Hydrodynamic tweezers: 1. Noncontact trapping of single cells using steady streaming microeddies

A key need for dynamic single-cell measurements is the ability to gently position cells for repeated measurements without perturbing their behavior. We describe a new method that uses a gentle secondary flow to trap and suspend single cells, including motile cells, at predictable locations in 3-D. Trapped cells can be more dense or less dense than the surrounding medium. The cells are suspended without surface contact in one of four steady streaming eddies created by audible-frequency fluid oscillation (< or =1000 Hz) in a microchannel containing a single fixed cylinder (radius = 125 microm). Comparison of measured trap locations to computations of the eddy flow show that each trap is located near the eddy center, and the location is controlled via the oscillation frequency. We use the motile phytoplankton cell (Prorocentrum micans) to experimentally measure the trapping force, which is controlled via the oscillation amplitude. Trapping forces up to 30 pN are generated while exerting moderate shear stresses (shear stresses < or = 1.5 N/m2) on the trapped cell. The magnitude of this trapping force is comparable to that of optical tweezers or dielectrophoretic traps, without requiring an external field outside the physiological range for cells (the shear stresses are comparable to those found in arterial blood flow). The unique combination of predictable 3-D positioning, insensitivity to cell and medium properties, strong adjustable trapping forces, and a gentle fluid environment makes hydrodynamic tweezers a promising new option for noncontact trapping of single cells in suspension.     

CFD simulation of micro-particle trapping under water tweezers

In early research,capture and manipulation of particles were mainly achieved by means of light,electricity and plasma in micro-fabrication and micro-assembly.A new method is proposed using micro-water jet to form water tweezers to capture solid particles and implement position control of micro-particles.This paper analyzes the basic principle of water tweezers,and the discrete element method and smoothed particle hydrodynamics method are employed to establish a solid-liquid coupling model used in analyzing the trapping mechanism.A flow field model is set up to simulate dynamic characteristic of water tweezers based on computational fluid dynamics(CFD).Selection of boundary conditions,initial guess,solver control and convergence strategies of the model are discussed.Velocity and pressure of streamline are predicted and discussed under certain input conditions.Simulation results demonstrate that it is an efficiently theoretical method to eventually trap solid particles by water tweezers.     

Defining the trapping limits of holographical optical tweezers

Abstract

We report on the limitations of using a spatial light modulator (SLM) within optical tweezers to produce both lateral and axial displacements. We find that lateral displacements of optical traps are limited by the optical efficiency of the SLM, whereas the axial displacements are limited by the abberations of the objective lens. In addition, we show the SLM can be used for correcting abberations arising from trapping deep within the sample. The maximum possible lateral and axial displacements were 50 μm and 40 μm, respectively.     

光镊中米氏微粒子的捕捉力仿真

为了使所建立的光镊中粒子捕捉力的数学模型适宜任意形状粒子,将米氏粒子表面离散为多个微面元,基于几何光学理论得到光线与微面元相互作用产生的光辐射压力及光线追迹方程.基于所建立的数学模型,实现了对任意形状米氏粒子(球形、圆锥形、多面体形和任意形状光滑曲面粒子)受力的计算机仿真.经过仿真分析,验证了仿真软件的正确性,从而为光镊技术在纳米测量和微细加工等的应用提供了有利的分析和设计工具.     

A study on evolutionary multi-objective optimization for flow geometry design

In this paper, we investigate three recently proposed multi-objective optimization algorithms with respect to their application to a design-optimization task in fluid dynamics. The usual approach to render optimization problems is to accumulate multiple objectives into one objective by a linear combination and optimize the resulting single-objective problem. This has severe drawbacks such that full information about design alternatives will not become visible. The multi-objective optimization algorithms NSGA-II, SPEA2 and Femo are successfully applied to a demanding shape optimizing problem in fluid dynamics. The algorithm performance will be compared on the basis of the results obtained.     

In situ microparticle analysis of marine phytoplankton cells with infrared laser-based optical tweezers

We describe the application of infrared optical tweezers to the in situ microparticle analysis of marine phytoplankton cells. A Nd:YAG laser (λ= 1064 nm) trap is used to confine and manipulate single Nannochloris and Synechococcus cells in an enriched seawater medium while spectral fluorescence and Lorenz-Mie backscatter signals are simultaneously acquired under a variety of excitation and trapping conditions. Variations in the measured fluorescence intensities of chlorophyll a (Chl a) and phycoerythrin pigments in phytoplankton cells are observed. These variations are related, in part, to basic intrasample variability, but they also indicate that increasing ultraviolet-exposure time and infrared trapping power may have short-term effects on cellular physiology that are related to Chl a photobleaching and laser-induced heating, respectively. The use of optical tweezers to study the factors that affect marine cell physiology and the processes of absorption, scattering, and attenuation by individual cells, organisms, and particulate matter that contribute to optical closure on a microscopic scale are also described.     

Axial optical trapping forces on two particles trapped simultaneously by optical tweezers

Optical tweezers, which utilize radiation pressure to control and manipulate microscopic particles, are used for a large number of applications in biology and colloid science. In most applications a single optical tweezers is used to control one single particle. However, two or more particles can be trapped simultaneously. Although this characteristic has been used in applications, no theoretical analysis of the trapping force or the status of the trapped particles is available to our knowledge. We present our calculation, using a ray optics model, of the axial trapping forces on two rigid particles trapped in optical tweezers. The spherical aberration that results from a mismatch of the refractive indices of oil and water is also considered. The results show that the forces exerted by the optical tweezers on the two particles will cause the two particles to touch each other, and the two particles can be stably trapped at a joint equilibrium point. We also discuss the stability of axial trapping. The calculation will be useful in applications of optical tweezers to trap multiple particles.     

Influence of a glass-water interface on the on-axis trapping of micrometer-sized spherical objects by optical tweezers

A systematic study of the influence of a glass-water interface on the on-axis trapping of micrometer-sized spherical objects by optical tweezers is presented. The ways in which the escape force and the trapping position, as well as the stiffness of the trap, depend on the focusing depth, the numerical aperture, and the degree of overfilling of the objective entrance pupil are investigated. It is concluded, among other things, that objectives with the highest numerical aperture and the use of large degrees of overfilling do not always provide the optimum trapping conditions at finite depths.     

Poly-lactide-co-glycolide microparticle sizes: a rational factorial design and surface response analysis

Microsphere size is a primary determinant of solute release velocity. We present here a rational way for producing PLGA microspheres with different and controlled sizes. The following process variables were studied: Stirring velocity during the second emulsion step, dispersed and continuous phases volume ratio, and poly(vinyl alcohol) concentration in the continuous phase. A full factorial experimental design 2(3) with triplicate at the central point was used to determine the influence of variables on PLGA microsphere mean size. The stirring velocity and poly(vinyl alcohol) concentration were the main variables at 0.95 significance level. An influence of PVA and stirring velocity on microspheres size is observed, there is no correlation for DP/CP volume ratio on size of microspheres. By combining the two variables--the stirring velocity and poly(vinyl alcohol) concentration, the surface response was analyzed. The increase of poly(vinyl alcohol) concentration with concomitant increase on stirring velocity produced microspheres with the lower sized. In contrast the lower poly(vinyl alcohol) concentration and the lower stirring velocity used produced the higher microspheres sized. Uniformly spherical and smooth microspheres (4-15 microm of diameter) were obtained. No significant difference was observed on Ponca S loading within the experimental region. Our results open the possibility of formulating PLGA microspheres with custom sizes performing a minimum of experiments as required for specific applications.     

Planar geometry for trapping and separating ions and charged particles

A planar quadrupole ion trap is proposed. We have demonstrated an extremely large operating range by trapping ions and particles with mass-to-charge ratio ranging from 10(2) to 10(9) at frequencies from 2.8 x 10(6) to 60 Hz at an operating pressure of 1.1 x 10(-4) to 760 Torr, respectively, using a trap radius of r1 = 1 mm. We have also performed mass spectrometry with a resolution of 1.2 amu with mass-to-charge range from 50 to 150. Our geometry is simple enough to be integrated into existing integrated circuits and microelectromechanical system devices, opening up the possibility of many novel hybrid applications and experiments.     

Measurement of the average shear rate around a microparticle in the shear thinning medium with laser tweezers

A novel method for the determination of the average shear rate around a microparticle moving in the fluid is presented. Although the shear rate around a particle moving in the fluid is a parameter of paramount importance in sedimentation studies, its determination is time-consuming, thus model-based solutions are preferred. However, the current literature models require still rigorous validation. The presented technique relies on optical tracking of a laser-tweezers trapped particle suspended in the liquid medium. The obtained experimental results for spherical particles of 1 micron diameter show a good agreement with the classical rheology and literature correlation models. The new method is a valuable tool for determination of sedimentation parameters as it reduces the time of experiments and the sample volume by order(s) of magnitude when compared to classical methods.     

Calculation of the radiation trapping force for laser tweezers by use of generalized Lorenz-Mie theory. II. On-axis trapping force

The efficiency of trapping an on-axis spherical particle by use of laser tweezers for a particle size from the Rayleigh limit to the ray optics limit is calculated from generalized Lorenz-Mie light-scattering theory and the localized version of a Gaussian beam that has been truncated and focused by a high-numerical-aperture lens and that possesses spherical aberration as a result of its transmission through the wall of the sample cell. The results are compared with both the experimental trapping efficiency and the theoretical efficiency obtained from use of the localized version of a freely propagating focused Gaussian beam. The predicted trapping efficiency is found to decrease as a function of the depth of the spherical particle in the sample cell owing to an increasing amount of spherical aberration. The decrease in efficiency is also compared with experiment.     

Hydrodynamic interaction between two red blood cells in simple shear flow: its impact on the rheology of a semi-dilute suspension

Blood is a suspension of red blood cells (RBCs) and its rheology is important when discussing the physiology of the cardiovascular system. In this study, we performed a numerical investigation of the rheological properties of an RBC suspension from the dilute to semi-dilute regime. RBCs were modelled as a capsule with a two-dimensional hyperelastic membrane. Large deformation of the thin membrane was calculated by a finite element method. Due to the small size of the RBC, fluid motion around the RBC was assumed to follow Stokes flow and was solved by a boundary element method. In the dilute limit, cell–cell interactions were omitted and the bulk stress of the suspension was calculated by the stresslet generated on a single RBC. Interestingly, the effective shear viscosity of the dilute suspension decreased with increasing viscosity of the internal liquid. In the semi-dilute regime, cells can be considered as showing pairwise interactions. The effective shear viscosity of the semi-dilute suspension shows a quadratic increase with respect to the volume fraction. These findings are important for understanding the complex phenomena of blood rheology.     

Hydrodynamic characteristics of ascending gas-liquid flow

A new equation for a cell model of bubble flow is obtained on the basis of a previously unused boundary condition. A comprehensive investigation of the hydrodynamic characteristics of this flow has been carried out on a specially built apparatus, enabling us to test experimentally both the newly derived equation and that published earlier by Marrucci. A comparison of the calculated and experimental data showed that the Marrucci equation describes the bubble flow more accurately.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 58, No. 5, pp. 753–760, May, 1990.     

High trapping forces for high-refractive index particles trapped in dynamic arrays of counterpropagating optical tweezers

We demonstrate the simultaneous trapping of multiple high-refractive index (n > 2) particles in a dynamic array of counterpropagating optical tweezers in which the destabilizing scattering forces are canceled. These particles cannot be trapped in single-beam optical tweezers. The combined use of two opposing high-numerical aperture objectives and micrometer-sized high-index titania particles yields an at least threefold increase in both axial and radial trap stiffness compared to silica particles under the same conditions. The stiffness in the radial direction is obtained from measured power spectra; calculations are given for both the radial and the axial force components, taking spherical aberrations into account. A pair of acousto-optic deflectors allows for fast, computer-controlled manipulation of the individual trapping positions in a plane, while the method used to create the patterns ensures the possibility of arbitrarily chosen configurations. The manipulation of high-index particles finds its application in, e.g., creating defects in colloidal photonic crystals and in exerting high forces with low laser power in, for example, biophysical experiments.     

Atomistic modeling and rational design of optothermal tweezers for targeted applications

Optical manipulation of micro/nanoscale objects is of importance in life sciences,colloidal science,and nanotechnology.Optothermal tweezers exhibit superior manipulation capability at low optical intensity.However,our implicit understanding of the working mechanism has limited the further applications and innovations of optothermal tweezers.Herein,we present an atomistic view of opto-thermo-electro-mechanic coupling in optothermal tweezers,which enables us to rationally design the tweezers for optimum performance in targeted applications.Specifically,we have revealed that the non-uniform temperature distribution induces water polarization and charge separation,which creates the thermoelectric field dominating the optothermal trapping.We further design experiments to systematically verify our atomistic simulations.Guided by our new model,we develop new types of optothermal tweezers of high performance using low-concentrated electrolytes.Moreover,we demonstrate the use of new tweezers in opto-thermophoretic separation of colloidal particles of the same size based on the difference in their surface charge,which has been challenging for conventional optical tweezers.With the atomistic understanding that enables the performance optimization and function expansion,optothermal tweezers will further their impacts.     

Optical tweezers: 20 years on

In 1986, Arthur Ashkin and colleagues published a seminal paper in Optics Letters, 'Observation of a single-beam gradient force optical trap for dielectric particles' which outlined a technique for trapping micrometre-sized dielectric particles using a focused laser beam, a technology which is now termed optical tweezers. This paper will provide a background in optical manipulation technologies and an overview of the applications of optical tweezers. It contains some recent work on the optical manipulation of aerosols and concludes with a critical discussion of where the future might lead this maturing technology.     

Hydrodynamic modeling of dilute and dense granular flow

We study numerically a continuum model for granular flow, which covers the regime of fast dilute flow as well as slow dense flow up to vanishing velocity. The constitutive relations at small and intermediate densities are equivalent to those derived from kinetic theory of granular flow. The existence of an inherent instability due to the vanishing kinetic or collisional pressure for small granular temperatures requires a cross over from a collisional pressure to an a thermal yield pressure at densities close to random close packing. Contrary to a kinetic viscosity, the viscosity turns into a function diverging for small temperatures analogous to the diverging viscosities of liquids close to the glass transition. In this respect the presented model is a simplified version of a model of Savage (J Fluid Mech 377:1–26, 1998), which nevertheless recovers many aspects of dense granular flow. As examples we show simulations of sandpiles with predictable slopes, hopper simulations with mass and core flow and angle dependent critical sand heights in flows down an inclined plane. We solve the system of the strongly nonlinear singular hydrodynamic equations with the help of a newly developed nonlinear time stepping algorithm together with a finite volume space discretization. The numerical algorithm is implemented using a finite volume solver framework developed by the authors which allows discretization on cell-centred bricks in arbitrary domains.     

Optical trapping of a spherically symmetric sphere in the ray-optics regime: a model for optical tweezers upon cells

Since their invention in 1986, optical tweezers have become a popular manipulation and force measurement tool in cellular and molecular biology. However, until recently there has not been a sophisticated model for optical tweezers on trapping cells in the ray-optics regime. We present a model for optical tweezers to calculate the optical force upon a spherically symmetric multilayer sphere representing a common biological cell. A numerical simulation of this model shows that not only is the magnitude of the optical force upon a Chinese hamster ovary cell significantly three times smaller than that upon a polystyrene bead of the same size, but the distribution of the optical force upon a cell is also much different from that upon a uniform particle, and there is a 30% difference in the optical trapping stiffness of these two cases. Furthermore, under a small variant condition for the refractive indices of any adjacent layers of the sphere, this model provides a simple approximation to calculate the optical force and the stiffness of an optical tweezers system.     

Hydrodynamic modeling of impact craters in ice

In this study, impact craters in water ice are modeled using the hydrodynamic code CTH. In order to capture impact cratering in ice, an equation of state and a material model were created and validated. Cratering simulation results correlated well with known experimental results found in the literature with some minor differences that are discussed. An important result from this study was that the simulations showed a proportional correlation between the damaged volume of the ice crater produced by an aluminum projectile and the projectile's momentum. Also, the identification of four distinct stages in the crater development of ice (contact and compression, initial damage progression, crater shaping, and ejected damaged material) is introduced and described.