid clamping-soldering approach-for cryogenically cooled thin diamond crystals crucial to stable operation of X-ray Free Electron Laser (XFEL) systems. While clamping methods exhibit temperature resilience and flexibility, meticulous design is required to prevent stress-induced warping and reduce thermal contact area. Soldering methods offer reliable mechanical and thermal bonding but encounter challenges due to the coefficient of thermal expansion mismatch at cryogenic temperatures. The hybrid method, integrating clamping and soldering with strain relief cuts, effectively mitigates overall distortion caused by mounting and XFEL thermal loads. These findings offer a novel mounting solution for high-performance x-ray optics in XFEL research and applications, ensuring stability and optimal functionality in cryogenic conditions.

Using HYDRUS-2D model to simulate the water flow and nitrogen transport in a paddy field with traditional flooded irrigation.

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 NH4+-N concentrations at different depths below soil surface at point A1 basically had the same trend, and the simulated NH4+-N concentrations in ponding water had better agreement with the measured data with RMSE = 1.323 mg/L, and NSE = 0.958. The measured NH4+-N concentrations at depths of 0.1 m, 0.2 m, and 0.3 m below soil surface at point A2 were larger than the simulated values, but they had the same trend on the whole. The simulated NH4+-N concentrations at different depths below soils' surface at point A3 did not fit well with the measured values. The overall trend of the simulated and measured NO3--N concentrations in ponding water on soil surface at point A1 was consistent, but the peak values of the simulated NO3--N concentrations were larger than the measured ones. The simulated and measured NO3--N concentrations at different depths below soil surface at points A2 and A3 did not agree well although they had the same trend, which became worse with the increase of soil depth. This indicated that the HYDRUS-2D model was effective in simulating water flow and nitrogen transport in TFI paddy fields. Sensitivity analysis suggested different simulated nitrogen concentrations in different water depths at different time were sensitive to different model parameters.

New mounting mechanism for cryogenically cooled thin crystal x-ray optics in high brightness high repetition rate free-electron laser applications.

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.

Two-stage reflective self-seeding scheme for high-repetition-rate X-ray free-electron lasers.

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.

Dynamic pulse-to-pulse thermal load effects in pulse-train-mode self-seeded X-ray free-electron laser.

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.

Analytical model for monochromator performance characterizations under thermal load.

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.

Adapting the Electron Beam from SEM as a Quantitative Heating Source for Nanoscale Thermal Metrology.

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.

Modes and break periods of electrowetting 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.

Publisher Correction: Trapping and manipulation of nanoparticles using multifocal optical vortex metalens.

A correction to this article has been published and is linked from the HTML version of this paper. The error has been fixed in the paper.

Six Stages of Microdroplet Detachment from Microscale Fibers.

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.

Trapping and manipulation of nanoparticles using multifocal optical vortex metalens.

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.

Plasmonic trapping of nanoparticles by metaholograms.

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.

Coalescence of sessile microdroplets subject to a wettability gradient on a solid surface.

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}=ß_{1}τ^{ß} and W_{m}=α_{1}τ^{α}. 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.

Multi-channel orbital angular momentum detection with metahologram.

Orbital angular momentum (OAM) is an intrinsic property of light that has attracted increasing attention recently. In a wide range of applications that involve OAM, it is often crucial to discern the OAM states with high fidelity. In this Letter, we propose a novel method to extend the detectable range of the OAM states by adopting a multi-sector metahologram. The incident light carrying OAM would be focused by the metahologram into surface plasmon waves with separated propagation directions that are spatially sampled by multiple subwavelength detectors. Through quantizing and mapping the detector signals into a lookup table, a wide range of OAM states could be distinguished. The principle reported in this Letter may find important applications in optical communications and information processing with the OAM states.

Numerical Simulations of the Digital Microfluidic Manipulation of Single Microparticles.

Single-cell analysis techniques have been developed as a valuable bioanalytical tool for elucidating cellular heterogeneity at genomic, proteomic, and cellular levels. Cell manipulation is an indispensable process for single-cell analysis. Digital microfluidics (DMF) is an important platform for conducting cell manipulation and single-cell analysis in a high-throughput fashion. However, the manipulation of single cells in DMF has not been quantitatively studied so far. In this article, we investigate the interaction of a single microparticle with a liquid droplet on a flat substrate using numerical simulations. The droplet is driven by capillary force generated from the wettability gradient of the substrate. Considering the Brownian motion of microparticles, we utilize many-body dissipative particle dynamics (MDPD), an off-lattice mesoscopic simulation technique, in this numerical study. The manipulation processes (including pickup, transport, and drop-off) of a single microparticle with a liquid droplet are simulated. Parametric studies are conducted to investigate the effects on the manipulation processes from the droplet size, wettability gradient, wetting properties of the microparticle, and particle-substrate friction coefficients. The numerical results show that the pickup, transport, and drop-off processes can be precisely controlled by these parameters. On the basis of the numerical results, a trap-free delivery of a hydrophobic microparticle to a destination on the substrate is demonstrated in the numerical simulations. The numerical results not only provide a fundamental understanding of interactions among the microparticle, the droplet, and the substrate but also demonstrate a new technique for the trap-free immobilization of single hydrophobic microparticles in the DMF design. Finally, our numerical method also provides a powerful design and optimization tool for the manipulation of microparticles in DMF systems.

An unsteady microfluidic T-form mixer perturbed by hydrodynamic pressure.

An unsteady microfluidic T-form mixer driven by pressure disturbances was designed and investigated. The performance of the mixer was examined both through numerical simulation and experimentation. Linear Stokes equations were used for these low Reynolds number flows. Unsteady mixing in a micro-channel of two aqueous solutions differing in concentrations of chemical species was described using a convection-dominated diffusion equation. The task was greatly simplified by employing linear superimposition of a velocity field for solving a scalar species concentration equation. Low-order-based numerical codes were found not to be suitable for simulation of a convection-dominated mixing process due to erroneous computational dissipation. The convection-dominated diffusion problem was addressed by designing a numerical algorithm with high numerical accuracy and computational-cost effectiveness. This numerical scheme was validated by examining a test case prior to being applied to the mixing simulation. Parametric analysis was performed using this newly developed numerical algorithm to determine the best mixing conditions. Numerical simulation identified the best mixing condition to have a Strouhal number (St)of 0.42. For a T-junction mixer (with channel width = 196 µm), about 75% mixing can be finished within a mixing distance of less than 3 mm (i.e. 15 channel width) at St = 0.42 for flow with a Reynolds number less than 0.24. Numerical results were validated experimentally by mixing two aqueous solutions containing yellow and blue dyes. Visualization of the flow field under the microscope revealed a high level of agreement between numerical simulation and experimental results.