Motion Of Microparticles Research Articles

Cell/particle manipulation using Bulk Acoustic Waves (BAWs) on centrifugal microfluidic platforms: A mathematical study

This study presents an integrated acoustic-aided centrifugal microfluidic system to focus and separate microparticles. A 3D numerical simulation was conducted to analyze microparticle movement by exploiting the simultaneous imposition of centrifugal forces and bulk acoustic waves (BAWs) on an electrified lab-on-a-disc device (eLOD). Accordingly, the movement of microparticles was analyzed in a radially positioned rectangular microchannel at various rotation speeds. The effect of physical parameters, including the distance of the microchannel to the center/radius, tilting angle (α), the oscillation amplitude of BAWs, the microchannel's dimension, and the particles’ diameter on particle trajectories and focusing efficiency, was studied. It was found that properly adjusting the microchannel's placement at α = 30° made it possible to direct the focused stream of microparticles toward the desired outlet. Higher values of applied oscillation amplitude of BAWs (0.3 nm) led to perfect focusing of microparticles toward the middle outlet in a 200-µm width microchannel at 80 rad/s rotation. Furthermore, the system's ability to separate the circulating tumor cells (CTC) from white blood cells (WBC) was also simulated. The results showed that a successful size-based separation of these bioparticles is achievable by adequately adjusting the microchannel's position or tilting angle at 286 rpm.

Field-Directed Motion, Cargo Capture, and Closed-Loop Controlled Navigation of Microellipsoids.

Microrobots have the potential for diverse applications, including targeted drug delivery and minimally invasive surgery. Despite advancements in microrobot design and actuation strategies, achieving precise control over their motion remains challenging due to the dominance of viscous drag, system disturbances, physicochemical heterogeneities, and stochastic Brownian forces. Here, a precise control over the interfacial motion of model microellipsoids is demonstrated using time-varying rotating magnetic fields. The impacts of microellipsoid aspect ratio, field characteristics, and magnetic properties of the medium and the particle on the motion are investigated. The role of mobile micro-vortices generated is highlighted by rotating microellipsoids in capturing, transporting, and releasing cargo objects. Furthermore, an approach is presented for controlled navigation through mazes based on real-time particle and obstacle sensing, path planning, and magnetic field actuation without human intervention. The study introduces a mechanism of directing motion of microparticles using rotating magnetic fields, and a control scheme for precise navigation and delivery of micron-sized cargo using simple microellipsoids as microbots.

Diffusiophoresis in Polymer and Nanoparticle Gradients.

Diffusiophoresis is the movement of the colloidal particles in response to a concentration gradient and can be observed for both electrolyte (e.g., salt) and nonelectrolyte (e.g., glucose) solutes. Here, we investigated the diffusiophoretic behavior of polystyrene (PS-carboxylate surface) microparticles in nonadsorbing charged and uncharged solute gradients [sodium polystyrenesulfonate (NaPSS), polyethylene glycol (PEG), and nanoscale colloidal silica (SiO2)] using a dead-end channel setup. We compared the diffusiophoretic motion in these gradient types with each other and to the case of using a monovalent salt gradient. In each of the nonadsorbing gradient systems (NaPSS, PEG, and SiO2 nanoparticles), the PS particles migrated toward the lower solute concentration. The exclusion distance values (from the initial position) of particles were recorded within the dead-end channel, and it was found that an increase in solute concentration increases exclusion from the main channel. In the polyelectrolyte case, the motion of PS microparticles was reduced by the addition of a background salt due to reduced electrostatic interaction, whereas it remained constant when using the neutral polymer. Particle diffusiophoresis in gradients of polyelectrolytes (charged macromolecules) is quite similar to the behavior when using a PEG gradient (uncharged macromolecule) in the presence of a background electrolyte. Moreover, we observed PS microparticles under different concentrations and molecular weights of PEG gradients. By combining the simulations, we estimated the exclusion length, which was previously proposed to be the order of the polymer radius. Furthermore, the movement of PS microparticles was analyzed in the gradient of silica nanoparticles. The exclusion distance was higher in silica nanoparticle gradients compared to similar-size PEG gradients because silica nanoparticles are charged. The diffusiophoretic transport of the PS microparticles could be simulated by considering the interaction between the PS microparticles and silica nanoparticles.

Capillary wave tweezer

Precise control of microparticle movement is crucial in high throughput processing for various applications in scalable manufacturing, such as particle monolayer assembly and 3D bio-printing. Current techniques using acoustic, electrical and optical methods offer precise manipulation advantages, but their scalability is restricted due to issues such as, high input powers and complex fabrication and operation processes. In this work, we introduce the concept of capillary wave tweezers, where mm-scale capillary wave fields are dynamically manipulated to control the position of microparticles in a liquid volume. Capillary waves are generated in an open liquid volume using low frequency vibrations (in the range of 10–100 Hz) to trap particles underneath the nodes of the capillary waves. By shifting the displacement nodes of the waves, the trapped particles are precisely displaced. Using analytical and numerical models, we identify conditions under which a stable control over particle motion is achieved. By showcasing the ability to dynamically control the movement of microparticles, our concept offers a simple and high throughput method to manipulate particles in open systems.

Propagation of elliptical Gaussian vortex beam based on angular spectrum representation

Vortex beams with hollow elliptical ring structure have attracted broad interest in applications such as micro-particle trapping and manipulation. The paper investigates the dependence of the hollow elliptic ring structure of vortex beams on the beam order and the topological charge, while propagating in the near- and far-field zones. The electric field of the beam is formulated by using the angular spectrum method, which satisfies rigorously Maxwell's equation. The electric field, linear momentum and orbital angular momentum are calculated. Numerical results show that the hollow elliptical ring structure approximately remains unchanged over a finite propagation distance, accompanied by a rotation whose angle depends on the topological charge. In the far-field zone, the ring structure is broken, leading to two bright spots and several dark islands. It is found that the beam profile in the near-field zone is mainly related to the beam order but in the far-field zone is primarily affected by the topological charge. The results presented in the paper may be useful in predicting and explaining the interaction between the elliptic hollow Gaussian vortex beam and the micro-particles, such as the movement of micro-particles along the orbit and the rolling rotation.

Enhanced contact flexibility from nanoparticles in capillary suspensions

Hypothesis: Sample-spanning particle networks are used to induce structure and a yield stress, necessary for 3D printing of porous ceramics and paints. In capillary suspensions, a small quantity of immiscible secondary fluid is incorporated into a suspension. By further adding nanoparticles with a range of hydrophobicities, the structure of the bridges and microparticle-microparticle contacts is expected to be modified, resulting in a tunable yield stress and shear moduli. Moreover, the compressibility of these samples, important in many processing and application steps, is expected to be sensitive to these changes.Experiment: The nanoparticle hydrophobicity was altered and their position relative to the microparticles and the bridges was examined using confocal microscopy where the correlation between bridge size and network structure was observed. A step-wise uniaxial compression test on the confocal was conducted to monitor the microparticle movement and structural changes between capillary suspension networks with and without nanoparticles.Findings: Our observation suggests that nanoparticles induce the formation of thin liquid films on the surface of the microparticles, mitigating contact line pinning and promoting internal liquid exchange. Additionally, nanoparticles at microparticle contact regions further diminish Hertzian contact, enhancing the capacity for rearrangement. These effects enhance microparticle movement, narrowing the bridge size distribution.

Comparisons of the acoustic radiation force of ultrasonic standing waves in half-wavelength and quarter-wavelength micro-resonators of cylindrical geometry

Ultrasonic standing waves with specific wavelengths generated in the multi-layered micro-resonators were numerically and experimentally analyzed. Using a three-dimensional scanning fluorescence microscope, the acoustophoretic motion of fluorescent microparticles within the micro-resonators was carefully and accurately measured. The manufactured micro-resonators were validated by comparing the location of the acoustic pressure nodal plane and the average energy density curves derived from numerical and experimental results. Results confirmed that the acoustic radiation force of the induced ultrasonic standing waves drives the microparticles vertically within the micro-resonators and their average energy density increases as the sinusoidal voltage applied to the piezoelectric transducer increases. Semi-empirical correlations were developed for the average energy density, based on experimental results for a wide range of the applied voltage amplitudes. The correlations were in good agreement, within less than 20 % of the experimental values measured for both the half-wavelength and quarter-wavelength micro-resonators.

Light-controllable liquid crystal platform for microparticle oscillations and transport.

Liquid crystal colloids manifest complex motion caused by external stimuli, but tunable and addressable control of microsized objects remains a challenge. This study aims to demonstrate light-driven trapping, transport, and sustained periodic motions of microparticles by employing liquid crystal films as a light-controllable colloidal platform. The diverse motions of microscopic particles result from Marangoni convection coupled with elastic deformations in free-surface liquid crystal films subjected to light beam heating. The specific mode of particle motion, including damped and sustained oscillations, also combined with sustained rotation, is defined by the liquid crystal chirality, particle surface treatment, film thickness, and the power of the tightly focused light beam. The results reveal that free-surface liquid crystals provide a unique platform for the indirect optical manipulation of microscopic objects, paving the way for novel applications in microfluidic tools, particle sorting and transport, micropatterning, and various micromachines.

Modeling the trajectories of floating and non-floating microplastic particles in the water column

The distribution and movement of microparticles of plastic (MP) in freshwater and marine environments are determined by the characteristics of both MP and the surrounding flow. The most common plastic polymers in the aquatic environment, such as polyethylene and polypropylene, have a density lower than the density of water. Therefore, based only on buoyancy, it is expected that most MP will be present in the surface layers of the aquatic environment. In both freshwater and marine environments, turbulence-induced mixing depends on factors such as velocity gradient and convective flow. Consequently, the surface wind, the deep temperature gradient and the breaking of waves in the surf zone can lead to the formation of turbulence and deep vertical mixing. In such conditions, in addition to gravity and buoyancy, the motion caused by turbulent mixing can affect the vertical transport and distribution of particles. The paper presents the results of modeling Lagrangian trajectories for floating particles and non-floating particles, under wave conditions they correspond to regular waves, while the particle sizes range from 10 microns to 5 mm, and the density ranges from 0.88 to 2.80 g/cm3, which is within some of the most common densities of microplastic particles.

Sensitivity of acoustofluidic particle manipulation to microchannel height in standing surface acoustic wave-based microfluidic devices

In this paper, the flow and particle trajectories, induced by standing surface acoustic waves (SSAWs) in a poly-dimethylsiloxane microchannel, are investigated by establishing a two-dimensional cross-sectional model with the finite element method and improved boundary conditions. Extensive parametric studies are conducted regarding the channel height, ranging from 0.2 to 4.0 times the spacing of the repetitive vertical interference pattern, to investigate its influences on the flow field and microparticle aggregation. The first-order flow field is found to be related to the channel height, exhibiting a periodic spatial distribution and oscillatory variation in its amplitude as the height changes. We theoretically analyze the propagation mechanism of the acoustic waves in the vertical direction and thus determine the periodicity of the wave interference pattern. Furthermore, we find that the speed of the particle aggregation is a function of the channel height, so the channel height can be optimized to maximize the strength of the first-order flow field and thus minimize the time of particle aggregation. The optimum heights can reduce the aggregation time by up to 76%. In addition, the acoustophoretic motions of microparticles exhibit a spatially dependent pattern when the channel height becomes larger than a quarter of the wavelength of the SAW, which can be explained by the change in the ratio between the radiation force and the streaming drag force from position to position. Our findings provide guidelines to the design and optimization of SSAW-based acoustofluidic devices.

Microparticle motion under dielectrophoresis: immersed boundary—Lattice Boltzmann-based multiphase model and experiments

Microparticle motion under dielectrophoresis: immersed boundary—Lattice Boltzmann-based multiphase model and experiments

Simultaneous and independent topological control of identical microparticles in non-periodic energy landscapes

Topological protection ensures stability of information and particle transport against perturbations. We explore experimentally and computationally the topologically protected transport of magnetic colloids above spatially inhomogeneous magnetic patterns, revealing that transport complexity can be encoded in both the driving loop and the pattern. Complex patterns support intricate transport modes when the microparticles are subjected to simple time-periodic loops of a uniform magnetic field. We design a pattern featuring a topological defect that functions as an attractor or a repeller of microparticles, as well as a pattern that directs microparticles along a prescribed complex trajectory. Using simple patterns and complex loops, we simultaneously and independently control the motion of several identical microparticles differing only in their positions above the pattern. Combining complex patterns and complex loops we transport microparticles from unknown locations to predefined positions and then force them to follow arbitrarily complex trajectories concurrently. Our findings pave the way for new avenues in transport control and dynamic self-assembly in colloidal science.

Micro-particle aggregation using vortices induced by vibration of a piezoelectric cantilever beam-probe structure

Large-scale nondestructive aggregation of micro-particles is essential for cell detection, microplastics removal and micro- and nano pharmaceuticals. To achieve this goal, this work proposes a method by creating a vortex region in the fluid field using low-frequency oscillation of a piezoelectric cantilever beam probe structure. Vibration of the probe agitates the liquid, creating a horseshoe-shaped vortex in the fluid field. This facilitates the movement of micro-particles and traps them. Using polystyrene microspheres with a diameter of 20 µm and pollen (20–40 µm) as aggregated objects, particle agglomeration tests and flow field PIV tests are performed. Experimental results confirm that the designed structure has the best performance with a driving voltage of 100 V and a driving frequency of 72 Hz. The maximum aggregation area of polystyrene and pollen is 106956 µm2 and 234750 µm2. There are symmetrical large vortices regions on both sides of the probe and small vortices zero vorticity near the free end of the probe. It is also confirmed that small vortices are the root cause of microparticle aggregation. The aggregation method proposed in this study has advantages of low cost, non-selectivity of characteristics for controlled samples, large manipulation area and low power consumption (0.42 W).

Parametric analysis of acoustically levitated droplet for potential microgravity application

Acoustic levitation is a versatile technique for the non-contact handling of both solid and liquid samples, yet its potential remains underutilized in container-less processing and lab-on-a-drop application. This study demonstrates the efficient use of acoustically levitated droplets as microgravity simulators on earth similar to clinostat. Using a simple and affordable non-resonant single-axis acoustic levitator called TinyLev, we first investigate the impact of applied voltage and droplet volume on the shape and rotational motion of levitated water droplets. Furthermore, we examine the rotational motion of suspended microparticles within the droplets. Our findings reveal the feasibility of using the levitated droplets as a miniaturized microgravity simulators on earth.

Studying turbulence in a fluid with background damping.

In this experimental paper, we demonstrate that turbulence can develop in a fluid system with background damping. For that purpose, we analyze dust acoustic waves, self-excited in a fluid complex plasma where the motion of individual microparticles was recorded with a high-speed video camera. We use the Wiener-Khinchin theorem to calculate the kinetic spectrum during different phases of the highly nonlinear periodic wave motion and show that a turbulent cascade develops at the phases of highest particle compression. We demonstrate that the energy cascade occurs despite the presence of a damping force due to the background neutral gas.

Floods enhance the abundance and diversity of anthropogenic microparticles (including microplastics and treated cellulose) transported through karst systems

Microplastics (plastics <5 mm) are emerging contaminants that have been detected in virtually all environments. While microplastic research in terrestrial surface waters has been proliferating, microplastic contamination in subsurface environments remains understudied. Karst terrains may be particularly susceptible to microplastic pollution because the presence of large dissolution openings allows fast transport of water through these systems, facilitating the introduction of surface contaminants into subsurface habitats. Furthermore, few studies address the prevalence and movement of microparticles composed of semisynthetic and modified natural materials, despite their known ecotoxicity. Our study therefore aims to identify anthropogenic (i.e., synthetic, semisynthetic, and treated natural) microparticle extent, sourcing, and transport in subsurface karst environments. To do so, we examined a cave spring under variable flow conditions, finding that anthropogenic microparticles were present in all samples and were most frequently fibrous and clear. The mean anthropogenic microparticle concentration during baseflow was 9.2 counts/L but increased up to 81.3 counts/L during floods, indicating their enhanced mobilization when relatively dilute, acidic, and sediment-rich event water entered the cave. These results suggest that anthropogenic microparticles may originate from surface recharge or sediment resuspension within the cave. When we analyzed a subset of microparticles with Fourier transform infrared spectroscopy (FTIR), we found that cellulose of known (i.e., dyed) and suspected (i.e., clear) anthropogenic origin was the most abundant material type. We nevertheless confirmed the presence of microplastics in the cave stream under all flow conditions, with the most common polymer being polyethylene. Both the concentrations and relative fractions of microplastics were higher during floods compared to baseflow, indicating their increased transport during high flow events. We also observed that microplastic polymer types diversified as discharge increased. Our study gives new insight into how anthropogenic microparticle contamination is transported through karst landscapes that can help inform debris mitigation strategies to protect ecosystems and water resources.

Programming Motion of Platinum Microparticles: From Linear to Orbital

Self-powered catalytic micromotors that swim through a fluid by harvesting energy from their environment have received increasing attention because of their potential applications in nanomachinery, nanoscale assembly, and robotics. However, persistent directional motion is required for micromotors to perform useful work. Herein, through modeling and experiments, we demonstrate the general design strategy for single-component platinum micromotors that, in the presence of hydrogen peroxide, can execute a variety of motion trajectories from extremely linear to orbital. This is achieved via a simple shape design wherein the speed and directionality of a particle are determined by overall symmetry elements. Shape design gives high reproducibility and allows a wide range of adjustments to fine-tune the desired particle motion. We observe strong agreement between experimental data and numerical predictions, indicating that our model can serve to predict directionality and speed of a particle prior to fabrication. Shape programming works on an individual-particle basis, thus enabling complex systems which require different modes of motion for the individual parts.

The optical measurement speed of moving bodies and the observer's position effect

From ancient times to the Newtonian mechanics system, people defaulted to the speed of light as infinity. People usually measure the speed of a moving object directly with the help of optical or electronic equipment and take the measured speed as the actual speed of the object. In fact, the speed of light is limited. When an object moves at high speed, we must correct the measured speed of the object. However, at present, people only use Einstein's special theory of relativity to deal with the high-speed motion of objects, and there is no theory to correct the measured speed of objects under the condition of high-speed motion. Here, we report that the optical measurement speed of an object is affected by the observer's position effect and obtain the relationship between the optical measurement speed and the actual speed. On this basis, this paper explains the reason why Newton's classical mechanics is suitable for the case of low-speed motion, and there is a large deviation between the optical measurement speed and the actual speed under the condition of high-speed motion. This paper also explains the velocity limit of high-energy particles and the phenomena of superluminal and negative velocity and analyzes the observation of microparticle motion. This paper provides a theory to explain and deal with the high-speed motion of objects, which is of great scientific significance.

Multidirectional motion control of microparticles by a focused vortex laser beam in air

We propose a novel method for controlling a single microparticle moving arbitrary direction motions in air by a single focused vortex optical tweezer (OT). In principle, the movement of microparticles can be controlled synchronously by modulating the movement direction and movement path of the focused vortex laser beam. The spatial light modulator (SLM) modulates the 532 nm wavelength continuous-wave (CW) laser to create a Gaussian vortex laser beam for capturing and manipulating a microparticle in air. We demonstrate that a about 1 µm diameter carbon particle can be successfully controlled in a linear reciprocating or circular motion by a single focused vortex laser beam. In the experiment, the microparticles can be moved at a uniform speed of 0.167 cm per second by the designed route. By establishing a theoretical interference framework and numerical calculations, we have also demonstrated the effectiveness of this novel approach. This approach to controlling a single microparticle is of significant value for studying the control of multi-microparticles and 3D display in air.

Three-dimensional modeling and experimentation of microfluidic devices driven by surface acoustic wave

Surface acoustic wave (SAW) technology is proving to be an effective tool for manipulating micro-nano particles. In this paper, we present a fully-coupled 3D model of standing SAW acoustofluidic devices for obtaining particle motion. The “improved limiting velocity method” (ILVM) was used to investigate the distribution of acoustic pressure and acoustic streaming in microchannel. The results show that the distribution of acoustic pressure and acoustic streaming on the piezoelectric substrate surface perpendicular to the acoustic wave propagation direction is inhomogeneous. The motion of micro-particles with diameters of 0.5-, 5-, and 10 μm is then simulated to investigate the interaction of acoustic radiation force and drag force caused by pressure and acoustic streaming. We demonstrate that micro and nanoparticles can move in three dimensions when acoustic radiation force and acoustic streaming interact. This result and method are critical for designing SAW microfluidic chips and controlling particle motion precisely.