Confined Catalytic Janus Swimmers in a Crowded Channel: Geometry‐Driven Rectification Transients and Directional Locking

Self‐propelled Janus particles, acting as microscopic vehicles, have the potential to perform complex tasks on a microscopic scale, suitable, e.g., for environmental applications, on‐chip chemical information processing, or in vivo drug delivery. Development of these smart nanodevices requires a better understanding of how synthetic swimmers move in crowded and confined environments that mimic actual biosystems, e.g., network of blood vessels. Here, the dynamics of self‐propelled Janus particles interacting with catalytically passive silica beads in a narrow channel is studied both experimentally and through numerical simulations. Upon varying the area density of the silica beads and the width of the channel, active transport reveals a number of intriguing properties, which range from distinct bulk and boundary‐free diffusivity at low densities, to directional “locking” and channel “unclogging” at higher densities, whereby a Janus swimmer is capable of transporting large clusters of passive particles.     

Noninvasive acoustic cell trapping in a microfluidic perfusion system for online bioassays

Techniques for manipulating, separating, and trapping particles and cells are highly desired in today's bioanalytical and biomedical field. The microfluidic chip-based acoustic noncontact trapping method earlier developed within the group now provides a flexible platform for performing cell- and particle-based assays in continuous flow microsystems. An acoustic standing wave is generated in etched glass channels (600x61 microm2) by miniature ultrasonic transducers (550x550x200 microm3). Particles or cells passing the transducer will be retained and levitated in the center of the channel without any contact with the channel walls. The maximum trapping force was calculated to be 430+/-135 pN by measuring the drag force exerted on a single particle levitated in the standing wave. The temperature increase in the channel was characterized by fluorescence measurements using rhodamine B, and levels of moderate temperature increase were noted. Neural stem cells were acoustically trapped and shown to be viable after 15 min. Further evidence of the mild cell handling conditions was demonstrated as yeast cells were successfully cultured for 6 h in the acoustic trap while being perfused by the cell medium at a flowrate of 1 microL/min. The acoustic microchip method facilitates trapping of single cells as well as larger cell clusters. The noncontact mode of cell handling is especially important when studies on nonadherent cells are performed, e.g., stem cells, yeast cells, or blood cells, as mechanical stress and surface interaction are minimized. The demonstrated acoustic trapping of cells and particles enables cell- or particle-based bioassays to be performed in a continuous flow format.     

Array-controlled ultrasonic manipulation of particles in planar acoustic resonator

Ultrasonic particle manipulation tools have many promising applications in life sciences, expanding on the capabilities of current manipulation technologies. In this paper, the ultrasonic manipulation of particles and cells along a microfluidic channel with a piezoelectric array is demonstrated. An array integrated into a planar multilayer resonator structure drives particles toward the pressure nodal plane along the centerline of the channel, then toward the acoustic velocity maximum centered above the subset of elements that are active. Switching the active elements along the array moves trapped particles along the microfluidic channel. A 12-element 1-D array coupled to a rectangular capillary has been modeled and fabricated for experimental testing. The device has a 300-μm-thick channel for a half-wavelength resonance near 2.5 MHz, with 500 μm element pitch. Simulation and experiment confirm the expected trapping of particles at the center of the channel and above the set of active elements. Experiments demonstrated the feasibility of controlling the position of particles along the length of the channel by switching the active array elements.     

Bandpass filters for 8 GHz using solidly mounted bulk acoustic wave resonators

Frequency shift, design, and fabrication issues have been investigated for the realization of 8 GHz bandpass filters based on AlN thin film bulk acoustic wave resonators. Fabrication includes well-textured AlN thin films on Pt (111) electrodes and SiO/sub 2//AlN Bragg gratings for the solidly mounted resonators. The chosen ladder filter design requires the tuning of the shunt resonators with respect to the series one. For this purpose, mass loading of the shunt resonators with aluminum (Al) and SiO/sub 2/ were studied. Design simulations showed that the channel bandwidth can be doubled by shifting more than the difference of resonance and antiresonance frequency. Bandpass filters at 8 GHz were successfully fabricated with -5.5 dB insertion loss, -26 dB out-of-band rejection, 99 MHz (1.2%) /spl plusmn/0.2 dB channel bandwidth, and 224 MHz (2.8%) 3 dB bandwidth. The group delay variations within any 30 MHz channel inside the channel bandwidth amounts to <0.2 ns. Comparisons with simulation calculations and single resonator characteristics show that each /spl pi/-section includes a parasitic series resistance and inductance.     

Effects of process parameters on surface morphology of metal powders produced by free fall gas atomization

There are a number of process parameters which affect the characteristics of metal powders produced by free fall gas atomization. In the following work effects of various process parameters like apex angle of atomizer, focal length of atomizer, number of nozzles, diameter of nozzles, diameter of liquid metal delivery tube, superheat of liquid metal and type of metal etc. were studied on their surface morphology. It was observed that shape of powder particles depends on apex angle, superheat of liquid metal, type of metal and particle size range within a powder collective. Other parameters like focal length of atomizer, number of nozzles, diameter of nozzles and diameter of liquid metal delivery tube were found to have no effect on the shape of powder particles. However, Surface porosity and solidification shrinkage were observed on almost all types of metal powders.     

3D electron microscopy study of metal particles inside multiwalled carbon nanotubes

The location of palladium nanoparticles on and inside the multiwalled carbon nanotubes channel is presented for the first time using electron tomography (3D TEM). The palladium salt precursor was rapidly sucked inside the nanotube channel by means of capillarity that is favored by the hydrophilic character of the tube wall after acidic treatment at low temperature. Statistical analysis indicates that the palladium particles were well dispersed and the palladium particle size was relatively homogeneous, ranging from 3 to 4 nm regardless of their location within the nanotube, within the resolution limit of the technique for our experimental conditions, i.e., about 2 nm. Three-dimensional TEM analysis also revealed that introduction of foreign elements inside the tube channel is strongly influenced by the diameter of the tube inner channel, i.e., easy filling seems to occur with a tube channel >or=30 nm , whereas with tubes having a smaller channel (<15 nm), almost no filling by capillarity occurred leading to the deposition of the metal particles only on the outer wall of the tube.     

Selective polarization imager for contrast enhancements in remote scattering media

Conventional intensity imaging through turbid media suffers from rapid loss of image contrast due to light scattering from particles or random variations of refractive index. This paper features the development of an active imaging, snapshot, system design and postprocessing algorithms that differentiate between radiation that scatters or reflects from remote, obscured objects and the radiation from the scattering media itself through a combination of polarization difference imaging, channel blurring, and Fourier spatial filtering. The produced sensor acquires and processes image data in real time, yielding improved image contrasts by factors of 10 or greater for dense water vapor obscurants.     

Mesoporous silica as micro/nano-carrier: From passive to active cargo delivery,a mini review

Mesoporous silica has been widely explored for biomedical applications due to its unique structure and good biocompatibility. In particular it exhibits superior properties as micro/nano-carriers in the biomedical field. We explore their potentials in controlled drug/gene co-delivery and photodynamic therapy for cancer treatment both in vitro and in vivo. By incorporating mesoporous silica nanoparticles(MSNP) with two-dimensional nanomaterial, graphene oxide nano-sheet, we utilize MSNP in cellular bio-imaging with squaraine dye. Meanwhile, through delicate combination between mesoporous silica micro/nano carriers with catalytic/bio-catalytic reactions, we manage to achieve self-propelled micro/nano-motors based on mesoporous silica that are capable of transporting cargos in an active manner. Especially, enzyme powered mesoporous silica motors can be powered by physiologically available fuels such as glucose and urea,which are advantageous for future biomedical use. Motion control on both velocity and movement direction provides a powerful tool for targeted drug delivery. Therefore, such mesoporous silica based active carriers pave way to the solution of targeted drug delivery for cancer treatment in future nano-medicine field.     

The Effect of Frequency Sweeping and Fluid Flow on Particle Trajectories in Ultrasonic Standing Waves

Particle concentration and separation in ultrasonic standing waves through the action of the acoustic radiation force on suspended particles are discussed. The acoustic radiation force is a function of the density and compressibility of the fluid and the suspended particles. A two-dimensional theoretical model is developed for particle trajectory calculations. An electroacoustic model is used to predict the acoustic field in a resonator, driven by a piezoelectric transducer. Second, the results of the linear acoustic model are used to calculate the acoustic radiation force acting on a particle suspended in the resonator. Third, a particle trajectory model is developed that integrates the equation of motion of a particle subjected to a buoyancy force, a fluid drag force, and the acoustic radiation force. Computational fluid dynamics calculations are performed to calculate the velocity field that is subsequently used to calculate fluid drag. For a fixed frequency excitation, the particles are concentrated along the stable node locations of the acoustic radiation force. Through a periodic sweeping of the excitation frequency particle translation is achieved. Two types of frequency sweeps are considered, a ramp approach and a step-change method. Numerical results of particle trajectory calculations are presented for two configurations of flow-through resonators and for two types of frequency sweeping. It is shown that most effective particle separation occurs when the fluid drag force is orthogonal to the acoustic radiation force.     

Fabrication of advanced particles and particle-based materials assisted by droplet-based microfluidics

Recent advances in the fabrication of complex particles and particle-based materials assisted by droplet-based microfluidics are reviewed. Monodisperse particles with expected internal structures, morphologies, and sizes in the range of nanometers to hundreds of micrometers have received a good deal of attention in recent years. Due to the capability of generating monodisperse emulsions and of executing precise control and operations on the suspended droplets inside the microchannels, droplet-based microfluidic devices have become powerful tools for fabricating complex particles with desired properties. Emulsions and multiple-emulsions generated in the microfluidic devices can be composed of a variety of materials including aqueous solutions, gels, polymers and solutions containing functional nanoparticles. They are ideal microreactors or fine templates for synthesizing advanced particles, such as polymer particles, microcapsules, nanocrystals, and photonic crystal clusters or beads by further chemical or physical operations. These particles are promising materials that may be applicable for many fields, such as photonic materials, drug delivery systems, and bio-analysis. From simple to complex, from spherical to nonspherical, from polymerization and reaction crystallization to self-assembly, this review aims to help readers be aware of the many aspects of this field.     

Preparation of Monodisperse Biodegradable Polymer Microparticles Using a Microfluidic Flow‐Focusing Device for Controlled Drug Delivery

Degradable microparticles have broad utility as vehicles for drug delivery and form the basis of several therapies approved by the US Food and Drug Administration. Conventional emulsion‐based methods of manufacturing produce particles with a wide range of diameters (and thus kinetics of release) in each batch. This paper describes the fabrication of monodisperse, drug‐loaded microparticles from biodegradable polymers using the microfluidic flow‐focusing (FF) devices and the drug‐delivery properties of those particles. Particles are engineered with defined sizes, ranging from 10 µm to 50 µm. These particles are nearly monodisperse (polydispersity index = 3.9%). A model amphiphilic drug (bupivacaine) is incorporated within the biodegradable matrix of the particles. Kinetic analysis shows that the release of the drug from these monodisperse particles is slower than that from conventional methods of the same average size but a broader distribution of sizes and, most importantly, exhibit a significantly lower initial burst than that observed with conventional particles. The difference in the initial kinetics of drug release is attributed to the uniform distribution of the drug inside the particles generated using the microfluidic methods. These results demonstrate the utility of microfluidic FF for the generation of homogenous systems of particles for the delivery of drugs.     

Numerical simulation of flow behavior of liquid and particles in liquid–solid risers with multi scale interfacial drag method

Formation of particle clusters in liquid–solid circulating fluidized beds significantly affects macroscopic hydrodynamic behavior of the system. A multi scale interfacial drag coefficient (MSD) is proposed to determine effects of particle clusters on the mesoscale structure, by taking momentum and energy balance of dense phase, dilute phase and interphase into account. Based on the transportation and suspension energy-minimization method, the multi scale interfacial drag coefficient model used in this work is combined with the Euler–Euler two fluid model to simulate the heterogeneous behaviors of liquid–solid circulating fluidized bed. It was found that the reduction in drag coefficient is at least an important factor for the simulation of clusters formation, and the core-annulus flow is observed in the riser. The liquid–solid flow regime was significantly affected by the down-flow of particles in the form of clusters near the walls of the riser. The calculated concentration of particles inside the riser compared reasonably well with the available experimental data obtained by Razzak et al.     

Frequency Characteristics of a Quartz Tuning Fork Immersed in He II

The effect of dissipation on frequency characteristics of tuning forks was measured, the dissipation being induced by acoustic radiation of different wavelengths, excited by tuning forks. The tuning forks have been immersed in the superfluid helium. The fork resonance frequencies 32, 77 and 99 kHz have been measured at T=370 mK in the pressure range between SVP and 24.9 atm. Most of the tuning forks have been studied in a commercial can. It is found that at wavelength λ>0.6 cm the frequency dependence is determined by the relationship between density and pressure. It is established that a decrease in wavelength enhances influence of the acoustic radiation on the fork oscillation frequency. In the case where the sound wavelength is equal to the can internal diameter an acoustic resonance occurs. The frequency reaches values higher than the fork frequency in vacuum. Further reduction of the sound wavelength leads to the situation when the resonant frequency is similar to the frequency at long wavelengths.     

Dense gas anti-solvent processes for pharmaceutical formulation

Conventional processes for manipulating the characteristics of particles used in drug formulation are limited in their flexibility. Recent studies in which supercritical/dense gas processes have been used for particle design and drug formulation show considerable promise. These developing dense gas technologies for drug formulation are described in this review. Issues relating to scale up on dense gas technology application for pharmaceutical active are also identified.     

Pair Interaction between Two Catalytically Active Colloids

Due to the intrinsically complex non-equilibrium behavior of the constituents of active matter systems, a comprehensive understanding of their collective properties is a challenge that requires systematic bottom–up characterization of the individual components and their interactions. For self-propelled particles, intrinsic complexity stems from the fact that the polar nature of the colloids necessitates that the interactions depend on positions and orientations of the particles, leading to a 2d − 1 dimensional configuration space for each particle, in d dimensions. Moreover, the interactions between such non-equilibrium colloids are generically non-reciprocal, which makes the characterization even more complex. Therefore, derivation of generic rules that enable us to predict the outcomes of individual encounters as well as the ensuing collective behavior will be an important step forward. While significant advances have been made on the theoretical front, such systematic experimental characterizations using simple artificial systems with measurable parameters are scarce. Here, two different contrasting types of colloidal microswimmers are studied, which move in opposite directions and show distinctly different interactions. To facilitate the extraction of parameters, an experimental platform is introduced in which these parameters are confined on a 1D track. Furthermore, a theoretical model for interparticle interactions near a substrate is developed, including both phoretic and hydrodynamic effects, which reproduces their behavior. For subsequent validation, the degrees of freedom are increased to 2D motion and resulting trajectories are predicted, finding remarkable agreement. These results may prove useful in characterizing the overall alignment behavior of interacting self-propelling active swimmer and may find direct applications in guiding the design of active-matter systems involving phoretic and hydrodynamic interactions.     

基于有效抽头和进化规划算法的自适应水声信道估计

为了实现稀疏水声信道的快速高效估计,提出一种新的基于有效抽头的信道参数模型降低运算复杂性,并采用进化算法以最小二乘误差为适应度函数对信道响应中有效抽头的阶数、位置和权重同时进行进化操作寻优,避免了有色输入造成的相邻抽头耦合导致的性能下降。仿真实验结果表明:与传统LMS算法及基于有效抽头检测的稀疏信道估计方法相比,该算法在有色输入信号下具有更优越的估计性能和收敛速度,为稀疏水声信道处理提供了一类新方法。     

渗透度对锯齿形尾缘喷嘴气动声学影响研究

在低马赫数条件下（Ma=0.12）,采用时间分辨TR-PIV以及远场噪声测量方法,对不同渗透度锯齿形尾缘喷嘴的气动声学特性进行实验研究。流场测量结果表明,锯齿形尾缘喷嘴缩短势核长度,增加势核内气流和周围环境气流的掺混,随渗透度的增加该趋势得到增强。远场声学测量结果表明,随着渗透度增加,锯齿形尾缘喷嘴对于噪声低频降低和高频增加的幅度都有所增强。低渗透度的锯齿形尾缘喷嘴有效降低噪声总声压级,在150°时总声压级降低达到1.3 dB,研究对于降低工厂排空噪声有现实意义。     

Acoustic destruction of a microcapsule having a hard plastic shell

Acoustic destruction of a microcapsule having a hard plastic shell is discussed. In an ultrasonic drug delivery system, microcapsules having thin elastic shells release drugs that are contained therein when the shell is destroyed. In this paper, two subjects related to capsule destruction are discussed: the driving pulse duration for capsule destruction and the frequency dependence of capsule destruction. Optical observation of microcapsule destruction is performed with a high-speed video camera. In the case of capsule destruction by a pulse wave, the internal gas of the microcapsule cannot be ejected completely, and a portion of the internal gas remains inside the broken shell. It is found that capsule destruction by pulse waves depends on both the amplitude of the driving pressure and the pulse duration. The frequency dependence of microcapsule destruction also is investigated. In the case of capsule destruction by a low-amplitude acoustic wave, the destruction rate under the resonance condition is higher than under nonresonance conditions. By controlling the driving frequency, selective capsule destruction can be achieved.     

电子膨胀阀内啸叫噪声特性及发声规律的实验研究

空调系统中电子膨胀阀节流时出现的尖锐啸叫噪声严重影响用户使用舒适性,了解制冷剂流经电子膨胀阀时的啸叫噪声产生机理及其发声规律是解决上述噪声问题的关键。本文设计了可以对阀前后制冷剂状态进行调节控制并对产生的啸叫噪声进行测量分析的实验系统,得到不同工况及阀开度下的啸叫噪声发声规律。研究表明：啸叫噪声来源于阀内流体高频压力脉动引起的流体周期性振荡,其发声特性是阀内环锥形节流通道与阀腔构成的亥姆霍兹共振腔结构对共振频率附近的噪声源信号选择性放大的结果。啸叫噪声声压级主要与阀内制冷剂流速及阀开度有关,阀开度为700 pls下制冷剂速度由2.5 m/s增至3 m/s时,噪声声压级提高了21%;阀内制冷剂流速决定了流体振荡频率,阀开度决定了阀内声腔共振频率;通过改变阀腔结构以增大共振频率,使常见空调工况下阀内制冷剂冲击引起的振荡频率低于共振频率,可以有效避免啸叫噪声产生。     

不同形状Janus颗粒自驱动特性数值模拟

Janus颗粒利用自身非对称的表面性质建立浓度梯度场,并在其作用下产生自驱动,在微机电系统、生物学、流体力学等领域具有重要的应用.本文首先建立了模拟这一过程的数值模型,并由Pt-SiO2型Janus微球的实验数据确定了迁移速率匹配常数.随后,研究了3种相同体积、不同形状的Janus颗粒的自驱动,结果表明,与相同体积的球形Janus颗粒相比,圆柱及椭球状Janus颗粒具有更快的自驱动速度,同时燃料消耗更多.对于圆柱状颗粒,研究了粗细程度对圆柱状颗粒自驱动性能的影响,结果表明存在最优的直径与长度比(d/l=0.28).这一研究可为Janus颗粒具体应用提供理论基础.