Jetting liquid marbles: study of the Taylor instability in immersed marbles

The behavior of liquid marbles encapsulated with various powders, immersed in oil, and exposed to a uniform DC field was investigated. At some critical value of the electric field, the Taylor instability of the marble shape took place, accompanied by the appearance of a cone and jetting a small droplet. The squared critical electric field was linear dependent on inverse of the size parameter of the marble. In some cases, the extrapolation of this linear dependence to the zero field gave the finite value of the spherical marble radius corresponding to the Rayleigh limit that meant that the marbles were charged. Lycopodium-coated marbles remained neutral under the action of a DC field, as well as a pure water droplet. Therefore, charging marbles is determined by their powder coverage. The data on effective surface tension at marble–oil interfaces were extracted from the above linear dependence for the uncharged marble. The effective surface tension was measured in parallel by the capillary rise method.     

Temperature dependence of fluid transport in nanopores

Understanding the temperature-dependent nanofluidic transport behavior is critical for developing thermomechanical nanodevices. By using non-equilibrium molecular dynamics simulations, the thermally responsive transport resistance of liquids in model carbon nanotubes is explored as a function of the nanopore size, the transport rate, and the liquid properties. Both the effective shear stress and the nominal viscosity decrease with the increase of temperature, and the temperature effect is coupled with other non-thermal factors. The molecular-level mechanisms are revealed through the study of the radial density profile and hydrogen bonding of confined liquid molecules. The findings are verified qualitatively with an experiment on nanoporous carbon.     

Anomalous Pore‐Density Dependence in Nanofluidic Osmotic Power Generation

Osmotic power generation in biomimetic nanofluidic systems has attracted considerable research interest owing to the enhanced performance and long‐term stability. Towards practical applications, when extrapolating the materials from single‐nanopore to multi‐pore membranes, conventional viewpoint suggests that, to gain high electric power density, the porosity should be as high as possible. However, recent experimental observations show that the commonly‐used linear amplification method largely overestimates the actual performance, particularly at high pore density. Herein, we provide a theoretical investigation to understand the reason. We find a counterintuitive pore‐density dependence in high porosity nanofluidic systems that, once the pore density approaches more than 1×10⁹ pores/cm², the overall output electric power goes down with the increasing pore density. The excessively high pore density impairs the charge selectivity and induces strong ion concentration polarization, which undermines the osmotic power generation process. By optimizing the geometric size of the nanopores, the performance degradation can be effectively relieved. These findings clarify the origin of the unsatisfactory performance of the current osmotic nanofluidic power sources, and provide insights to further optimize the device.     

The possibility of different time scales in the dynamics of pore imbibition

To model the imbibition of liquids into porous solids, use is often made of the Lucas-Washburn equation, which relates the distance of penetration of a liquid at a given time to the pore radius, the viscosity and surface tension of the liquid, and the effective contact angle between the liquid and the solid. In this paper, we extend previous large-scale molecular dynamics simulations to show how this tool can be used to study the details of liquid imbibition, including the impact of the contact angle on the dynamics of penetration and the evolution of the internal flow field. In particular, we show that the asymptotic behavior of the contact angle versus time for a completely wetting liquid is given by approximately t(-1/4).     

The influence of electric field-induced orientation on the retraction of a liquid crystal droplet immersed in a flexible polymer matrix

We measured the apparent interfacial tension between a liquid crystal and a flexible polymer by deformed droplet retraction method. An external electric field is applied to change the director orientation in liquid crystal droplet. The deformation and recovery of a single liquid crystal droplet dispersed in a polydimethylsiloxane (PDMS) matrix were realized by a transient shear flow and observed by polarized optical microscope. In order to control the director orientation in LC droplet, the electric field is applied perpendicular and parallel to the flow field, respectively. The different orientation induced by electric field in liquid crystal droplet has different behavior during droplet retraction and affect the apparent interfacial tension between liquid crystal and flexible polymer.     

A Surfactant Bridge Model for the Nonlinear Electrorheological Effects of Surfactant-Activated ER Suspensions

In surfactant-activated electrorheological (ER) suspensions it is observed that the ER response shows linear ER behavior (F~E(2)) at small surfactant concentrations and nonlinear ER behavior (F~E(n), n approximately 1) at large surfactant concentrations. Here, a surfactant bridge model is developed to explain the nonlinear ER behavior of surfactant-activated ER suspensions. The model shows that the formation and size of a surfactant bridge depend on various variables, especially the electric field strength, the surfactant surface tension, and the initially adsorbed amount of surfactants on particles. The predicted dependence of the formation and size of a surfactant bridge on the electric field strength and the initially adsorbed amount of surfactants is consistent with the observations. Also, the model indicates that there is a critical minimum electric field E(crit) for the formation of a surfactant bridge, and the estimated E(crit) shows good agreement with the observations. The force acting between particles is composed of the electrostatic force and force associated with surface tension. However, it is found that the contribution of the force associated with surface tension can be ignored and the electrostatic force is dominant regardless of the formation of surfactant bridges between particles. When surfactant bridges are formed between particles, the predicted force shows nonlinear ER behavior (F~E(n), n approximately 1), consistent with the observed nonlinear ER behavior at large surfactant concentrations. When no surfactant bridge is formed, the predicted force is proportional to the electric field squared (F~E(2)), consistent with the interfacial polarization. The model can successfully predict the nonlinear ER behavior at large surfactant concentrations, confirming that the nonlinear ER behavior of surfactant-activated ER suspensions arises from the observed formation of surfactant bridges between particles. Copyright 2001 Academic Press.     

Mass transport in nanofluidic devices

Nanofluidics is a recent appearing research field, introduced in 1995 as an analogue of the field of microfluidics, and has been becoming popular in the past few years. The proximity of the channel dimension, the Debye length, and the size of biomolecules such as DNA and proteins gives the unique features of nanofluidic devices. Of various unique properties of the nanofluidics, mass transport in nanochannel plays determining roles in fundamental reaches and practical applications of nanofluidic device. Thus, much work including numerical and experimental researches has been performed to investigate the mass transport behaviors in nanofluidic devices. This review summarizes the fabrication technologies for nanofluidic devices, the mass transport behaviors in nanochannel, and their applications in bioanalysis. The main focus will be laid on the effects of nanochannel size and surface charge on mass transport including electrokinetic transport of charged analytes, diffusion of electric neutral molecules, ionic current rectification, concentration polarization, nonlinear electrokinetic flow at the micro-nanofluidic interfaces.     

The in-plane switching of homogeneously aligned nematic liquid crystals

We have investigated the electro-optical effects and physical switching principle of homogeneously aligned nematic liquid crystals when applying an in-plane electric field with interdigital electrodes. By using the in-plane switching (IPS) of the liquid crystals which is achieved by the in-plane electric field, the viewing angle characteristics of the electro-optical effects were confirmed to be far superior to those of the conventional twisted nematic mode in which the electric field is applied along the direction perpendicular to the substrates. The non-reversal region of grey scales was extremely wide in which a high contrast ratio was kept, even along quite an oblique direction in the IPS mode. In order to clarify the switching principle of the liquid crystals in the IPS mode, a simplified expression describing the threshold behaviour of the device was derived with the assumption that a uniform in-plane electric field was applied along a direction perpendicular to the director and parallel to the homogeneously aligned nematic slab, and found to be sufficiently able to explain the experimental results. First, a critical field at which the liquid crystals just began to twist, was found to be proportional to the reciprocal of the cell gap. Second, it was the electric field and not the voltage that drives the liquid crystals. This relationship was due to the independence of the electric field regarding the liquid crystal layer normal direction. So the threshold voltage in the IPS mode was strongly dependent on the variation of the cell gap. For the dynamical response mechanism of the liquid crystals to the in-plane electric field, the switching on and off processes of the liquid crystals were analysed quantitatively. The relaxation time of the liquid crystals when removing the electric field could be described as proportional to the square of the cell gap. A thinner cell gap also proved to be effective in obtaining a fast response time in the IPS mode. In contrast, the switching on time when applying the in-plane electric field was found to be inversely proportional to the difference between the square of the electric field strength and the square of the critical electric field strength at which the liquid crystals began to deform.     

Effect of wall roughness on fluid transport resistance in nanopores

Using non-equilibrium molecular dynamics simulations, we investigate the effect of wall roughness on the transport resistance of water molecules inside modified carbon nanotubes. The effective shear stress, which characterizes the strong interaction between liquid molecules and solid wall, is a quantity that dominates the nanofluidic transport resistance. Both the effective shear stress and nominal viscosity arise with the increase of the amplitude or the decrease of the wavelength of roughness. The effect of roughness is also relatively more prominent in smaller nanotubes. The molecular mechanism is elucidated through the study of the radial density profile, hydrogen bonding, and velocity field of the confined water molecules.     

Numerical simulation of droplet coalescence behavior in gas phase under the coupling of electric field and flow field

The coalescence behavior of droplets in an electric field belongs to the important research contents of electrohydrodynamics. Based on the phase field method of the Cahn–Hilliard equation, the electric field and the flow field are coupled to establish the numerical model of twin droplet coalescence in a coupled field. The effects of flow rate, electric field strength, droplet diameter, and interfacial tension on the coalescence behavior of droplets during the coalescence process were investigated. The results show that the dynamic behavior of the droplets is divided into coalescence, after coalescence rupture, and no coalescence under the coupling of electric field and flow field. The proper increase of the electric field strength will accelerate the coalescence of the droplets, and the high electric field strength causes the droplets to burst after coalescence. Excessive flow rates make droplets less prone to coalescence. Under the coupling field, the larger the droplet interface tension, the smaller the droplet diameter, the smaller the flow rate, and the shorter the droplet coalescence time. The results provide a theoretical basis for the application of electrostatic coalescence in gas–liquid separation technology.     

线性聚丙烯酰胺凝胶毛细管电泳的迁移特性

使用线性聚丙烯酰胺作为筛分介质,对片段长度为80～584bp的标准DNA样品进行毛细管电泳,利用激光诱导荧光方法检测信号,荧光染料为溴化乙啶。改变电场强度100～375V/cm,得到的迁移率曲线与电场强度和DNA片段长度成复杂的函数关系,已有的经典理论模型:Ogston模型、Reptation无拉伸模型和Reptation拉伸模型都不能正确地描述实验观察到的迁移率随电场强度和DNA片段长度的变化情况。因此,提出一种修正的Ogston筛分理论,假定迁移的DNA分子在电场强度方向延展拉伸,如同小分子穿过凝胶筛孔。在该修正模型中,DNA的迁移率仅依赖于电场强度、筛分介质浓度和片段长度,很好地解释了实验现象。     

Pore nucleation in mechanically stretched bilayer membranes

We report a computer-simulation study of the free-energy barrier for the nucleation of pores in the bilayer membrane under constant stretching lateral pressure. We find that incipient pores are hydrophobic but as the lateral size of the pore nucleus becomes comparable with the molecular length, the pore becomes hydrophilic. In agreement with previous investigations, we find that the dynamical process of growth and closure of hydrophilic pores is controlled by the competition between the surface tension of the membrane and the line tension associated with the rim of the pore. We estimate the line tension of a hydrophilic pore from the shape of the computed free-energy barriers. The line tension thus computed is in a good agreement with available experimental data. We also estimate the line tension of hydrophobic pores at both macroscopic and microscopic levels. The comparison of line tensions at these two different levels indicates that the "microscopic" line tension should be carefully distinguished from the "macroscopic" effective line tension used in the theoretical analysis of pore nucleation. The overall shape of the free-energy barrier for pore nucleation shows no indication for the existence of a metastable intermediate during pore nucleation.     

Fluidic communication between multiple vertically segregated microfluidic channels connected by nanocapillary array membranes

Hybrid microfluidic/nanofluidic devices offer unique capabilities for manipulating and analyzing minute volumes of expensive or hard-to-obtain samples. Here, multilayer poly-(methyl methacrylate) microchips, with multiple spatially isolated microfluidic channels interconnected by nanocapillary array membranes (NCAMs), are fabricated using an adhesive contact printing process. The NCAMs, positioned between the microfluidic channel layers, add functionality to the inter-microchannel fluid transfer unit operation. They do so because the transport of specific analytes through the NCAM can be controlled by adjusting the ionic strength, the polarity of the applied bias, the surface charge density, and the pore size. A simplified, floating injection technique for NCAM-coupled nanofluidic devices is described and compared with conventional biased injection. In the floating injection approach, a voltage is applied across the injection channel and the slight electric field extension at the cross-section is used to transfer analytes through the nanopores to the separation channel. Floating injection excels in plug reproducibility, separation resolution, and operation simplicity, although it decreases assay throughput relative to biased injection. Floating injection can avoid the uneven distribution of analytes in the microfluidic channel that sometimes results from biased injection because of the volume mismatch between NCAM nanopore transport capacity and the supply of fluid. Moreover, the pressure-driven flow caused by the mismatch of the EOFs in the microfluidic channels connected by an NCAM must be considered when using NCAMs with pore diameters below 50 nm.     

Nuclear magnetic resonance study of the vapor contribution to diffusion in silica glasses with micrometer pores partially filled with liquid cyclohexane or water

The contribution of the vapor phase to molecular diffusion in porous silica glass (Vitrapor#5; mean pore diameter 1 micrometer) partially filled with cyclohexane (nonpolar) or water (polar) was investigated with the aid of field-gradient NMR diffusometry. Due to the vapor phase, the effective diffusion coefficient of cyclohexane increased up to ten times relative to the value in bulk liquid upon reduction of the pore space filling factor. On the other hand, the effective diffusion coefficient of water first decreases and then increases when the liquid content is reduced. A two-phase exchange theory is presented accounting well for all experimental diffusion features. The diffusion behavior in the samples with micrometer pores under investigation here is in contrast to previous findings for the same solvents in a material with nanometer pores (Vycor; mean pore diameter 4 nm) where the fast-exchange limit had to be assumed [Ardelean et al., J. Chem. Phys. 119, 10358 (2003)]. It is concluded that the pore size plays a crucial role for the relevance of molecular exchange limits relative to the experimental diffusion/exchange time.     

Numerical simulation of deformation behavior of droplet in gas under the electric field and flow field coupling

The water droplets in the process of electrostatic coalescence are important when studying electrohydrodynamics. In the present study, the electric field and flow field are coupled through the phase field method based on the Cahn–Hilliard formulation. A numerical simulation model of single droplet deformation under the coupling field was established. It simulated the deformation behavior of the movement of a droplet in the continuous phase and took the impact of droplet deformation into consideration which is affected by two-phase flow velocity, electric field strength, the droplet diameter, and the interfacial tension. The results indicated that under the single action of the flow field, when the flow velocity was lower, the droplet diameter was greater as was the droplet deformation degree. When the flow velocity was increased, the droplet deformation degree of a small-diameter droplet was at its maximum size, the large-diameter droplet had a smaller deformation degree, and the middle-diameter droplet was at a minimum deformation degree. When the flow velocity was further increased, the droplet diameter was smaller, and the droplet deformation degree was greater. Under the coupled effect of the electric field and flow field, the two-phase flow velocity and the electric field strength were greater, and the degree of droplet deformation was greater. While the droplet diameter and interfacial tension were smaller, the degree of droplet deformation was greater. Droplet deformation degree increased along with the two-phase flow velocity. The research results provided a theoretical basis for gas–liquid separation with electrostatic coalescence technology.     

Fluids in nanospaces: molecular simulation studies to find out key mechanisms for engineering

We have analyzed various phenomena that occur in nanopores, focusing on elucidating their key mechanisms, to advance the effective engineering use of nanoporous materials. As ideal experimental systems, molecular simulations can effectively provide information at the molecular level that leads to mechanistic insight. In this short review, several of our recent results are presented. The first topic is the critical point depression of Lennard-Jones fluid in silica slit pores due to finite size effects, studied by our original Monte Carlo (MC) technique. We demonstrate that the first layers of adsorbed molecules in contact with the pore walls act as a “fluid wall” and impose extra finite size effects on the fluid confined in the central portion of the pore. We next present a new kernel for pore size distribution (PSD) analysis, based entirely on molecular simulation, which consists of local isotherms for nitrogen adsorption in carbon slit pores at 77 K. The kernel is obtained by combining grand canonical Monte Carlo (GCMC) method and open pore cell MC method that was developed in the previous study. We show that overall trends of the PSDs of activated carbons calculated with our new kernel and with conventional kernel from non-local density functional theory are nearly the same; however, apparent difference can be seen between them. As the third topic, we apply a free energy analysis method with the aid of GCMC simulations to investigate the gating behavior observed in a porous coordination polymer, and propose a mechanism for the adsorption-induced structural transition based on both the theory of equilibrium and kinetics. Finally, we construct an atomistic silica pore model that mimics MCM-41, which has atomic-level surface roughness, and perform molecular simulations to understand the mechanism of capillary condensation with hysteresis. We calculate the work required for the gas–liquid transition from the simulation data, and show that the adsorption branch with hysteresis for MCM-41 arise from spontaneous capillary condensation from a metastable state.     

Capillary filling in nanostructured porous silicon

An experimental study on the capillary filling of nanoporous silicon with different fluids is presented. Thin nanoporous membranes were obtained by electrochemical anodization, and the filling dynamics was measured by laser interferometry, taking advantage of the optical properties of the system, related with the small pore radius in comparison to light wavelength. This optical technique is relatively simple to implement and yields highly reproducible data. A fluid dynamic model for the filling process is also proposed including the main characteristics of the porous matrix (tortuosity, average hydraulic radius). The model was tested for different ambient pressures, porous layer morphology, and fluid properties. It was found that the model reproduces well the experimental data according to the different conditions. The predicted pore radii quantitatively agree with the image information from scanning electron microscopy. This technique can be readily used as nanofluidic sensor to determine fluid properties such as viscosity and surface tension of a small sample of liquid. Besides, the whole method can be suitable to characterize a porous matrix.     

Parachors of liquid/vapor systems: A set of critical amplitudes

In light of the available experimental data and of our current understanding of liquid–vapor critical phenomena, we examine the values of the parachors and of the parachor exponent, which are commonly used to estimate surface tension from the density difference between coexisting liquid and vapor phases. This is a controversial issue, as values for the parachor exponent ranging from 3.5 to 4 have been proposed in the literature. The parachor exponent and parachors can be viewed as a critical exponent and critical amplitudes, respectively. The Ising value, equal to 3.88, should be observed for the exponent “close enough” to the liquid/vapor critical point, i.e., for “low enough” tensions and densities. However, a review of experimental data for several fluids suggests an effective value in the range of 3.6, in line with the effective values observed for the exponents that describe the vanishing of the density difference and capillary length with the distance to the critical temperature. In fact, the asymptotic Ising regime is not reached experimentally, as confirmed by an estimation of the parachors very near the critical point. Those (Ising) parachors can be inferred from other critical amplitudes corresponding to bulk properties by using two-scale factor universality. Their values exceed those deduced from off-critical tension and density data by more than 10%, corresponding to surface tension differences larger than 50%. We argue that effective parachors (i.e., corresponding to an exponent in the range of 3.6) can be utilized in combination with two-scale-factor universality for determining the critical behavior of fluid systems in an extended range around their liquid/vapor critical point.     

Polyelectrolytes in multivalent salt solutions

We study conformational and electrophoretic properties of polyelectrolytes (PEs) in tetravalent salt solutions under the action of electric fields by means of molecular dynamics simulations. Chain conformations are found to have a sensitive dependence on the salt concentration C(s). As C(s) is increased, the chains first shrink to a globular structure and subsequently re-expand above a critical concentration C(s)*. An external electric field can further alter the chain conformation. If the field strength E is larger than a critical value E*, the chains are elongated. E* is shown to be a function of C(s) by using two estimators E(I)* and E(II)* through the study of the polarization energy and the onset point of chain unfolding, respectively. The electrophoretic mobility of the chains depends strongly on C(s), and the magnitude increases significantly, accompanying the chain unfolding, when E>E(II)*. We study the condensed ion distributions modified by electric fields and discuss the connection of the modification with the change of chain morphology and mobility. Finally, E* is studied by varying the chain length N. The inflection point is used as a third estimator E(III)*. E(III)* scales as N(-0.63(4)) and N(-0.76(2)) at C(s) =0.0 and C(s)*, respectively. E(II)* follows a similar scaling law to E(III)* but a crossover appears at C(s) =C(s)* when N is small. The E(I)* estimator fails to predict the critical field, which is due to oversimplifying the critical polarization energy to the thermal energy. Our results provide valuable information to understand the electrokinetics of PE solutions at the molecular level and could be helpful in micro/nanofluidic applications.     

Effects of electric field on confined electrolyte in a hexagonal mesoporous silica

In an electrowetting experiment on a surface treated hexagonal mesoporous silica, it is noticed that the effective solid-liquid interfacial tension is quite insensitive to the applied voltage, while the accessible nanopore volume decreases significantly as the voltage is increased. When the voltage is higher than 900 V, the liquid infiltration cannot be detected. The liquid defiltration is quite insensitive to the electric field. These unique phenomena may be attributed to the field responsive ion behaviors in the confining nanoenvironment.