Surface-acoustic waves (SAWs) induce a time-averaged electroosmotic flow (EOF) in an aqueous electrolyte confined in a narrow channel that may exceed flow actuation via acoustic streaming. For a parallel-plate channel the EOF can be maximized by using two SAWs on both channel walls that have the same frequency but are phase-shifted by 180\ifmmode^\circ\else\textdegree\fi{}. The proposed actuation might be a viable alternative for driving liquid electrolytes through narrow channels, without the need for electric interconnects and electrodes.

The Faraday instability in a system of two conjugated immiscible liquid layers with disparate thicknesses is investigated. The top layer is relatively thick and undergoes short-wavelength instabilities, while the bottom layer is thin and undergoes long-wavelength instabilities. The two layers are coupled by the kinematic and dynamic relations at the interface. Through linear stability analysis, a lubrication effect, which significantly reduces the destabilization threshold, is identified. Especially when the vibration frequency is low, the lubrication effect is seen to influence the transition between the harmonic and subharmonic instability modes. It is studied how far the system with two layers can be approximated by a single-layer system with a Navier-slip boundary condition at the bottom. In corresponding experiments it is found that the time-periodic excitation of the system creates a steady-state deformation of the bottom layer. This indicates nonlinear dynamics of the system and the violation of reversibility. The excellent agreement between experimental and theoretical results for the onset of the instability underpins the validity of the linear stability analysis.

The stability and coupling of liquid films coating the walls of a parallel-plate channel and sheared by a pressure-driven gas flow along the channel center plane is studied. The films are susceptible to a long-wavelength instability. For sufficiently low Reynolds numbers and thick gas layers, the dynamic behavior is found to be described by two coupled nonlinear partial differential equations. A linear stability analysis is conducted under the condition that the material properties and the initial undisturbed liquid-film thicknesses are equal. The linear analysis is utilized to determine whether the interfaces are predominantly destabilized by the variations of the shear stress or by the pressure gradient acting upon them. The analysis of the weakly nonlinear equations performed for this case shows that instabilities corresponding to a vanishing Reynolds number are absent from the system. Moreover, for this configuration, the patterns emerging along the two interfaces are found to be identical in the long-time limit, implying that the films are fully synchronized. A different setup, where the liquid films have identical material properties but their undisturbed thicknesses differ, is studied numerically. The results show that, even for this configuration, the interfacial waves remain phase-synchronized and closely correlated for an extended period of time. These findings are particularly relevant for gaseous flow through narrow ducts with liquid-coated walls.

The effect of an axial temperature gradient on the flow profile and the induced streaming potential of a pressure-driven symmetric electrolyte in a slit channel is investigated. Based on the non-isothermal Nernst–Planck equations, as well as the Poisson equation in the lubrication approximation, expressions for the ion distribution in the electric double layer (EDL) are derived. It is found that thermophoretic ion motion and a temperature-dependent electrophoretic ion mobility increase the local EDL thickness with temperature, whereas a temperature-dependent permittivity shrinks the EDL. Within the Debye–Hückel approximation, the Navier–Stokes equation with the corresponding electric body force terms is solved. Analytical expressions for the flow profile and the induced (streaming) field under non-isothermal conditions are derived. It is shown that for such a situation the induced electric field is the linear superposition of at least seven individual contributions. For very wide channels, only the thermoelectric field typically present in bulk electrolytes when subjected to a temperature gradient (Soret equilibrium) as well as the conventional pressure-induced streaming field are of importance. Counterintuitively, for the latter, while still being affected by the temperature dependence of the dielectric permittivity and local salt concentration, the temperature dependencies of the viscosity, Fickian diffusion coefficients and ion electromobilities exactly cancel each other. For narrow channels, five additional contributions become relevant, which – similar to the Soret voltage – do not vanish in the case that the externally applied pressure gradient is removed. The first is caused by selective thermo-electromigration driven by the interplay between the temperature-dependent electrophoretic ion mobility and the interaction of the ions with the surface wall charge. This non-advective effect is at its maximum under extreme confinement. For channels whose widths are of the same order as the EDL thickness, four thermoosmotic effects become significant. Besides the well-known thermoosmosis due to the temperature dependence of the dielectric permittivity in the (extended) Korteweg–Helmholtz force, it is demonstrated that – by contrast to isothermal conditions – a thermal gradient renders the ion cloud in the EDL out of mechanical equilibrium. In this context it is shown that a thermophoretic ion motion (i.e. the intrinsic Soret effect of the ions) and a temperature-dependent ion electromobility as well as a temperature-dependent permittivity not only cause an axial gradient of the EDL potential, but simultaneously lead to a pressure of thermal origin, which sets the fluid into an advective motion. Corresponding phenomena were not previously discussed in the literature and may be interpreted as an apparent, thermally induced slip velocity within the EDL. Subsequently, the ion advection affiliated with such thermoosmotic flow may induce a thermoelectric field of a similar order of magnitude to that caused by more conventional thermal effects.

A method to shape thin polymer films into periodic array of lenses is presented. A liquid layer placed on top of a much thinner polymer film is exposed to a transverse temperature gradient. The upper liquid layer undergoes the short-wavelength Bénard-Marangoni instability, which leads to periodic shear stresses at the liquid-liquid interface and corresponding interfacial deformations. The structures formed are solidified by curing with ultraviolet light. In comparison to previously reported single-layer patterning techniques, the presented method achieves structures of very high horizontal planform symmetry (highly ordered structures). Furthermore, other shapes of technical interest can be easily fabricated by engineering the temperature distribution at the liquid-gas interface.

The electric field in an extended phase of a liquid electrolyte exposed to a temperature gradient is attributed to different thermophoretic mobilities of the ion species. As shown herein, such Soret-type ion thermodiffusion is not required to induce thermoelectricity even in the simplest electrolyte if it is confined between charged walls. The space charge of the electric double layer leads to selective ion diffusion driven by a temperature-dependent electrophoretic ion mobility, which-for narrow channels-may cause thermovoltages larger in magnitude than for the classical Soret equilibrium.

In this work, the shear-induced electrokinetic streaming potential present in free-surface electrolytic flows subjected to a gradient in surface tension is assessed. Firstly, for a Couette flow with fully resolved electric double layer (EDL), the streaming potential per surface stress as a function of the Debye parameter and ζ -potential is analyzed. By contrast to the Smoluchowski limit in pressure-driven channel flow, the shear-induced streaming potential vanishes for increasing Debye parameter (infinitely thin EDL), unless the free surface contains (induced) surface charge or the flow at the charged, solid wall is permitted to slip. Secondly, a technical realization of surface-tension-induced streaming is proposed, with surface stress acting on the free (slipping) surfaces of a micro-structured, superhydrophobic wall. The streaming potential is analyzed with respect to the slip parameter and surface charge. Finally, the surface tension is assumed to vary with temperature (thermocapillarity) or with surfactant concentration (destillocapillarity). The maximal thermal efficiency is derived and compared to the Carnot efficiency. For large thermal Marangoni number, the efficiency is severely limited by the large heat capacity of aqueous solvents. By contrast, destillocapillary flows may reach conversion efficiencies similar to pressure-driven flow.

The coupled dynamics of two conjugated liquid layers of disparate thicknesses, which coat a solid substrate and are subjected to a transverse temperature gradient, is investigated. The upper liquid layer evolves under the short-wavelength Bénard–Marangoni instability, whereas the lower, much thinner film undergoes a shear-driven long-wavelength deformation. Although the lubricating film should reduce the viscous stresses acting on the up to one hundred times thicker upper layer by only 10 %, it is found that the critical Marangoni number of marginal stability may be as low as if a stress-free boundary condition were applied at the bottom of the upper layer, i.e. much lower than the classical value of 79.6 known for a single film. Furthermore, it is experimentally verified that the deformation of the liquid–liquid interface, albeit small, has a non-negligible effect on the temperature distribution along the liquid–gas interface of the upper layer. This stabilizes the hexagonal pattern symmetry towards external disturbances and indicates a two-way coupling of the different layers. The experiments also demonstrate how convection patterns formed in a liquid film can be used to pattern a second conjugated film. The experimental findings are verified by a numerical model of the coupled layers.

A novel approach to calibrate the sensitivity of a differential thermometer, consisting of several thermocouples connected in series (thermopile), has been developed. The goal of this method is to increase the accuracy of small temperature difference measurements (Delta T <= 1 K), without invoking higher sensor complexity. To this end, a method to determine the optimal temperature difference employed during the differential measurement of thermoelectric sensitivities has been developed. This calibration temperature difference is found at the minimum of combined measurement and linearization error for a given mean temperature. The developed procedure is demonstrated in an illustrative example calibration of a nine-junction thermopile. For mean temperatures between -10 degrees C and +15 degrees C, the thermoelectric sensitivity was measured with an uncertainty of less than +/- 2%. Subsequently, temperature differences as low as 0.01 K can be resolved, while the thermometer used for the example calibration was accurate only to +/- 0.3 K. This and higher degrees of accuracy are required in certain research applications, for example to detect heat flux modulations in bifurcating fluidic systems. (C) 2015 Elsevier Inc. All rights reserved.

The effects of thermal coupling between two thin liquid layers separated by a gas layer are discussed. The liquid layers undergo long-wavelength instabilities driven by gravitational and thermocapillary stresses. To study the dynamics, both a linear stability analysis and a full numerical solution of the thin-film equations are performed. The results demonstrate that the stability properties of the combined system differs substantially from the case where both layers evolve independently from each other. Most prominently, oscillatory instabilities, not present in single-liquid layer configurations, may occur.

Environmental context Understanding the molecular composition and chemical transformations of organic aerosols during atmospheric aging is a major challenge in atmospheric chemistry. Ultra-high resolution mass spectrometry can provide detailed information on the molecular composition of organic aerosols. Aerosol samples collected in summer and winter at an urban site are characterised and compared in detail with respect to the elemental composition of their components, especially nitrogen- and sulfur-containing compounds, and are discussed with respect to atmospheric formation processes. Abstract Organic compounds are major constituents of atmospheric aerosol particles. The understanding of their chemical composition, their properties and reactivity are important for assessing aerosol effects upon both global climate change and human health. The composition of organic aerosols is poorly understood, mainly due to its highly complex chemical composition of several thousand compounds. There is currently no analytical technique available covering a wide enough chemical space to characterise this large number of organic compounds. In recent years ultra-high resolution mass spectrometry has been increasingly used to explore the chemical complexity in organic aerosols from laboratory and ambient samples. In the present study ambient particles <1 µm were collected at an urban site in Cambridge, UK, from August to December 2009. The water-soluble organic fraction of the filters was separated from inorganic ions following a procedure developed for humic-like substance isolation. Ultra-high resolution mass spectrometry analyses were performed in negative and positive polarity. Data in the mass range of m/z 50–350 were analysed for their elemental composition. Summer samples generally contained more components than winter samples. The large number of compounds was subdivided into groups according to their elemental composition. Up to 80 % of the peaks contain nitrogen and sulfur functional groups and only ~20 % of the compounds contain only C, H and O atoms. In summer the fraction of compounds with oxidised nitrogen and sulfur groups increases compared with winter indicating a photo-chemical formation route of these multifunctional compounds. In addition to oxidised nitrogen compounds a large number of amines was identified.

Several experimental groups have reported spontaneous formation of periodic pillar arrays in molten polymer nanofilms confined within closely spaced substrates held at different temperatures. These formations have been attributed to a radiation pressure instability caused by interface reflection of acoustic phonons. We demonstrate here how variations in thermocapillary stress at the air/polymer interface can produce significant periodic protrusions in any viscous film no matter how small the transverse thermal gradient. The linear stability analysis of the interface evolution equation corresponds to an extreme limit of Bénard–Marangoni flow peculiar to films of nanoscale dimensions—deformation amplitudes are small in comparison to the pillar spacing and hydrostatic forces are negligible. Finite element simulations of the full nonlinear equation provide estimates of the array pitch and growth rates beyond the linear regime. Results of the Lyapunov free energy as a function of time also confirm that pillarlike elongations are energetically preferred in nanofilms, in contrast to cellular instabilities in macroscopically thick films. If not mass limited, fluid elongations continue to grow until contact with the cooler substrate is achieved. These predictions should facilitate the fabrication of extended arrays for nanoscale optical, photonic, and biological applications.

Experiments by several groups during the past decade have shown that a molten polymer nanofilm subject to a large transverse thermal gradient undergoes spontaneous formation of periodic nanopillar arrays. The prevailing explanation is that coherent reflections of acoustic phonons within the film cause a periodic modulation of the radiation pressure which enhances pillar growth. By exploring a deformational instability of particular relevance to nanofilms, we demonstrate that thermocapillary forces play a crucial role in the formation process. Analytic and numerical predictions show good agreement with the pillar spacings obtained in experiment. Simulations of the interface equation further determine the rate of pillar growth of importance to technological applications.

Submitted for the DFD09 Meeting of The American Physical Society Study of Pattern Evolution in Nanoscale Viscous Films Subject to Pillar Instability by Thermocapillary Stresses SANDRA TROIAN, MATHIAS DIETZEL, California Institute of Technology, 1200 E. California Blvd., MC 128-95, Pasadena, CA 91125 — Nanofilms of molten polymer subject to a large transverse thermal gradient are known to undergo instability to arrays of elongated pillars separated by a few microns. The simplicity with which such large area patterns can be formed offers a new type of resistless, non-contact lithography in which pillar spacing and amplitude can be controlled by plate separation distance, initial film thickness and thermal gradient. Here, we investigate the evolution of pillar formation from the linear to non-linear regime by examining an unexplored limit of Benard instability accessible to films of nanoscale dimensions. Given the small plate spacings used in experiment, we examine the influence of initial conditions on the wave number selection process. For intermediate values of plate separation, the pillar spacing is well predicted by linear stability analysis and unaffected by the type of initial disturbance applied. For larger plate separation, the pillar arrays evolve toward hexagonal symmetry while subsequent depletion of fluid in the interstitial regions leads to formation of subharmonic protrusions. These bifurcations are suggestive of cascade processes observed in other thin film evolution equations. Sandra Troian California Institute of Technology, 1200 E. California Blvd., MC 128-95, Pasadena, CA 91125 Date submitted: 11 Aug 2009 Electronic form version 1.4