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When solute molecules in a liquid evaporate at the surface, concentration gradients can lead to surface tension gradients and provoke fluid convection at the interface, a phenomenon commonly known as the Marangoni effect. Here, we demonstrate that minute quantities of ethanol in concentrated sodium hydroxide solution can induce pronounced and long-lasting Marangoni flow upon evaporation at room temperature. By employing particle image velocimetry and gravimetric analysis, we show that the mean interfacial speed of the evaporating solution sensitively increases with the evaporation rate for ethanol concentrations lower than 0.5 mol %. Placing impermeable objects near the liquid-gas interface enforces steady concentration gradients, thereby promoting the formation of stationary flows. This allows for contact-free control of the flow pattern as well as its modification by altering the objects shape. Analysis of bulk flows reveals that the energy of evaporation in the case of stationary flows is converted to kinetic fluid energy with high efficiency, but reducing the sodium hydroxide concentration drastically suppresses the observed effect to the point where flows become entirely absent. Investigating the properties of concentrated sodium hydroxide solution suggests that ethanol dissolution in the bulk is strongly limited. At the surface, however, the co-solvent is efficiently stored, enabling rapid adsorption or desorption of the alcohol depending on its concentration in the adjacent gas phase. This facilitates the generation of large surface tension gradients and, in combination with the perpetual replenishment of the surface ethanol concentration by bulk convection, to the generation of long-lasting, self-sustaining flows. ispartof: Langmuir vol:39 issue:22 pages:7804-7810 ispartof: location:United States status: Published online

Current nucleation models propose manifold options for the formation of crystalline materials. Exploring and distinguishing between different crystallization pathways on the molecular level however remain a challenge, especially for complex porous materials. These usually consist of large unit cells with an ordered framework and pore components and often nucleate in complex, multiphasic synthesis media, restricting in-depth characterization. This work shows how aluminosilicate speciation during crystallization can be documented in detail in monophasic hydrated silicate ionic liquids (HSILs). The observations reveal that zeolites can form via supramolecular organization of ion-paired prenucleation clusters, consisting of aluminosilicate anions, ion-paired to alkali cations, and imply that zeolite crystallization from HSILs can be described within the spectrum of modern nucleation theory. doi: 10.1021/acs.chemmater.2c00418 Current nucleation models propose manifold options for the formation of crystalline materials. Exploring and distinguishing between different crystallization pathways on the molecular level however remain a challenge, especially for complex porous materials. These usually consist of large unit cells with an ordered framework and pore components and often nucleate in complex, multiphasic synthesis media, restricting in-depth characterization. This work shows how aluminosilicate speciation during crystallization can be documented in detail in monophasic hydrated silicate ionic liquids (HSILs). The observations reveal that zeolites can form via supramolecular organization of ion-paired prenucleation clusters, consisting of aluminosilicate anions, ion-paired to alkali cations, and imply that zeolite crystallization from HSILs can be described within the spectrum of modern nucleation theory. ispartof: Chemistry Of Materials vol:34 issue:16 pages:7139-7149 ispartof: location:United States status: Published online

Probing nucleation and growth of porous crystals at a molecular level remains a cumbersome experimental endeavour due to the complexity of the synthesis media involved. In particular, the study of zeolite formation is hindered as these typically form in multiphasic synthesis media, which restricts experimental access to crystallisation processes. Zeolite formation from single phasic hydrated silicate ionic liquids (HSiL) opens new possibilities. In this work, HSiL zeolite crystallisation is investigated

An online condition monitoring system based on the measurement of viscosity and density of liquids is presented and applied to three different measuring tasks relevant for industrial and automotive applications. One topic is oil characterization in hydraulic systems. It is shown that by measuring over varying temperature and pressure, additional physical properties can be made available for online condition monitoring, which are difficult to measure otherwise. These include, for example, the coefficient of thermal expansion and the bulk modulus, which is also related to the proportion of dissolved air. In the second application we investigate the efficiency of a passive oil defoamer and estimate the percentage of free air. Finally, the suitability of the measurement system for the determination of the diesel fraction in the engine oil as caused by the regeneration cycles of the diesel particulate filter is demonstrated.

We have devised a method that employs moving electrode electrochemical impedance spectroscopy to monitor the sedimentation of particles in conductive suspensions. In contrast to standard electrochemical cells with a fixed geometry, our cell has a flexible design with a movable counter electrode that allows precise adjustment of the electrode distance. Measuring the electrical impedance at various electrode spacings and utilizing the linear dependence of this function on the electrode displacement enables probing of a small section of the sample. This has considerable advantages when heterogenous liquids (e.g., suspensions) are to be analyzed. We applied our moving electrode approach to various test cases and obtained the following results: (i) We demonstrated by experiment that the bulk conductivity can be measured correctly even if particle sediments cover the electrode surface. (ii) We studied monodisperse suspensions of various compositions and investigated the effect of particle concentrations and size on conductivity. (iii) We monitored the particle sedimentation process and, by combining experimental and theoretical results, identified a correlation between the growing mass of the sedimentation layer and the impedance measured. The intended application of our approach is to monitor crystallization processes in ionic liquids for use in zeolite synthesis.

A possible application of self-propelling particles is the transport of microscopic cargo. Maximizing the collection and transport efficiency of particulate matter requires the area swept by the moving particle to be as large as possible. One such particle geometry are rods propelled perpendicular to their long axis, that act as “sweepers” for collecting particles. Here we report on the required Janus coating to achieve such motion, and on the dynamics of the collection and transport of microscopic cargo by sideways propelled Janus rods. ispartof: Soft Matter vol:17 issue:9 ispartof: location:England status: Published online

A measurement cell for the use of accurate conductivity measurements of corrosive ionic media is presented. Based on the concept of moving electrode electrochemical impedance spectroscopy, exceptional measurement accuracy is achieved in a large conductivity range. Extensive testing with corrosive ionic media demonstrated the robust operation of the cell under harsh chemical conditions, up to temperatures of 130 °C. The novel design allows monitoring small conductivity changes during chemical reactions in ionic media, for instance, zeolite formation from hydrated ionic liquids. ispartof: ACS SENSORS vol:5 issue:11 pages:3392-3397 ispartof: location:United States status: published

In this work, a resonant quartz tuning fork sensor based measurement system is employed to study the settlement of small particles in suspension as they pass the sensing element. The resonator features varying sensitivity to fluid parameter changes along its length, with the tip being most sensitive. The spatial averaging effect is analyzed theoretically and measured by passing the sensor through a fluid–fluid interface. Subsequently, the averaging function is used for particle distribution determination from recorded resonance frequency and quality factor measurements. The associated inverse problem is of Fredholm type and is solved numerically. Based on the system specifications, detection limits, required temperature stability and expected precision are assessed and particle distribution functions of defined test dispersions are measured.

We present the design of a microfluidic pumping device based on the effect of boundary layer driven acoustic streaming. One wall of the channel is made from a flexible material and hosts a flexural travelling wave, which induces a directed flow of the fluid inside the channel. A flexible printed circuit board was chosen as the oscillating wall, which makes the manufacturing process easy and could potentially enable the fabrication of low-cost disposable devices for the use in e.g. biomedical applications. Numerical studies based on an approach utilizing perturbation theory were conducted, where a comparison with the result of a time-dependent simulation of the full Navier-Stokes equations is provided. Based on the numerical analysis, a quadratic dependency of the flow velocity on the deflection amplitude of the membrane was identified. A ring-shaped membrane and channel were considered to be most practical for the experimental setup, where the idea and design process will be discussed. The flow velocity was measured using particle tracking velocimetry and the results show the same quadratic dependency of the flow velocity, which is in agreement with the theory.

We devised a method to monitor the sedimentation of particles from conductive solutions by means of moving electrode electrochemical impedance spectroscopy. Measuring impedance spectra at different electrode spacings allows an isolated view on the electrolyte properties while simultaneously studying sedimentation processes. We present an experimental setup and propose application, for instance, to monitor crystallization processes in ionic liquids for the use of zeolite synthesis.

We present a viscosity and density measurement principle based on diamagnetically stabilized levitation of a test object (a floating magnet) in the fluid to be characterized. Miniaturized resonant viscosity sensors are usually operated at relatively high frequencies, ranging from the lower kHz to tens of MHz, which utilize shear waves penetrating the liquid in the close vicinity of the vibrating surface thus only enabling the sensing of a small liquid film on the surface. With our approach, we reduce the resonance frequency which increases the penetration depth of the shear wave. Due to the freely levitated measurement body and the magnetic readout no mechanical or electrical connections into the measurement chamber are necessary, making the setup particularly useful for sterile, toxic or poisonous fluids. The design of the setup allows different modes of operation for the floater magnet, e.g., linear oscillations along the (vertical) z-axis and rotational oscillations around the (horizontal) x- or y-axis. In this contribution we analyze the rotational oscillation mode and present a theoretical model. Different additional features such as frequency tunability or the influence of the levitation height are examined. Measurements are discussed that prove the theoretical model and demonstrate the functionality of the principle. The use of rotatory oscillations instead of linear movements leads to a reduced measurement time and less influence of the boundaries.

We introduce a recently devised approach for viscosity and viscoelasticity measurement using a device based on the excitation of acoustic shear waves in fluids, which opens up opportunities for rheological applications in the low kilohertz range. Two in-plane plate resonators, which are driven and read out electromagnetically, are aligned in parallel and are separated by a well-defined gap filled with the fluid sample. The lower plate is actuated, generating a shear wave in the viscous or viscoelastic fluid. The response on the second side is recorded in a frequency range, where intrinsic resonances of this setup are observed. The coupling of the two resonators increases with viscosity, yielding a, for resonator viscosity sensors unprecedented, high viscosity measurement range (measured up to $17.6~\textrm {Pa} \cdot \textrm {s}$ ). Analytical modeling and experimental results are presented.

Electromechanical resonators are used as viscosity sensors where the effect of the fluid loading on the resonance parameters is evaluated. A complete numerical analysis is often unrealistic due to the large computational effort. We introduce a reduced-order fluid impedance model valid for a large class of geometries. The fluid structure interaction is characterized by three parameters, an effective volume, an effective interaction surface, and an effective length. For idealized geometries these parameters can be derived analytically. We derive scaling laws for rectangular cross sections which are relevant for quartz tuning fork resonators.

We present a microfluidic device for moving a fluid in a ring-shaped channel that utilizes boundary layer driven acoustic streaming as its driving mechanism. One wall of the channel is made of a flexible material and acts as the acoustic source. To help in the design process, a numerical study based on perturbation theory was conducted. A flexible printed circuit board is used as an actuator, which is excited magnetically using Lorentz forces to introduce a flexural travelling wave. The resulting streaming velocity is measured using particle tracking velocimetry and its dependence on the deflection amplitude is identified to verify the numerical results.

In this contribution, we present a symmetric arrangement of electromagnetically actuated, resonating plates for viscosity and density measurement. Regarding the oscillation mode, the design resembles a tuning fork and is based on previously introduced resonating plate designs. It exhibits similar sensitivity to viscosity as previously characterized single plate resonators, while having two shear modes at different frequencies, that can be both actuated with the same setup allowing a multi-frequency analysis of viscoelastic liquids. We introduce the new design and present the results of FE simulations in order to determine the associated eigenmodes. We investigate the effects of the actuation method on the sensors performance by actuating with two different magnetic field configurations and by trying different configurations of the actuation and readout circuit, from which we show that with a particular configuration, Q-factors of up to 5000 can be reached for the symmetric shear mode. From these results we then proceed to suggesting a simplified lumped mechanical oscillator model explaining the dependence of the resonator’s Q-factor while operated in air on induced eddy currents damping. Measurement results are presented which show the general dependence of the new design’s actuated modes, the anti-symmetric and symmetric mode, on viscosity and density. We apply two different models to our data: a newly developed generalized model and a simplified version of that model which better describes the observed relation of our measurements to the square root of the product viscosity density. We then proceed to perform an estimated error analysis on viscosity, density, and the square root of their products based on the above mentioned applied models. From this error analysis several conclusions are drawn, mainly that the symmetric mode is more accurate than the anti-symmetric one, the sensor is generally more accurate for lower viscosity liquids, and lastly that the square root of the viscosity density product is a more suited value for the description of the sensor's behavior than the viscosity or the density alone.

In this contribution a conceptual study for torsional oscillators, which are electromagnetically driven and read out, is presented. The aim is to experimentally investigate the basic feasibility of a torsional resonator with application to viscosity and mass density sensing in liquids. Such a device is particularly interesting as cylindrical, torsional resonators for fluid sensing applications are hardly reported but unlike many other devices, yield pure shear wave excitation in the liquid. The design of first conceptual demonstrators for measurements in air as well as in liquids and their benefits and disadvantages are discussed in detail. A closed form as well as a reduced order model and measurement results obtained with first demonstrators are presented.

The feasibility of using commercially available steel tuning forks for viscosity and mass density sensing is investigated. For this task, the tuning forks are electromagnetically driven and read out to record their frequency responses containing the fundamental resonant mode upon immersion in a sample liquid. Evaluated resonance frequencies and quality factors are then related to the liquids’ mass density and viscosity. The used electromagnetic actuation and readout principle allows that only the tuning fork which is placed in the center of a glass tube gets wetted with the liquid to be examined. All excitation and read out related structures and electronics are placed outside the glass tube and thus, are not affected or influenced by the liquid. A generalized model relating evaluated quality factors and resonance frequencies to viscosity and mass density is used to describe the tuning forks’ sensitivities and furthermore to estimate required stabilities of apparent quality factors and resonance frequencies to achieve measurement accuracies similar to those of laboratory instruments. It is shown that relative accuracies in the order of 1% in viscosity and 0.1 mg/cm3 in mass density are achievable.

The synovial fluid is found in joints of vertebrates and contains a high molecular weight polysaccharide (hyaluronan) acting as a modifier to ensure proper lubrication. Both, its concentration and molecular weight are subject to pathological alterations in certain diseases. Laboratory analysis of physical properties is difficult because of the limited amount of sample volume (usually below 100 μl). We present a sensor platform which requires volumes below 10 μl to measure the viscoelastic properties in the kilohertz range and the electrochemical impedance spectrum (EIS) from 102 to 106 Hz. Two electrodynamic acoustic shear wave transducers are used to measure both the single-sided viscoelastic shear impedance and the transmission of shear waves. The same elements are used as electrodes for measuring the EIS with a precision impedance analyser. The device is supposed to be used for clinical diagnosis and therapy tracking.

Acoustic streaming is a promising way for transporting fluid or particles suspended in a fluid in lab-on-a-chip devices and microfluidics in general. In literature usually water is considered as the working medium. In this contribution we present a framework based on perturbation theory, which implements viscoelastic fluid behavior to the acoustic streaming effect expanding the applicability beyond simple Newtonian fluids. Numerical simulations were carried out by finite element analysis. As an example we consider the steady flow generated in a channel by acoustic streaming using the upper-convected Maxwell (UCM) model to describe the viscoelastic behavior.

Acoustic streaming effects are usually treated using perturbation theory by writing the field variables as perturbation expansions utilizing a small scale parameter and solving the ensuing equation systems. When conducting numerical simulations, this approach can help reducing the computational effort drastically. In most cases, the zero-order velocity is assumed to be zero, which is an applicable assumption, if the fluid is considered to be at rest in the unperturbed state. If this assumption is not fulfilled, the zero-order velocity has to be taken into account in the calculation. In this contribution we investigate the influence of such a background velocity field on the resulting acoustic streaming flow by finite element analysis. The zero-order velocity is therefore included in a numerical simulation scheme, which is based on perturbation theory. The impact of the zero-order velocity is investigated in two examples taken from literature.

A piezoelectric tuning fork sensor is evaluated as a physical excitation and sensing device aiming at the detection of crystals and their nucleation behavior in joint fluids, as observed in gout. This device allows to study the crystallization tendency at varying temperatures or at increased uric acid concentrations as predominant in real human joints. In this work, a tuning fork resonator is characterized to enable spatially resolved viscosity and density measurements by calculating the mode shape of the vibration from resonance frequency measurements when immersed in test liquids.

We introduce an analysis platform for the viscoelastic properties of biological fluids such as the synovia found in articular joints. Small sample volumes are available for diagnostic purpose. Our approach uses a thickness shear mode quartz crystal integrated in an electrodynamically actuated spring structure. This combines the sensitivity of the quartz resonator operating at several overtones in the MHz range with previously introduced low kHz frequency multimode resonators. The latter is not only advantageous for characterizing viscoelastic fluids but is also used as an acoustic streaming actuator. Particulate matter in the sample thus can be resuspended and their settling is observed by tracking the resonance frequencies and Q-factors of both the quartz and electrodynamic resonator sensors.

Electrochemical impedance spectroscopy (EIS) was used to detect and investigate nucleation in silicalite-1 clear solutions. Although zeolite nucleation was previously assumed to be a step event, inducing a sharp discontinuity around a Si/OH– ratio of 1, complex bulk conductivity measurements at elevated temperatures reveal a gradual decay of conductivity with increased silicon concentrations. Inverse Laplace transformation of the complex conductivity allows the observation of the chemical exchange phenomena governing nanoaggregate formation. At low temperatures, the fast exchange between dissociated ions and ion pairs leads to a gradual decay of conductivity with an increasing silicon content. Upon heating, the exchange rate is slower and the residence time of ion pairs inside of the nanoaggregates is increasing, facilitating the crystallization process. This results in a bilinear chemical exchange and gives rise to the discontinuity at the Si/OH– ratio of 1, as observed by Fedeyko et al. EIS allows the o...

A generalized reduced order model for resonant viscosity and mass density sensors is presented and experimentally verified. Thereduced expressions for the resonance frequency and quality factor, respectively, are mathematically valid for in-plane oscillatingplates, oscillating spheres and laterally oscillating cylinders. However, as shown for measurements obtained with a tuning forkresonator with rectangular cross-section, the model can also be applied for resonating structures, for which closed form solutionsof the fluid structure interaction are not available. Benefits of the presented model are amongst others first, its simplicity, whichrequires no more than three calibration measurements for parameter identification and second, its general applicability for resonantmass density and viscosity sensors which furthermore facilitates the comparison of dierent resonant mass density and viscositysensors in terms of sensitivity and measurement noise propagation.

A resonant sensor for a fluid's viscosity and mass density, employing an out-of-plane vibrating U-shaped tungsten wire is presented. The motivation for such a design is based on four major aspects, which are: operation in the low kilohertz range, circular cross-section of the resonator, electrodynamic actuation and read-out by means of Lorentz forces and low cross-sensitivity of the device's resonance frequency to temperature. The setup is described in detail, an analytical model is presented and results from experiments using acetone–isopropanol and DI-water–glycerol solutions are discussed to show the sensor's performance.

In this contribution we discuss a generalized, reduced order model for resonant viscosity and mass density sensors which considers also the devices’ cross sensitivities to temperature. The applicability of the model is substantiated by experimental results from measurements obtained with a circular steel tuning fork in various liquids and temperatures. Advantages of this model are its simplicity, its general applicability for resonant mass density and viscosity sensors which furthermore facilitates the comparison of different sensors.

A resonating sensor to measure liquid density and viscosity is investigated. The sensor consists of a sample cell sealed by two polymer membranes carrying conductive paths for Lorentz force excitation and inductive readout. The setup is modeled by a semi-numeric method in the spectral domain. The model is used to reproduce and to explain experimentally demonstrated and previously published results. From the verified numerical model, simpler approximate equations are derived using energy methods. Furthermore, adverse effects which can impair the proper operation are discussed.

This contribution presents the analysis of an earlier proposed double membrane sensor for measuring mass density and rheological properties of liquids with respect to different driving modes. Concerning practical implementation the sensor mounting and the stability of the polyethylene foil, currently used as membrane material, are investigated. The sensor is based on two opposed membranes vibrating in parallel where a sample liquid is enclosed between the membranes. The excitation and read-out mechanisms of the membrane vibration are based on Lorentz forces induced in a static magnetic field. Each membrane carries three conductive paths for excitation, which can be separately connected to the excitation currents. This allows the excitation of the first and second modes of vibration and enables prestressing the second mode of oscillation. Analyzing the material-stability of the used polyethylene foil shows a strong long-term drift of the modulus of elasticity and an increase of internal damping with increasing temperature. Comparing the resonance frequency of the fundamental mode with earlier measurements achieved with the second mode of resonance indicates an increased sensitivity to density featuring a reasonably sustained quality factor for high viscosities. Thereby, the sensitivity can be adjusted by varying the distance between the membranes.

The response to extensional deformation of aqueous polymeric solutions depends strongly on the molecular weight and concentration of the solute. The molecular weight of hyaluronic acid in the synovial fluid is a critical parameter for its physiological condition. We propose a novel method for the characterization of these kinds of fluids using a fluidic two-phase system where a dispensed droplet is elongated by electrostatic forces from a voltage source of up to two kilovolts. This way a filament is formed comparable to laboratory capillary breakup extensional rheometers. High-speed imaging is used to investigate the rapid deformation process. Using a mathematical model for the dynamics of the filament evolution, the fluid parameters like the shear viscosity and elastic modulus are derived. The deformation rate can be controlled over a wide range by the magnitude and slope of the applied voltage.

We present a special operation mode of a levitating sphere viscometer which allows steady rotations of a levitated magnet around its vertical axis. In contrast to miniaturized oscillatory viscosity sensors, this mode of operation allows to obtain viscosity measurements more comparable to standard laboratory viscometers. Moreover, the measurement range is increased to higher viscosities, where no resonance peak is detectable in with oscillatory sensors. Using electromagnetical actuation and the readout, all fluids except ferrofluids are measurable. Due to the levitation and the contactless readout, only the measurement body has to be in contact with the fluid, all readout and actuation parts can be placed outside the measurement cell, which makes the setup particularly useful for, e.g., sterile, toxic or poisonous fluids.

A method for the fast calculation of vertical displacement fields, observed on thin piezoelectric layers of aluminium nitride (AlN) type deposited on n-doped silicon wafers, is shown. The basic findings are that the surface bending effect due to d 31 , d 32 is very strong but is largely compensated by d33 in the demonstrated case. The displacements are very sensitive on the piezoelectric properties and therefore the field computation must be very accurate in this respect. Furthermore, the anisotropy of the silicon wafer results in noticeable deviations when assuming a rotational symmetry for material properties. The demonstrated approach is based on an electromechanical Green's function (GF) and the assumption of a uniform charge distribution at the electrodes, which is shown to be acceptable when the thickness of the piezoelectric layer is much smaller than the lateral electrode dimensions. The infinite integral encountered in the rigorous field computations is approximated by a truncated series that can be calculated efficiently. The validity of the applied approximations is demonstrated by a comparison with results of a rigorous field calculation and with measurement results obtained with a laser Doppler vibrometer.

Hitherto zeolite formation is not fully understood. Although electrochemical impedance spectroscopy has proven to be a versatile tool for characterizing ionic solutions, it was never used for monitoring zeolite growth. We show here that EIS can quantitatively monitor zeolite formation, especially during crucial early steps where other methods fall short. crosscheck: This document is CrossCheck deposited related_data: Supplementary Information identifier: E. Breynaert (ORCID) copyright_licence: The Royal Society of Chemistry has an exclusive publication licence for this journal copyright_licence: This article is freely available. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence (CC BY 3.0) history: Received 3 February 2016; Accepted 19 March 2016; Accepted Manuscript published 21 March 2016; Advance Article published 29 March 2016; Version of Record published 7 April 2016 ispartof: Chemical Communications vol:52 issue:31 pages:5478-5481 ispartof: location:England status: published

A complete model of a resonating sensor for fluid density and viscosity is obtained by applying spectral domain techniques. It describes a sample chamber sealed by two polymer membranes which carry conductive paths for Lorentz force excitation and inductive readout. The model is used to reproduce and to explain experimentally demonstrated and previously published results. From the verified numerical model simpler approximate equations are derived and tested in a representative parameter range. In this paper the results of several contributions dealing with the described and with similar setups are summarized to show a comprehensive picture of the state of the art.

Miniaturized physical sensors for precise density and viscosity measurement are required as supplement or replacement for complex and expensive laboratory instruments. Utilizing miniature mechanical resonators as transducers, one can greatly reduce the necessary liquid volume, measurement time, and complexity of the experimental setup. In this contribution, we describe the development of a miniature, modular measurement setup for a large viscosity range. We devised and fabricated an inexpensive flow-through cell using milling technology of multiple PCB layers and Parafilm or Teflon sealing for easy assembly and disassembly. The mechanical resonator is designed as a suspended plate using electrodynamic Lorentz force excitation and movement induction detection. Quality factor and frequency shift of the resonator are evaluated and related to the density–viscosity product of different test liquids at multiple resonant modes of vibration at low operating frequencies for different temperatures and over a wide range of different viscosities.