Confined liquid: Simultaneous observation of a molecularly layered structure and hydrodynamic slip

A Review of Nanofluids as Coolants for Thermal Management Systems in Fuel Cell Vehicles

With the development of high-power fuel cell vehicles, heat dissipation requirements have become increasingly stringent. Although conventional cooling techniques improve the heat dissipation capacity by increasing the fan rotating speed or radiator dimensions, high energy consumption and limited engine compartment space prevent their implementation. Moreover, the insufficient heat transfer capacity of existing coolants limits the enhancement of heat dissipation performance. Therefore, exploring novel coolants to replace traditional coolants is important. Nanofluids composed of nanoparticles and base liquids are promising alternatives, effectively improving the heat transfer capacity of the base liquid. However, challenges remain that prevent their use in fuel cell vehicles. These include issues regarding the nanofluid stability and cleaning, erosion and abrasion, thermal conductivity, and electrical conductivity. In this review, we summarize the nanofluid applications in oil-fueled, electric, and fuel cell vehicles. Subsequently, we provide a comprehensive literature review of the challenges and future research directions of nanofluids as coolants in fuel cell vehicles. This review demonstrates the potential of nanofluids as an alternative thermal management system that can facilitate transition toward a low-carbon, energy-secure economy. It will serve as a reference for researchers to focus on new areas that could drive the field forward.

Electroviscous Dissipation in Aqueous Electrolyte Films with Overlapping Electric Double Layers

We use dynamic atomic force microscopy (AFM) to investigate the forces involved in squeezing out thin films of aqueous electrolyte between an AFM tip and silica substrates at variable pH and salt concentration. From amplitude and phase of the AFM signal we determine both conservative and dissipative components of the tip sample interaction forces. The measured dissipation is enhanced by up to a factor of 5 at tip-sample separations of ≈ one Debye length compared to the expectations based on classical hydrodynamic Reynolds damping with bulk viscosity. Calculating the surface charge density from the conservative forces using Derjaguin-Landau-Verwey-Overbeek (DLVO) theory in combination with a charge regulation boundary condition we find that the viscosity enhancement correlates with increasing surface charge density. We compare the observed viscosity enhancement with two competing continuum theory models: (i) electroviscous dissipation due to the electrophoretic flow driven by the streaming current that is generated upon squeezing out the counterions in the diffuse part of the electric double layer, and (ii) visco-electric enhancement of the local water viscosity caused by the strong electric fields within the electric double layer. While the visco-electric model correctly captures the qualitative trends observed in the experiments, a quantitative description of the data presumably requires more sophisticated simulations that include microscopic aspects of the distribution and mobility of ions in the Stern layer.

Solid-like heat transfer in confined liquids

The aim of this research is to identify possible mechanisms that govern heat transport at a solid-liquid interface using molecular dynamics. The study reveals that, unlike its bulk analogue, a liquid in a nanochannel sustains long-lived collective vibrations, phonons, which propagate over longer timescales and distances. The larger phonon mean free path in nanochannels is attributed to the greater structural order of the liquid atoms and to the larger liquid relaxation time-the time in which the liquid structure remains unchanged and solid-like. For channels of height less than 10 σ , long-range phonons are the dominant means of heat transfer in the directions parallel to the channel walls. The present findings are in agreement with experiments, which have observed significantly increased liquid relaxation times for the same range of channel heights. Finally, it is argued that confinement introduces additional transverse modes of vibration that also contribute to the thermal conductivity enhancement.

True non-contact atomic force microscopy imaging of heterogeneous biological samples in liquids: topography and material contrast

The present work analyses how the tip-sample interaction signals critically determine the operation of an Atomic Force Microscope (AFM) set-up immersed in liquid. On heterogeneous samples, the conservative tip-sample interaction may vary significantly from point to point - in particular from attractive to repulsive - rendering correct feedback very challenging. Lipid membranes prepared on a mica substrate are analyzed as reference samples which are locally heterogeneous (material contrast). The AFM set-up is operated dynamically at low oscillation amplitude and all available experimental data signals - the normal force, as well as the amplitude and frequency - are recorded simultaneously. From the analysis of how the dissipation (oscillation amplitude) and the conservative interaction (normal force and resonance frequency) vary with the tip-sample distance we conclude that dissipation is the only appropriate feedback source for stable and correct topographic imaging. The normal force and phase then carry information about the sample composition ("chemical contrast"). Dynamic AFM allows imaging in a non-contact regime where essentially no forces are applied, rendering dynamic AFM a truly non-invasive technique.

Nanofluidics of thin polymer films: linking the slip boundary condition at solid-liquid interfaces to macroscopic pattern formation and microscopic interfacial properties

If a thin liquid film is not stable, different rupture mechanisms can be observed causing characteristic film morphologies: spinodal dewetting and dewetting by nucleation of holes. This rupturing entails liquid flow and opens new possibilities to study microscopic phenomena. Here we use this process of dewetting to gain insight on the slip boundary condition at the solid-liquid interface. Having established hydrodynamic models that allow for the determination of the slip length in a dewetting experiment based on nucleation, we move on to the quantification and molecular description of slip effects in various systems. For the late stage of the dewetting process involving the Rayleigh-Plateau instability, several distinct droplet patterns can be observed. We describe the importance of slip in determining what pattern may be found. In order to control the slip length, we use polymeric liquids on different hydrophobic coatings of silicon wafers. We find that subtle changes in the coating can lead to large changes in the slip length. Thus, we gain insight into the question of how the structure of the substrate affects the slip length.

Interfacial slip on rough, patterned and soft surfaces: a review of experiments and simulations

Advancements in the fabrication of microfluidic and nanofluidic devices and the study of liquids in confined geometries rely on understanding the boundary conditions for the flow of liquids at solid surfaces. Over the past ten years, a large number of research groups have turned to investigating flow boundary conditions, and the occurrence of interfacial slip has become increasingly well-accepted and understood. While the dependence of slip on surface wettability is fairly well understood, the effect of other surface modifications that affect surface roughness, structure and compliance, on interfacial slip is still under intense investigation. In this paper we review investigations published in the past ten years on boundary conditions for flow on complex surfaces, by which we mean rough and structured surfaces, surfaces decorated with chemical patterns, grafted with polymer layers, with adsorbed nanobubbles, and superhydrophobic surfaces. The review is divided in two interconnected parts, the first dedicated to physical experiments and the second to computational experiments on interfacial slip of simple (Newtonian) liquids on these complex surfaces. Our work is intended as an entry-level review for researchers moving into the field of interfacial slip, and as an indication of outstanding problems that need to be addressed for the field to reach full maturity.

Interactions of hyaluronan layers with similarly charged surfaces: the effect of divalent cations

We used colloidal probe atomic force microscopy to measure the normal forces between the surface of a silica colloidal particle and a sparse layer of hyaluronan (hyaluronic acid, HA, MW ≈ 10(6) Da) covalently attached to a planar silica surface, both across pure water and following the addition of 1 mM MgCl2. It was found that in the absence of salt the HA layer repelled the colloidal silica surface during both approach and retraction. The addition of the MgCl2, however, changes the net force between the negatively charged HA layer and the opposing negatively charged silica surface from repulsion to adhesion. This interaction reversal is attributed to the bridging effect of the added Mg(2+) ions. Our results provide first direct force data to support earlier simulation and predictions that such divalent cations could bridge between negative charges on opposing surfaces, leading to an overall reversal of force from repulsion to attraction.

Reconciling slip measurements in symmetric and asymmetric systems

In the past decade, the slip of simple liquids on solid surfaces has been demonstrated by many groups. However, the slip of liquids on wettable surfaces is heavily debated. Using colloid probe atomic force microscopy (AFM), we found the slip length of di-n-octylphthalate in a symmetric wettable system (silica) to be around 11 nm, which raises the question of what the measured slip length in an asymmetric hydrophilic-hydrophobic system would be. To answer this question, we investigated liquid slip in one symmetric nonwettable system (hydrophobic DCDMS or OTS) and in one asymmetric hydrophilic (silica)-hydrophobic (DCDMS) system by the same method at driving velocities of between 10 and 80 μm/s. The slip results obtained from the three systems are in agreement with each other, and this comparison provides a means to self-assess the accuracy and reproducibility of the measured force curves and the fitted slip length in our systems. Furthermore, this method provides access to reliable values of the actual slip length on any investigated flat surface in an asymmetric system, avoiding the difficulty of preparing a symmetric probe/flat surface system in a colloid probe AFM force measurement.

Spectrum of position fluctuations of a Brownian particle bound in a harmonic trap near a plane wall

The spectrum of position fluctuations of a Brownian particle bound in a harmonic trap near a plane wall is calculated from an approximate result for the Fourier transform of the velocity autocorrelation function. Both a no-slip and a perfect slip boundary condition at the wall are considered. In both cases at low frequency the calculated spectrum differs markedly from recent experimental data. It is suggested that a partial slip boundary condition with a frequency-dependent slip coefficient may explain the experimental results.

Hydrodynamic force on a particle oscillating in a viscous fluid near a wall with dynamic partial-slip boundary condition

The hydrodynamic force on a particle oscillating in a viscous fluid near a wall with partial-slip boundary condition is studied on the basis of the linearized Navier-Stokes equations. Both incompressible and compressible fluids are considered. It is assumed that the slip length characterizing the partial-slip boundary condition depends on frequency. The consequences of this assumption for the spectrum of Brownian motion near a wall are investigated and compared with a recent experiment.

Non-DLVO silica interaction forces in NMP-water mixtures. I. A symmetric system

Despite the success of DLVO theory, there exist numerous examples of interactions that do not follow its predictions. One prominent example is the interaction between hydrophilic surfaces in mixtures of water with another polar, associating solvent. Interactions of such surfaces are still poorly understood yet play a key role in a wide variety of processes in nature, biology, and industry. The interaction forces between a silica sphere and a glass plate in N-methyl-2-pyrrolidone (NMP)-water binary mixtures were measured using the AFM technique. The interactions in pure NMP and pure water agreed qualitatively with DLVO theory. In contrast, the addition of NMP to water drastically altered the interactions, which no longer followed DLVO predictions. An unusually strong, long-range (50-80 nm), multistepped attractive force was observed on the approach of hydrophilic surfaces in the NMP concentration range of 30-50 vol %, where the adhesive pull-off force was also maximized. The maximum attractive force was observed at an NMP concentration near 30 vol %, consistent with the formation of a strong hydrogen-bonded complex between NMP and water near the solid surface. The analysis of force profiles, zeta potentials, solution viscosity, and contact angles suggests that attraction arises from the bridging of surface-adsorbed macrocluster layers known to form on hydrophilic surfaces in mixtures of associating liquids.

Depletion at solid/liquid interfaces: flowing hexadecane on functionalized surfaces

We present a neutron reflectivity study on interfaces in contact with flowing hexadecane, which is known to exhibit surface slip on functionalized solid surfaces. The single crystalline silicon substrates were either chemically cleaned Si(100) or Si(100) coated by octadecyl-trichlorosilane (OTS), which resulted in different interfacial energies. The liquid was sheared in situ and changes in reflectivity profiles were compared to the static case. For the OTS surface, the temperature dependence was also recorded. For both types of interfaces, density depletion of the liquid at the interface was observed. In the case of the bare Si substrate, shear load altered the structure of the depletion layer, whereas for the OTS covered surface no effect of shear was observed. Possible links between the depletion layer and surface slip are discussed. The results show that, in contrast to water, for hexadecane the enhancement of the depletion layer with temperature and interfacial energy reduces the amount of slip. Thus a density depletion cannot be the origin of surface slip in this system.

Nanoscale discontinuities at the boundary of flowing liquids: a look into structure

When downsizing technology, confinement and interface effects become enormously important. Shear imposes additional anisotropy on a liquid. This may induce inhomogeneities, which may have their origin close to the solid interface. For advancing the understanding of flow, information on structures on all length scales and in particular close to the solid interface is indispensable. Neutron scattering offers an excellent tool to contribute in this context. In this work, surface sensitive scattering techniques were used to resolve the structure of liquids under flow in the vicinity of a solid interface. Our results are summarized as follows. First, for a Newtonian liquid we report a depletion distance on the order of nanometers which is far too small to explain the amount of surface slip, on the order of micrometers, found by complementary techniques. Second, for a grafted polymer brush we find no entanglement-disentanglement transition under shear but the grafted film gets ripped off the surface. Third, by evaluating the local structure factor of a micellar solution close to the solid interface it turns out that the degree of order and local relaxation depends critically on the surface energy of the solid surface.

Improved in situ spring constant calibration for colloidal probe atomic force microscopy

In colloidal probe atomic force microscopy (AFM) surface forces cannot be measured without an accurate determination of the cantilever spring constant. The effective spring constant k depends upon the cantilever geometry and therefore should be measured in situ; additionally, k may be coupled to other measurement parameters. For example, colloidal probe AFM is frequently used to measure the slip length b at solid/liquid boundaries by comparing the measured hydrodynamic force with Vinogradova slip theory (V-theory). However, in this measurement k and b are coupled, hence, b cannot be accurately determined without knowing k to high precision. In this paper, a new in situ spring constant calibration method based upon the residuals, namely, the difference between experimental force-distance data and V-theory is presented and contrasted with two other popular spring constant determination methods. In this residuals calibration method, V-theory is fitted to the experimental force-distance data for a range of systematically varied spring constants where the only adjustable parameter in V-theory is the slip length b. The optimal spring constant k is that value where the residuals are symmetrically displaced about zero for all colloidal probe separations. This residual spring constant calibration method is demonstrated by studying three different liquids (n-decanol, n-hexadecane, and n-octane) and two different silane coated colloidal probe-silicon wafer systems (n-hexadecyltrichlorosilane and n-dodecyltrichlorosilane).

"Freezing" of nanoconfined fluids under an electric field

The problem of the solidlike transition of fluids in a nanogap has drawn much fundamental and practical attention. Here, we directly observed the disappearance of the fluidity of liquids confined within a gap with a surface separation of >10 nm under an EF in a ball-plate system, which is called the "freezing" of liquids. The flow of the nanoconfined liquid became very weak as the EF intensity was increased to a critical value and was correlated with the liquid polarity and the film thickness. It is deduced that the EF can induce more liquid molecules to be aligned to form more ordered layers in the nanogap.

Direct studies of liquid flows near solid surfaces by total internal reflection fluorescence cross-correlation spectroscopy

We present a new method to study flow of liquids near solid surface: Total internal reflection fluorescence cross-correlation spectroscopy (TIR-FCCS). Fluorescent tracers flowing with the liquid are excited by evanescent light, produced by epi-illumination through the periphery of a high numerical aperture oil-immersion objective. The time-resolved fluorescence intensity signals from two laterally shifted observation volumes, created by two confocal pinholes are independently measured. The cross-correlation of these signals provides information of the tracers' velocities. By changing the evanescent wave penetration depth, flow profiling at distances less than 200 nm from the interface can be performed. Due to the high sensitivity of the method fluorescent species with different size, down to single dye molecules can be used as tracers. We applied this method to study the flow of aqueous electrolyte solutions near a smooth hydrophilic surface and explored the effect of several important parameters, e.g. tracer size, ionic strength, and distance between the observation volumes.

Molecular momentum transport at fluid-solid interfaces in MEMS/NEMS: a review

This review is focused on molecular momentum transport at fluid-solid interfaces mainly related to microfluidics and nanofluidics in micro-/nano-electro-mechanical systems (MEMS/NEMS). This broad subject covers molecular dynamics behaviors, boundary conditions, molecular momentum accommodations, theoretical and phenomenological models in terms of gas-solid and liquid-solid interfaces affected by various physical factors, such as fluid and solid species, surface roughness, surface patterns, wettability, temperature, pressure, fluid viscosity and polarity. This review offers an overview of the major achievements, including experiments, theories and molecular dynamics simulations, in the field with particular emphasis on the effects on microfluidics and nanofluidics in nanoscience and nanotechnology. In Section 1 we present a brief introduction on the backgrounds, history and concepts. Sections 2 and 3 are focused on molecular momentum transport at gas-solid and liquid-solid interfaces, respectively. Summary and conclusions are finally presented in Section 4.

Colloidal interaction in ionic liquids: effects of ionic structures and surface chemistry on rheology of silica colloidal dispersions

To understand the important factors that dominate colloidal stability in ionic liquids (ILs), rheology of the dispersions of hydrophilic and hydrophobic silica nanoparticles were investigated in ILs with different ionic structures. The dispersion of hydrophilic silica nanoparticles in [BF(4)] anion-based ILs and in an IL containing a hydroxyl group, 1-(2-hydroxyethyl)-3-methylimidazolium bis(trifluoromethane sulfonyl)amide ([C(2)OHmim][NTf(2)]), showed an intriguing shear thickening response. Nonflocculation of the hydrophilic silica nanoparticles in the [BF(4)] anion-based ILs and in [C(2)OHmim][NTf(2)], where the interparticle electrostatic repulsion appears to be depressed, suggests that an IL-based steric hindrance or solvation force provides an effective repulsive barrier for the colloidal aggregation. On the other hand, the other dispersions presented shear thinning behavior with an increase in shear rates and gelled at relatively low particle concentrations. The elastic modulus (G') of the gels formed by the hydrophilic silica was correlated with the polarity scale, lambda(Cu), of the ILs, indicating that the silica-IL interactions, especially the silica-anion interactions, appear to affect the rheological behavior, even in flocculated systems. All the ILs used in this study can be solidified by the addition of hydrophobic silica particles. The rheological behavior of the silica colloidal dispersions was strongly affected by the ionic structure of the ILs and the surface structure of the silica particles.

Direct measurement of forces between cell-coating polymers and chiral crystal surfaces: the enantioselectivity of

The interactions of hyaluronan (HA), a high-molecular-weight linear polysaccharide present in many pericellular coatings, with different facets of chiral calcium tartrate (CT) crystal surfaces are investigated using a molecular force probe. Forces between {011} and {110} facets of (R,R) and (S,S) CT crystals and a HA-bearing surface have been measured in saturated CT solutions. It has been observed that hyaluronan binds most strongly to the {011} facet of the (R,R) crystal, compared with the other facets examined, which is consistent with earlier observations of the adhesion of HA-coated cells to chiral CT crystals. The variation of binding strength among the facets studied is tentatively attributed to the surface structure difference between the {011} and {110} facets as well as to the preferential matching of the local hyaluronan H-bond network to the -OH groups on the {011} facet of the (R,R) enantiomer.

Zeta potential and electroosmotic mobility in microfluidic devices fabricated from hydrophobic polymers: 2. Slip and interfacial water structure

We discuss the structure of water at hydrophobic interfaces from the standpoint of its impact on electrokinetic phenomena in microfluidic devices fabricated from hydrophobic polymers such as Teflon or Zeonor. Water structuring at hydrophobic interfaces has been described as a source of interfacial charge (see Part 1, this issue), and dewetting phenomena, whether via depletion layers or nanobubbles, contribute to slip and enhanced apparent electrokinetic potentials. Issues concerning the impact of hydrodynamic slip and the role of diffuse interfacial structures are discussed. These issues are coupled with each other and with interfacial charge concerns, providing challenges for measurements of individual parameters.

Role of solvation forces in the gelation of fumed silica–alcohol suspensions

Aggregation and gelation kinetics of fumed silica were investigated by altering the solvent-surface interactions. Native and surface-modified (hydrophobic) fumed silica particles were dispersed in short-chain linear alcohols. Based on the kinetics of aggregation and gelation, we show that the solvent-surface interactions have a tremendous impact on the bulk suspension properties. The gelation kinetics were qualitatively similar in all of the fumed silica-alcohol samples, and the gel times for all the alcohols were captured on a master curve requiring two parameters. The two parameters, the stability ratio and critical volume fraction, describe the two regimes of gelation. At low concentrations, gelation occurs due to aggregation of the particles diffusing over a potential barrier (15-25 kT). The rate of aggregation and time to gelation then scales with the stability ratio. At high particle loadings, gelation occurs at a critical volume fraction due to localization in a secondary minimum with a depth of 3-4 kT. These observations are supported by evidence of hydrogen bonding between the solvent and the particle, creating oscillatory solvation forces that govern the magnitude of these two parameters.

Effect of temperature on the nanomechanics of lipid bilayers studied by force spectroscopy

The effect of temperature on the nanomechanical response of supported lipid bilayers has been studied by force spectroscopy with atomic force microscopy. We have experimentally proved that the force needed to puncture the lipid bilayer (Fy) is temperature dependent. The quantitative measurement of the evolution of Fy with temperature has been related to the structural changes that the surface undergoes as observed through atomic force microscopy images. These studies were carried out with three different phosphatidylcholine bilayers with different main phase transition temperature (TM), namely, 1,2-dimyristoyl-sn-glycero-3-phosphocholine, 1,2-dipalmitoyl-sn-glycero-3-phosphocholine, and 2-dilauroyl-sn-glycero-3-phosphocholine. The solid-like phase shows a much higher Fy than the liquid-like phase, which also exhibits a jump in the force curve. Within the solid-like phase, Fy decreases as temperature is increased and suddenly drops as it approaches TM. Interestingly, a "well" in the Fy versus temperature plot occurs around TM, thus proving an "anomalous mechanical softening" around TM. Such mechanical softening has been predicted by experimental techniques and also by molecular dynamics simulations and interpreted in terms of water ordering around the phospholipid headgroups. Ion binding has been demonstrated to increase Fy, and its influence on both solid and liquid phases has also been discussed.

Boundary slip on smooth hydrophobic surfaces: intrinsic effects and possible artifacts

We report an accurate determination of the hydrodynamic boundary condition of simple liquids flowing on smooth hydrophobic surfaces using a dynamic surface force apparatus equipped with two independent subnanometer resolution sensors. The boundary slip observed is well defined and does not depend on the scale of investigation from one to several hundreds of nanometers, nor on shear rate up to 5 x 10(3)s(-1). The slip length of 20 nm is in good agreement with theory and numerical simulations concerning smooth nonwetting surfaces. These results disagree with previous data in the literature reporting very high boundary slip on similar systems. We discuss possible origins of large slip length on smooth hydrophobic surfaces due to their contamination by hydrophobic particles.

Molecular dynamics study of the effect of atomic roughness on the slip length at the fluid-solid boundary during shear flow

A systematic study into the effect of solid roughness on the slip boundary condition during shear flow is presented. Atomic roughness is modeled by varying the size and spacing between solid atoms at constant packing fraction while the interaction parameters and the thermodynamic state of the fluid are kept constant. It is shown that the fluid structure as manifest in the amplitude of the density oscillations increases with increasing smoothness of the surfaces. The fluid-solid slip length is shown to exhibit nonmonotonic behavior as the solid structure is varied from smooth to rough. Slip occurs for both smooth and rough surfaces, and stick occurs only for surfaces commensurate with the fluid.