Effect of electric-field-induced capillary attraction on the motion of particles at an oil-water interface

Here, we investigate experimentally and theoretically the motion of spherical glass particles of radii 240-310 microm attached to a tetradecane-water interface. Pairs of particles, which are moving toward each other under the action of lateral capillary force, are observed by optical microscopy. The purpose is to check whether the particle electric charges influence the particle motion, and whether an electric-field-induced capillary attraction could be detected. The particles have been hydrophobized by using two different procedures, which allow one to prepare charged and uncharged particles. To quantify the hydrodynamic viscous effects, we developed a semiempirical quantitative approach, whose validity was verified by control experiments with uncharged particles. An appropriate trajectory function was defined, which should increase linearly with time if the particle motion is driven solely by the gravity-induced capillary force. The analysis of the experimental results evidences for the existence of an additional attraction between two like-charged particles at the oil-water interface. This attraction exceeds the direct electrostatic repulsion between the two particles and leads to a noticeable acceleration of their motion.     

Interface motion of capillary-driven flow in rectangular microchannel

In microchannel flow, gas-liquid interface behavior is important for developing a wide range of microfluidic applications, especially in passive microfluidic systems. This paper presents a discussion of interface motion driven by capillary action in a microchannel. We have extended the theory beyond the previous theory of capillary rise problem for a circular tube, to a rectangular microchannel. The same formula for the relation between nondimensional time and interface position is obtained as for a circular tube. We examined rectangular microchannels with several sizes (about 50 to 100 microm square) of glass capillaries and 85 x 68 microm and 75 x 45 microm polydimethylsiloxane (PDMS) microchannels fabricated by photolithography technique, respectively. We observed movement of the gas-liquid interface position and compared it to the dimensionless relation. We obtained the value of a dimensionless variable of driving force that is related to dynamic contact angles for glass-water, glass-ethanol, and PDMS-ethanol. Using this variable, interface motion can be predicted for any size of rectangular channels.     

Acoustic control of suspended particles in micro fluidic chips

A method to separate suspended particles from their medium in a continuous mode at microchip level is described. The method combines an ultrasonic standing wave field with the extreme laminar flow properties obtained in a silicon micro channel. The channel was 750 microm wide and 250 microm deep with vertical side walls defined by anisotropic wet etching. The suspension comprised "Orgasol 5 microm" polyamide spheres and distilled water. The channel was perfused by applying an under pressure (suction) to the outlets. The channel was ultrasonically actuated from the back side of the chip by a piezoceramic plate. When operating the acoustic separator at the fundamental resonance frequency the acoustic forces were not strong enough to focus the particles into a well defined single band in the centre of the channel. The frequency was therefore changed to about 2 MHz, the first harmonic with two pressure nodes in the standing wave, and consequently two lines of particles were formed which were collected via the side outlets. Two different microchip separator designs were investigated with exit channels branching off from the separation channel at angles of 90 degrees and 45 degrees respectively. The 45 degrees separator displayed the most optimal fluid dynamic properties and 90% of the particles were gathered in 2/3 of the original fluid volume.     

Dynamic self-assembly of polymer colloids to form linear patterns

A dynamic self-assembling process is reported which involves drying a droplet of positively charged colloidal suspension on a flat negatively charged hydrophilic surface. This extremely simple method affords lines of colloidal particles with regular 1.5-4.5 microm line spacing and smaller than 2 microm line width over a broad surface area. The ordered region diffracts light to display an iridescent appearance and generates first-order diffraction spots when illuminated by a He-Ne laser. A periodic stick-slip motion of the drying liquid front is observed during the drying process using optical microscopy. The periodic motion must be related to the periodic particle deposition. We propose that the simultaneous deposition of the particles at a fixed distance (i.e., the line spacing) behind the previous line of particles where the contact line is pinned is in turn responsible for the periodic stick-slip motion. The key distinguishing feature of the present system is the attractive interaction between the particles and the surface, which instigates the periodicity of the particle deposition.     

Ultrafast studies of gold, nickel, and palladium nanorods

Steady state and ultrafast transient absorption studies have been carried out for gold, nickel, and palladium high aspect ratio nanorods. For each metal, nanorods were fabricated by electrochemical deposition into approximately 6 microm thick polycarbonate templates. Two nominal pore diameters(10 and 30 nm, resulting in nanorod diameters of about 40 and 60 nm, respectively) were used, yielding nanorods with high aspect ratios (>25). Static spectra of nanorods of all three metals reveal both a longitudinal surface plasmon resonance (SPR(L)) band in the mid-infrared as well as a transverse band in the visible for the gold and larger diameter nickel and palladium nanorods. The appearance of SPR(L) bands in the infrared for high aspect ratio metal nanorods and the trends in their maxima for the different aspect ratios and metals are consistent with calculations based on the Gans theory. For the gold and nickel samples, time resolved studies were performed with a subpicosecond resolution using 400 nm excitation and a wide range of probe wavelengths from the visible to the mid-IR as well as for infrared excitation (near 2000 cm(-1)) probed at 800 nm. The dynamics observed for nanorods of both metals and both diameters include transients due to electron-phonon coupling and impulsively excited coherent acoustic breathing mode oscillations, which are similar to those previously reported for spherical and smaller rod-shaped gold nanoparticles. The dynamics we observe are the same within the experimental uncertainty for 400 nm and infrared (5 microm) excitation probed at 800 nm. The transient absorption using 400 nm excitation and 800 nm probe pulses of the palladium nanorods also reveal coherent acoustic oscillations. The results demonstrate that the dynamics for high aspect ratio metal nanorods are similar to those for smaller nanoparticles.     

Droplet formation in a microchannel network

A method is given for generating droplets in a microchannel network. With oil as the continuous phase and water as the dispersed phase, pico/nanoliter-sized water droplets can be generated in a continuous phase flow at a -junction. The channel for the dispersed phase is 100 microm wide and 100 microm deep, whereas the channel for the continuous phase is 500 microm wide and 100 microm deep. For given experimental parameters, regular-sized droplets are reproducibly formed at a uniform speed. The diameter of these droplets is controllable in the range from 100-380 microm as the flow velocity of the continuous phase is varied from 0.01 m s(-1) to 0.15 m s(-1).     

Dielectrophoretic manipulation of suspended submicron particles

Planar and three-dimensiònal multi-electrode systems with dimensions of 2 - 40 microm were fabricated by IC technology and used for trapping and aggregation of microparticles. To achieve negative dielectrophoresis (repelling forces) in aqueous solution, radiofrequency (RF) electric fields were used. Experimentally, particles down to 100 nm in diameter were enriched and trapped as aggregates in field cages and dielectrophoretic microfilters and observed using confocal fluorimetry. Theoretically, single particles with an effective diameter down to about 35 nm should be trappable in micron field cages. Due to the unavoidable Ohmic heating, RF electric fields can induce liquid streaming in extremely small channels (12 microm in height). This can be used for pumping and particle enrichment but it enhances Brownian motion and counteracts dielectrophoretic trapping. Combining Brownian motion with ratchet-like dielectrophoretic forces enables the creation of Brownian pumps that could be used as sensitive separation devices for submicron particles if liquid pumping is avoided in smaller structures.     

Diffusion of nanoparticles in monolayers is modulated by domain size

Langmuir monolayers are often used as simple models for biological membranes. The possibility to change their composition and phase state in a very controlled manner as well as access to a large observation area makes them a versatile tool for the investigation of membrane-related interactions. Inspired by experiments in our group, we investigate the interaction of single, partially charged nanoparticles with lipid microdomains by Monte Carlo simulations. Condensed domains in inhomogeneous Langmuir monolayers exhibit an electric dipole field interacting attractively with the nanoparticle's dipole moment. With increasing domain size, the resulting electric field changes from single dipole to semi-infinite domain characteristics, significantly influencing the motion of the particle. Small immobile domains (R = 1 microm) confine the movement of the tracer to the boundary of the domain whereas for large domains (R > or = 10 microm) its motion is only temporarily hindered. This suggests a powerful mechanism for controlling diffusive transport in lipid membranes.     

Dye-sensitized solar cells based on oriented TiO2 nanotube arrays: transport, trapping, and transfer of electrons

Dye-sensitized solar cells fabricated using ordered arrays of titania nanotubes (tube lengths 5, 10, and 20 microm) grown on titanium have been characterized by a range of experimental methods. The collection efficiency for photoinjected electrons in the cells is close to 100% under short circuit conditions, even for a 20 microm thick nanotube array. Transport, trapping, and back transfer of electrons in the nanotube cells have been studied in detail by a range of complementary experimental techniques. Analysis of the experimental results has shown that the electron diffusion length (which depends on the diffusion coefficient and lifetime of the photoinjected electrons) is of the order of 100 microm in the titania nanotube cells. This is consistent with the observation that the collection efficiency for electrons is close to 100%, even for the thickest (20 microm) nanotube films used in the study. The study revealed a substantial discrepancy between the shapes of the electron trap distributions measured experimentally using charge extraction techniques and those inferred indirectly from transient current and voltage measurements. The discrepancy is resolved by introduction of a numerical factor to account for non-ideal thermodynamic behavior of free electrons in the nanostructured titania.     

Molecular dynamics simulations of electroosmosis in perfluorosulfonic acid polymer

An atomistic MD simulation method has been developed to study the electroosmotic drag in the hydrated perfluorosulfonic acid polymer. The transport characteristics of the hydroniums and water molecules are evaluated from their velocity distribution functions with an electric field applied. It is shown that the microstructure of the hydrated perfluorosulfonic acid polymer is not perturbed significantly by the electric field up to 2 V/microm, and the velocity distribution functions obey the peak shifted Maxwell velocity distribution functions. The evaluated peak shifting velocities are only about 1% of the average thermal motion. The hydronium flow and water flow are evaluated from the average transport velocities or the peak shifting velocities. The electroosmotic drag coefficients from the MD simulations are in good correspondence with the experimental values. It is also shown that the electroosmotic drag coefficient has no or weak temperature dependence.     

Micromachined bulk PZT tissue contrast sensor for fine needle aspiration biopsy

This paper describes a micromachined piezoelectric sensor, integrated into a cavity at the tip of a biopsy needle, and preliminary experiments to determine if such a device can be used for real-time tissue differentiation, which is needed for needle positioning guidance during fine needle aspiration (FNA) biopsy. The sensor is fabricated from bulk lead zirconate titanate (PZT), using a customized process in which micro electro-discharge machining is used to form a steel tool that is subsequently used for batch-mode ultrasonic micromachining of bulk PZT ceramic. The resulting sensor is 50 microm thick and 200 microm in diameter. It is placed in the biopsy needle cavity, against a steel diaphragm which is 300 microm diameter and has an average thickness of 23 microm. Devices were tested in materials that mimic the ultrasound characteristics of human tissue, used in the training of physicians, and with porcine fat and muscle tissue. In both schemes, the magnitude and frequency of an electrical impedance resonance peak showed tissue-specific characteristics as the needle was inserted. For example, in the porcine tissue, the impedance peak frequency changed approximately 13 MHz from the initial 163 MHz, and the magnitude changed approximately 1600 Omega from the initial 2100 Omega, as the needle moved from fat to muscle. Samples including oils and saline solution were tested for calibration, and an empirical tissue contrast model shows an approximately proportional relationship between measured frequency shift and sample acoustic impedance. These results suggest that the device can complement existing methods for guidance during biopsies.     

Acoustofluidics 7: The acoustic radiation force on small particles

In this paper, Part 7 of the thematic tutorial series "Acoustofluidics-exploiting ultrasonic standing waves, forces and acoustic streaming in microfluidic systems for cell and particle manipulation", we present the theory of the acoustic radiation force; a second-order, time-averaged effect responsible for the acoustophoretic motion of suspended, micrometre-sized particles in an ultrasound field.     

Continuous hydrophoretic separation and sizing of microparticles using slanted obstacles in a microchannel

We report a microfluidic separation and sizing method of microparticles with hydrophoresis--the movement of suspended particles under the influence of a microstructure-induced pressure field. By exploiting slanted obstacles in a microchannel, we can generate a lateral pressure gradient so that microparticles can be deflected and arranged along the lateral flows induced by the gradient. Using such movements of particles, we completely separated polystyrene microbeads with 9 and 12 microm diameters. Also, we discriminated polystyrene microbeads with diameter differences of approximately 7.3%. Additionally, we measured the diameter of 10.4 microm beads with high coefficient of variation and compared the result with a conventional laser diffraction method. The slanted obstacle as a microfluidic control element in a microchannel is analogous to the electric, magnetic, optical, or acoustic counterparts in that their function is to generate a field gradient. Since our method is based on intrinsic pressure fields, we could eliminate the need for external potential fields to induce the movement of particles. Therefore, our hydrophoretic method will offer a new opportunity for power-free and biocompatible particle control within integrated microfluidic devices.     

Using ratchets and sorters to fractionate motile cells of Escherichia coli by length

This paper describes the fabrication of a composite agar/PDMS device for enriching short cells in a population of motile Escherichia coli. The device incorporated ratcheting microchannels, which directed the motion of swimming cells of E. coli through the device, and three sorting junctions, which isolated successively shorter populations of bacteria. The ratcheting microchannels guided cells through the device with an average rate of displacement of (32 +/- 9) microm s(-1). Within the device, the average length of the cells decreased from 3.8 microm (Coefficient of Variation, CV: 21%) at the entrance, to 3.4 microm (CV: 16%) after the first sorting junction, to 3.2 mum (CV: 19%) after the second sorting junction, to 3.0 mum (CV: 19%) after the third sorting junction.     

The electrochemical detection of droplets in microfluidic devices

This paper presents a new electrochemical method for the detection and characterisation of aqueous droplets in an organic carrier fluid (1,2-dichloroethane) formed in flow-focusing microfluidic devices. The devices consist of a conventional flow-focusing channel 250 microm wide and 250 microm deep cast out of poly(dimethylsiloxane) (PDMS) which is sealed onto a glass substrate containing a set of microelectrodes 100 microm long. Chronoamperometric analysis of a suitable electrolyte contained in the organic phase is presented for characterising the droplet frequency and size. This chronoamperometric method is then extended to a dual working electrode approach in order to determine the velocity of the droplet. Good agreement between experimental measurements and theory was observed.     

Non-contact microsphere--surface adhesion measurement via acoustic base excitations

A non-contact adhesion measurement technique based on acoustic base excitation and laser interferometery has been introduced and demonstrated. The vibrational motion of 21.4-microm polystyrene latex particles (PSL) microspheres on surfaces were excited in the frequency range of 0-3.5 MHz, and their axial displacement responses were measured by an interferometer. It is shown that the rolling motion is dominant compared to the axial displacement of the bond. Using a formula for the rotational moment resistance of the particle-surface adhesion bond and the equation of rotational motion, the natural frequency of the rotational motion is related to the work of adhesion of the particle and substrate materials. The substrate materials used in the experiments include copper, aluminum, tantalum, and silicon. Measured work of adhesion values are compared to the data reported in the literature and good agreement is found.     

Microparticle collection and concentration via a miniature surface acoustic wave device

The ability to detect microbes, pollens and other microparticles is a critically important ability given the increasing risk of bioterrorism and emergence of antibiotic-resistant bacteria. The efficient collection of microparticles via a liquid water droplet moved by a surface acoustic wave (SAW) device is demonstrated in this study. A fluidic track patterned on the SAW device directs the water droplet's motion, and fluid streaming induced inside the droplet as it moves along is a key advantage over other particle collection approaches, because it enhances microparticle collection and concentration. Test particles consisted of 2, 10, 12 and 45 microm diameter monodisperse polystyrene and melamine microparticles; pollen from the Populus deltoides, Kochia scoparia, Secale cerale, and Broussonetia papyrifera (Paper Mulberry) species; and Escherichia coli bacteria. The collection efficiency for the synthetic particles ranged from 16 to 55%, depending on the particle size and surface tension of the collection fluid. The method was more effective in collecting pollen and the bacteria with an efficiency of 45-68% and 61.0-69.8%, respectively. Pollen collection was strongly influenced by its diameter, size, and surface geometry in a manner contrary to initial expectations. Reasons for the consistent yet unexpected collection results include leaky SAW pressure boundary segregation and shear-induced concentration of larger particles, and the subtle effects of wetting interactions. These results demonstrate a new method for collecting microparticles requiring only about one second per run, and illustrate the inadequacy of using synthetic microparticles as a substitute for their biological counterparts in experiments studying particle collection and behavior.     

Micro pumping with cardiomyocyte-polymer hybrid

This paper presents a hybrid micropump actuated by the up-down motion of a dome shaped cell-polymer membrane composite. The contractile force induced from self-beating cardiomyocytes cultured on the membrane causes shrinkage and relaxation of a microchamber, leading to a flow in a microchannel. Flow direction is controlled by the geometry of diffuser/nozzle in the microchannel. The fabrication process is noninvasive to cells, thus, cardiomyocytes can robustly maintain their activity for a long time. The fluid motion in the microchannel was monitored by tracking 2 microm polystyrene beads. A net flow rate of 0.226 nl min(-1) was obtained in our microscale device. Our device demonstrates a unique performance of a cell-microdevice hybrid lab-on-a-chip that does not require any external power source, preventing electrical or heat shock to analytes.     

Measuring the electrophoretic mobility of concentrated suspensions in nonaqueous media

We present a new method of measuring the electrophoretic mobility of a particle in a concentrated suspension. The method is used to measure the electrophoretic mobility of PMMA particles (diameter 10 microm) suspended in a mixture of liquid hydrocarbons. The particle volume fraction of the suspension is varied from 0 up to 0.30 and the resulting variation of the electrophoretic mobility is discussed. The suspending liquid is such that its refractive index is very close to that of the particles. Thus the suspension is almost transparent and it is possible to follow through a microscope the motion of one particle. The suspension is subjected to a low-frequency electric field (0.5 Hz). The cell containing the suspension is mounted on a piezoelectric crystal. The displacement that compensates for the particle motion (when the particle image is steady) is determined.     

Ultrasonic characterization of proteins and blood cells

Ultrasound changes its intensity and speed when propagating through a liquid or a suspension containing particles. In addition it generates a weak electric signal by altering the motion of ions and charged particles. Hence acoustic and electroacoustic measurements provide information about the properties of suspended particles and molecules. Here we present both acoustic and electroacoustic results on blood suspensions and protein solutions, relevant to life sciences. For blood cells a strong increase in acoustic attenuation with volume fraction is found, from which the speed of sound in an erythrocyte is found to be about 1900 m/s, assuming the attenuation is due to scattering only. A similar value of 1700 m/s is found from the increase in sound speed of the dispersion with concentration. Electroacoustic measurements on bovine serum albumin (BSA) yield a charge of about seven elementary charges per BSA molecule. These results show the power and usefulness of acoustic and electroacoustic measurement techniques for biological systems.