Multi-particle interaction in AC electric field driven by dielectrophoresis force

When the dielectrophoresis technology is used to manipulate micron-sized particles, the interaction between particles should not be ignored because of the particle-particle interaction. Especially, when multiple particles (number of particles is above 2) are simultaneously manipulated, the interaction between neighboring particles will affect the results of the manipulation. This research investigates the interaction of particles caused dielectrophoresis effect by the Arbitrary Lagrangian-Eulerian (ALE) method based on the hypothesis of the thin layer of the electric double layer at the microscale. The mathematics model can be solved simultaneously by the finite element method for the AC electric field, the flow field around the suspended particles and the particle mechanics at the micrometer scale. In this study, the particle conductivity and the direction of the electric field are investigated, we find that particle conductivity and electric field direction pose an impact on particle movement, and the research reveal the law of microparticle dielectrophoresis movement, which could offer theoretical and technology support to profoundly understand the precise manipulation of particles in microfluidic chips by the dielectrophoresis effect.     

Photonic electric field sensor based on polymeric microspheres

The detection of electric field by monitoring the optical whispering gallery mode shifts of polymeric microspheres is demonstrated. Two types of spheres are considered; (i) a polydimethylsiloxane (PDMS) sphere with 60 parts base silicon elastomer-to-1 part polymer curing agent by volume and; (ii) a silica sphere coated with a PDMS (uncured) base. The optical mode shifts are caused by perturbations to the resonator morphology induced by electrostriction effect in the presence of an external electric field. Preliminary experiments show that the latter microsphere yields higher sensitivity (0.027 pm/V m⁻¹) with a measurement precision of ∼1.8 V/m. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 276–279     

Drop formation via breakup of a liquid bridge in an AC electric field

Experimental results are presented for the study of drop formation mechanism in a newly proposed electrohydrodynamic (EHD) method of drop generation in an AC electric field. In the method, a small drop is generated in two stages. A pendant drop is elongated with large oscillation by an electric force in the first stage. Then, it undergoes formation and breakup of a liquid bridge between the upper nozzle and the insulator-coated lower flat plate in the second stage. It is found that there exists a resonant frequency for maximum oscillation, which leads to an efficient drop formation in the latter stage. It is also found that breakup of liquid bridge is accelerated by the electrowetting tension acting on the drop perimeter contacting the insulator-coated flat plate. Thus the whole procedure of drop formation depends heavily on the frequency of AC field and the properties of the insulator such as hydrophilicity, thickness, and the dielectric constant. It is demonstrated that a wide range of drop size, from picoliter to nanoliter, can be obtained by controlling such key parameters without changing the nozzle diameter.     

AC electric field induced helix unwinding in planar texture of a ferroelectric liquid crystal

The helix unwinding induced by an AC electric field in a thick ferroelectric S*_C sample with planar orientation has been investigated experimentally. Four patterns of instabilities have been observed below the unwinding critical field E_c. The corresponding thresholds and are found to be dependent on frequency. The formation of two instabilities (λ pattern and rhombic lattice pattern) is discussed on the basis of the present theory. The behaviour above the critical field is briefly described.     

Transient electroosmotic flow induced by AC electric field in micro-channel with patchwise surface heterogeneities

This paper investigates two-dimensional, time-dependent electroosmotic flow driven by an AC electric field via patchwise surface heterogeneities distributed along the micro-channel walls. The time-dependent flow fields through the micro-channel are simulated for various patchwise heterogeneous surface patterns using the backwards-Euler time stepping numerical method. Different heterogeneous surface patterns are found to create significantly different electrokinetic transport phenomena. The transient behavior characteristics of the generated electroosmotic flow are then discussed in terms of the influence of the patchwise surface heterogeneities, the direction of the applied AC electric field, and the velocity of the bulk flow. It is shown that the presence of oppositely charged surface heterogeneities on the micro-channel walls results in the formation of localized flow circulations within the bulk flow. These circulation regions grow and decay periodically in phase with the applied periodic AC electric field intensity. The location and rotational direction of the induced circulations are determined by the directions of the bulk flow velocity and the applied electric field.     

Two-dimensional crystallization of carboxylated benzene oligomers

Various carboxylic acid substitution patterns on the 1,3,5-triphenylbenzene nucleus were explored, and their influence on the symmetry of the resulting two-dimensional (2D) crystal structures was assessed. The symmetry of 1,3,5-benzenetribenzoic acid (H(3)BTB) was reduced by modifying the substitution pattern of the arene and/or adding an additional carboxylic acid. Four analogues belonging to various point groups were studied. Comparison of the monolayers of the analogues to that of H(3)BTB shows that plane group symmetry and molecular symmetry are not correlated: H(3)BTB and its analogues exhibit the same plane group p2 at the heptanoic acid/graphite interface. The 2D crystal structure of the H(3)BTB analogues is more strongly controlled by the geometry of hydrogen-bonding interactions rather than molecular symmetry. Other significant observations in this study include porosity, uncommon hydrogen-bonding motifs, and an unusually high number of inquivalent molecules (Z' = 3) present in the 2D crystal of the lowest symmetry analogue. This research demonstrates that reduction of molecular symmetry based on geometric modification of noncovalent interactions allows for control over porosity of the 2D crystals (close-packed structures to nanoporous networks) without changing the core shape of the molecule.     

Modulation of the translocation of peptides through nanopores by the application of an AC electric field

The interaction of two peptides with the α-hemolysin pore was studied in the presence of a MHz AC field. For an α-helical peptide the proportion of bumping events increased with increasing AC field whereas for a linear peptide with no dipole moment only small changes in the event profiles were observed.     

Folding and assembly of fibroin driven by an AC electric field: effects on film properties

Silk fibroin from Bombyx mori is a high-molecular-weight protein, largely employed in the biomaterials field. Several parameters can affect the folding and assembly of fibroin heavy and light chains. The present work has shown that anisotropic and water-stable films are produced when fibroin solution is cast under an alternating electric field (AC). The treatment can affect the mechanical, thermal and surface properties of fibroin films. These effects have been related to the alignment of molecular dipoles and the formation of oriented supramolecular assemblies. Cell response is affected by this novel processing: MRC5 fibroblasts, cultured on anisotropic fibroin films, preferentially spread parallel to the field direction 6 h after seeding. [Figure: see text].     

AC frequency characteristics of coplanar impedance sensors as design parameters

Glass-based microchannel chips were fabricated using photolithographic technology, and Pt thin-film microelectrodes, as coplanar impedance sensors, were integrated on them. Longitudinal design parameters, such as interelectrode spacing and electrode width, of coplanar impedance sensors were changed to determine AC frequency characteristics as design parameters. Through developing total impedance equations and modeling equivalent circuits, the dominant components in each frequency region were illustrated for coplanar impedance sensors and the measured results were compared with fitted values. As the ionic concentration increased, the value of the frequency-independent region decreased and cut-off frequencies increased. As the interelectrode spacing increased, cut-off frequencies decreased and total impedance increased. However, the width of each frequency-independent region was similar. As the electrode area increased, f(low) decreased but f(high) was fixed. We think that the decrease in R(Sol) dominated over the influence of other components, which resulted in heightening f(low) and f(high). The interelectrode spacing is a more significant parameter than the electrode area in the frequency characteristics of coplanar sensors. The deviation of experimentally obtained results from theoretically predicted values may result from the fringing effect of coplanar electrode structure and parasitic capacitance due to dielectric substrates. We suggest the guidelines of dominant components for sensing as design parameters.     

Experimental study of the growth of cholesteric fingers subjected to an AC electric field and a temperature gradient

We study the behaviour of cholesteric fingers of the first and second types (CF1 and CF2, respectively) which form in cholesteric samples treated for homeotropic anchoring when they are subjected to the combined action of an AC electric field and a temperature gradient. We investigate under which conditions each type of finger can drift and form a spiral. We then show that the ends of the growing CF1 follow circular trajectories, the curvatures of which depend linearly on the temperature gradient. This observation opens the possibility of measuring the thermal Lehmann coefficient in any cholesteric liquid crystal at all temperatures.     

Dynamic light scattering of an ionic SDS micellar solution under an AC electric field

The phase separation behavior of near-critical ionic sodium-dodecyl-sulfate (SDS) micellar solution under a sinusoidal electric field was investigated by phase-contrast optical microscopy and by the small-angle dynamic light scattering method. The sinusoidal electric field significantly deformed the concentration domains and shifted the phase separation temperature. The autocorrelation function under a sinusoidal electric field was measured in the vicinity of a phase separation temperature range of 0.10 K < T_p – T < 0.97 K, where T_p – T is the temperature distance from the phase separation temperature in quiescent state T_p. The correlation function with an oscillatory part in the longer correlation time region was observed. The occurrence of the oscillatory mode, which depended on both the applied field frequency and the ambient temperature, indicates deformation of concentration fluctuation domains by dynamical coupling between the phase separation and the applied sinusoidal electric field. Received: 9 June 2000 Accepted: 31 August 2000     

Drift of spiral waves controlled by a polarized electric field

The drift behavior of spiral waves under the influence of a polarized electric field is investigated in the light that both the polarized electric field and the spiral waves possess rotation symmetry. Numerical simulations of a reaction-diffusion model show that the drift velocity of the spiral tip can be controlled by changing the polarization mode of the polarized electric field and some interesting drift phenomena are observed. When the electric field is circularly polarized and its rotation follows that of the spiral, the drift speed of the spiral tip reaches its maximal value. On the contrary, opposite rotation between the spiral and electric field locks the drift of the spiral tip. Analytical results based on the weak deformation approximation are consistent with the numerical results. We hope that our theoretical results will be observed in experiments, such as the Belousov-Zhabotinsky reaction.     

Relaxation of a liquid-crystalline order induced by an electric field

The influence of a strong electric field on isotropic melts of 6-cyanobiphenyl and comb-shaped polyacrylate with mesogenic groups in its side chains is investigated. It is established that the electric field induces the isotropic liquid-nematic liquid crystal phase transition in melts of these compounds. A relaxation process was discovered that destroys the nematic ordering induced in substances by electric fields. It was established that, for polymer liquid crystals, the time of transition from the ordered state to the isotropic phase is several orders longer then that for low-molecular liquid crystal.     

Two-dimensional crystallization of hard sphere particles at a liquid–liquid interface

A method for studying crystallization of hard sphere like particles in two dimensions is presented. The method involves trapping the particles at the interface between two immiscible liquids. Particles at the interface undergo 2D Brownian motion, and at sufficiently high densities crystallization is observed. The pseudo hard sphere nature of the particle interactions under these conditions is maintained, as demonstrated by the area density at which crystallization occurs. In contrast to established techniques for studying crystallization in pseudo 2D hard spheres, the particles trapped at the interface undergo no vertical motion, so the system is in principle closer to a true 2D system. The method is therefore amenable to the study of the effects of polydispersity on crystallization behaviour. The advantages and disadvantages of the method are discussed.     

Transient electroosmotic flow induced by DC or AC electric fields in a curved microtube

This study investigates transient electroosmotic flow in a rectangular curved microtube in which the fluid is driven by the application of an external DC or AC electric field. The resultant flow-field evolutions within the microtube are simulated using the backwards-Euler time-stepping numerical method to clarify the relationship between the changes in the axial-flow velocity and the intensity of the applied electric field. When the electric field is initially applied or varies, the fluid within the double layer responds virtually immediately, and the axial velocity within the double layer tends to follow the varying intensity of the applied electric field. The greatest net charge density exists at the corners of the microtube as a result of the overlapping electrical double layers of the two walls. It results in local maximum or minimum axial velocities in the corners during increasing or decreasing applied electric field intensity in either the positive or negative direction. As the fluid within the double layer starts to move, the bulk fluid is gradually dragged into motion through the diffusion of momentum from the double layer. A finite time is required for the full momentum of the double layer to diffuse to the bulk fluid; hence, a certain phase shift between the applied electric field and the flow response is inevitable. The patterns of the axial velocity contours during the transient evolution are investigated in this study. It is found that these patterns are determined by the efficiency of momentum diffusion from the double layer to the central region of the microtube.     

Mapping electric fields generated by microelectrodes using optically trapped charged microspheres

In this work, we show a new technique to measure the direction and amplitude of the electric field generated by microelectrodes in a liquid environment, as often used in microfluidic devices. The method is based on the use of optical tweezers as a force transducer. A trapped, charged particle behaves as a probe. With this technique, it is possible to obtain a detailed map of the electric field, even for very complex electrode structures with a resolution below a micrometre and with a sensitivity as low as a few hundreds of V m(-1).     

Effect of crystallization on the morphologies of block copolymer/homopolymer blends cast in an electric field

Blends of polystyrene (PS) and poly(styrene-b-ethylene oxide) (PS-b-PEO) were cast from a ternary solvent mixture containing 85% toluene, 10% tetrahydrofuran, and 5% methanol under conditions that favor crystallization of the PEO phase. Electric fields (2–14 kV/cm) were applied during casting to explore the possibility of morphology control by the field. It was observed that films cast in the absence of an electric field, in the temperature range of 0–25°C, from solutions initially cooled to 0°C were translucent. Their transmission electron micrographs exhibited thread-like, fibrillar structures. Micrographs of films cast in dc fields of 2–14 kV/cm at 16.3 ± 0.4°C also showed fibrillar structures, with the fibrils in the presence of fields greater than 8 kV/cm being substantially oriented in the field direction. We suggest that the morphologies developed under these conditions result from crystallization from preexisting crystal nuclei in the cooled solutions with the fibrillar crystals being oriented by the electric field. This method provides a possible way of processing anisotropic polymer blends. © 1996 John Wiley & Sons, Inc.     

磁场影响聚合物本体结晶动力学的机理分析

在经典的热力学理论基础上,探讨了磁场对聚合物本体结晶过程的成核与生长的影响,建立了相关结晶动力学理论方程.初步认为,磁场产生的＂磁结晶效应＂可能是由于晶相与非晶相之间磁化率差异导致了两相之间磁化能的差异,也可能由于聚合物体系在结晶前会形成一种有序相,减小了体系的熵值,进而改变了结晶过程中的体系自由能,影响其成核与晶体生长,乃至整个结晶动力学方程.利用Matlab软件结合PLLA的各结晶参数值,绘制了结晶自由能与各成核临界参数之间的函数图像.结果表明,在低过冷度下,较小的自由能扰动可能导致较大的晶核临界参数变化.