Publications by authors named "Diethelm Johannsmann"

58 Publications

Kinetics of viscoelasticity in the electric double layer following steps in the electrode potential studied by a fast electrochemical quartz crystal microbalance (EQCM).

Analyst 2021 Apr 5;146(7):2160-2171. Epub 2021 Feb 5.

Institute of Physical Chemistry, Clausthal University of Technology, Arnold-Sommerfeld-Str. 4, D-38678 Clausthal-Zellerfeld, Germany.

Changes in the viscoelasticity of the electric double layer following steps in electrode potential were studied with an electrochemical quartz crystal microbalance (EQCM). The overtone scaling was the same as in gravimetry (-Δf/n≈ const with Δf the frequency shift and n the overtone order). Changes in half-bandwidth were smaller than changes in frequency. This Sauerbrey-type behaviour can be explained with either adsorption/desorption or with changes of the (Newtonian) viscosity of the diffuse double layer. While the QCM data alone cannot distinguish between these two processes, independent information supports the explanation in terms of double layer viscosity. Firstly, the magnitudes of the frequency response correlated with the expected changes of the viscosity-density product in the diffuse double layer. With regard to viscosity, these expectations are based on the viscosity B-coefficients as employed in the Jones-Dole equation. Expected changes in density were estimated from the densities of the respective salts. Secondly, the explanation in terms of liquid-like response matches the kinetic data. The response times of frequency and bandwidth were similar to the response times of the charge as determined with electrochemical impedance spectroscopy (EIS). Rearrangements in the Helmholtz layer should have been slower, given this layer's rigidity. Kinetic information obtained with a QCM can aid the understanding of processes at the electrode-electrolyte interface.
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http://dx.doi.org/10.1039/d0an01965hDOI Listing
April 2021

A Quartz Crystal Microbalance, Which Tracks Four Overtones in Parallel with a Time Resolution of 10 Milliseconds: Application to Inkjet Printing.

Sensors (Basel) 2020 Oct 20;20(20). Epub 2020 Oct 20.

Institute of Physical Chemistry, Clausthal University of Technology, Arnold-Sommerfeld-Str. 4, D-38678 Clausthal-Zellerfeld, Germany.

A quartz crystal microbalance (QCM) is described, which simultaneously determines resonance frequency and bandwidth on four different overtones. The time resolution is 10 milliseconds. This fast, multi-overtone QCM is based on multi-frequency lockin amplification. Synchronous interrogation of overtones is needed, when the sample changes quickly and when information on the sample is to be extracted from the comparison between overtones. The application example is thermal inkjet-printing. At impact, the resonance frequencies change over a time shorter than 10 milliseconds. There is a further increase in the contact area, evidenced by an increasing common prefactor to the shifts in frequency, Δ, and half-bandwidth, ΔΓ. The ratio ΔΓ/(-Δ), which quantifies the energy dissipated per time and unit area, decreases with time. Often, there is a fast initial decrease, lasting for about 100 milliseconds, followed by a slower decrease, persisting over the entire drying time (a few seconds). Fitting the overtone dependence of Δ() and ΔΓ() with power laws, one finds power-law exponents of about 1/2, characteristic of semi-infinite Newtonian liquids. The power-law exponents corresponding to Δ() slightly increase with time. The decrease of ΔΓ/(-Δ) and the increase of the exponents are explained by evaporation and formation of a solid film at the resonator surface.
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http://dx.doi.org/10.3390/s20205915DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7589769PMC
October 2020

Compressional-Wave Effects in the Operation of a Quartz Crystal Microbalance in Liquids:Dependence on Overtone Order.

Sensors (Basel) 2020 Apr 29;20(9). Epub 2020 Apr 29.

Institute of Physical Chemistry, Clausthal University of Technology, Arnold-Sommerfeld-Str. 4, D-38678 Clausthal-Zellerfeld, Germany, (F.M.).

The operation of the quartz crystal microbalance (QCM) in liquids is plagued by small flexural admixtures to the thickness-shear deformation. The resonator surface moves not only in the transverse direction, but also along the surface normal, thereby emitting compressional waves into the liquid. Using a simple analytical model and laser Doppler vibrometry, we show that the flexural admixtures are stronger on the fundamental mode than on the overtones. The normal amplitude of motion amounts to about 1% of the transverse motion on the fundamental mode. This ratio drops by a factor of two on the overtones. A similar dependence on overtone order is observed in experiments, where the resonator is immersed in a liquid and faces an opposite planar wall, the distance of which varies. Standing compressional waves occur at certain distances. The amplitudes of these are smaller on the overtones than on the fundamental mode. The findings can be rationalized with the tensor form of the small-load approximation.
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http://dx.doi.org/10.3390/s20092535DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7249204PMC
April 2020

Fast pH-mediated changes of the viscosity of protein solutions studied with a voltage-modulated quartz crystal microbalance.

Biointerphases 2020 03 24;15(2):021004. Epub 2020 Mar 24.

Institute of Physical Chemistry, Clausthal University of Technology, Arnold-Sommerfeld-Str. 4, D-38678 Clausthal-Zellerfeld, Germany.

An electrochemical quartz crystal microbalance is described, which achieves a time resolution down to 100 μs. Accumulation and averaging over a few hours bring the noise down to about 30 mHz. The application examples are pH-driven viscosity changes in albumin solutions. The pH was switched with the electrode potential. The characteristic response time is in the millisecond range. The focus is on experimental aspects as well as advantages and limitations of the technique.
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http://dx.doi.org/10.1116/1.5140619DOI Listing
March 2020

An ultrafast quartz crystal microbalance based on a frequency comb approach delivers sub-millisecond time resolution.

Rev Sci Instrum 2019 Nov;90(11):115108

Advanced Wave Sensors S.L., Calle Algepsers 24-1, 46988 Paterna, Valencia, Spain.

Quartz crystal microbalance with dissipation monitoring (QCMD) is a simple and versatile sensing technique with applications in a wide variety of academic and industrial fields, most notably electrochemistry, biophysics, quality control, and environmental monitoring. QCMD is limited by a relatively poor time resolution, which is of the order of seconds with conventional instrument designs at the noise level usually required. In this work, we present a design of an ultrafast QCMD with submillisecond time resolution. It is based on a frequency comb approach applied to a high-fundamental-frequency (HFF) resonator through a multifrequency lock-in amplifier. The combination allows us to reach data acquisition rates >10 kHz. We illustrate the method using a toy model of a glass sphere dropped on the resonator surfaces, bare or coated with liposomes, in liquid. We discuss some interesting features of the results obtained with the dropped spheres, such as bending of the HFF resonators due to the impact, sphere bouncing (or the absence of it), and contact aging.
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http://dx.doi.org/10.1063/1.5115979DOI Listing
November 2019

Environmental-Stress-Induced Increased Softness of Electroactive Biofilms, Determined with a Torsional Quartz Crystal Microbalance.

Anal Chem 2019 11 29;91(22):14476-14481. Epub 2019 Oct 29.

Institute of Physical Chemistry , Clausthal University of Technology , 38678 Clausthal-Zellerfeld , Germany.

Electroactive biofilms are intensely studied not only for energy conversion and electrosynthesis, but also as sensing systems. The electrical current produced by the layer is largely proportional to the rate of metabolism and therefore decreases when the biofilm experiences adverse environmental conditions. Acoustic measurements may complement this approach. The layer's softness can be inferred from shifts of resonance frequency and resonance bandwidth of a quartz crystal microbalance (QCM) contacting these layers. The layer's softness responds to the environment. Both negative potentials of the electrode (the equivalent of "suffocation") and lack of nutrient supply (the equivalent of "starvation") were studied. For comprehensive analysis, torsional resonators operating on three different modes of vibration are suited best. Such data can be fitted with a viscoelastic model, leading to a quantitative estimate of the shear modulus. On a more empirical level, one might also use the ratio of the shift in bandwidth to the negative shift in frequency as an indicator of stress. For ease of operation, one might even replace the torsional resonators with thickness-shear resonators.
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http://dx.doi.org/10.1021/acs.analchem.9b03204DOI Listing
November 2019

Gaseous "nanoprobes" for detecting gas-trapping environments in macroscopic films of vapor-deposited amorphous ice.

J Chem Phys 2019 Oct;151(13):134505

Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, United Kingdom.

Vapor-deposited amorphous ice, traditionally called amorphous solid water (ASW), is one of the most abundant materials in the universe and a prototypical material for studying physical vapor-deposition processes. Its complex nature arises from a strong tendency to form porous structures combined with complicated glass transition, relaxation, and desorption behavior. To gain further insights into the various gas-trapping environments that exist in ASW and hence its morphology, films in the 25-100 μm thickness range were codeposited with small amounts of gaseous "nanoprobes" including argon, methane, helium, and carbon dioxide. Upon heating in the 95-185 K temperature range, three distinct desorption processes are observed which we attribute to the gas desorption out of open cracks above 100 K, from internal voids that collapse due to the glass transition at ∼125 K and finally from fully matrix-isolated gas induced by the irreversible crystallization to stacking disordered ice (ice Isd) at ∼155 K. Nanoscale films of ASW have only displayed the latter desorption process which means that the first two desorption processes arise from the macroscopic dimensions of our ASW films. Baffling the flow of water vapor toward the deposition plate greatly reduces the first desorption feature, and hence the formation of cracks, but it significantly increases the amount of matrix-isolated gas. The complex nature in which ASW can trap gaseous species is thought to be relevant for a range of cosmological processes.
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http://dx.doi.org/10.1063/1.5113505DOI Listing
October 2019

Latex films with gradients in crosslink density created by small-molecule-based auto-stratification.

Eur Phys J E Soft Matter 2019 Feb 21;42(2):21. Epub 2019 Feb 21.

Institute of Physical Chemistry, Clausthal University of Technology, 38678, Clausthal-Zellerfeld, Germany.

A suitable balance of convective and diffusive transport of small molecules contained in the liquid phase of a drying latex film leads to auto-stratification and to functionally graded films. Differing from blends of latex particles, which may also experience drying-induced segregation, small molecules retain their mobility after the particles have touched and have formed an elastically coupled network. The use of a thickener, which turns the dispersion into a weak gel and prevents the free flow of particles, is compatible with this approach (and even advantageous). A problem with small molecules is fast diffusive equilibration of concentration differences. For this reason, composition gradients along the lateral direction, where the characteristic length scale is centimeters, are more easily achieved than gradients along the vertical. Addition of a thickener slows down the diffusion, which aids the development of gradients along the vertical. The application example chosen was the crosslinking agent adipic dihydrazide, ADH, which takes part in keto-hydrazide coupling. Its heterogeneous distribution produces a spatially variable crosslink-density in the dry film as evidenced by Raman microscopy. A side aspect of the work is an inward flow of serum, which is observed for high-T films. An explanation for this "anti-coffee-ring effect" --based on pore collapse driven by the polymer-water interfacial energy combined with finite polymer elasticity-- is proposed.
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http://dx.doi.org/10.1140/epje/i2019-11781-3DOI Listing
February 2019

Determination of the Shear Modulus of Thin Polymer Films with a Quartz Crystal Microbalance: Application to UV-Curing.

Anal Chem 2019 Jan 4;91(2):1595-1602. Epub 2019 Jan 4.

Institute of Physical Chemistry , Clausthal University of Technology , 38678 Clausthal-Zellerfeld , Germany.

The photoinduced curing of a light-sensitive varnish was followed, based on a change of the film's shear modulus, G, as determined with a quartz crystal microbalance (QCM). The film thickness was in the range of a few hundred nanometers. Both the storage modulus, G', and the loss modulus, G″, were obtained. The analysis is based on a perturbation calculation. The equations differ from the more commonly used set of equations derived from the small-load approximation and the acoustic multilayer formalism (sometimes termed Voigt-model). The discussion revisits assumptions, accuracy, and limits of the technique. Critical to the analysis is a knowledge of the thickness of the electrode underneath the film.
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http://dx.doi.org/10.1021/acs.analchem.8b05037DOI Listing
January 2019

Use of torsional resonators to monitor electroactive biofilms.

Biosens Bioelectron 2018 Jul 22;110:225-232. Epub 2018 Mar 22.

Institute of Physical Chemistry, Clausthal University of Technology, Arnold-Sommerfeld-Str. 4, D-38678 Clausthal-Zellerfeld, Germany; Institute of Environmental and Sustainable Chemistry, Technische Universität Braunschweig, Hagenring 30, D-38106 Braunschweig, Germany. Electronic address:

Whereas the study of interfaces and thin films with the quartz crystal microbalance (QCM) is well established, biofilms have proven to be a difficult subject for the QCM. The main problem is that the shear wave emanating from the resonator surface does not usually reach to the top of the sample. This problem can be solved with torsional resonators. These have a resonance frequency in the range of tens of kHz, which is much below the frequency of the thickness-shear QCMs. The depth of penetration of the shear wave is correspondingly larger. Data acquisition and data analysis can proceed in analogy to the conventional thickness-shear QCM. Torsional resonators may also be operated as electrochemical QCMs (EQCMs), meaning that a DC electrical potential may be applied to the active electrode and that shifts of frequency and bandwidth may be acquired in parallel to the electrical current. Here we report on the formation of mixed-culture biofilms dominated by the microorganism Geobacter anodireducens. The viscoelastic analysis evidences an increase in rigidity as the films grows. Potential sweeps on electroactive biofilms reveal a softening under negative potentials, that is, under conditions, where the layer's metabolism was slowed down by insufficient oxidative activity of the substrate. For comparison, biofilms were monitored in parallel with a conventional thickness-shear QCM.
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http://dx.doi.org/10.1016/j.bios.2018.03.046DOI Listing
July 2018

Characterizing protein-protein-interaction in high-concentration monoclonal antibody systems with the quartz crystal microbalance.

Phys Chem Chem Phys 2017 Dec;19(48):32698-32707

Boehringer Ingelheim Pharma GmbH and Co. KG, Protein Science, Birkendorfer Str. 65, 88397 Biberach/Riss, Germany.

Making use of a quartz crystal microbalance (QCM), concentrated solutions of therapeutic antibodies were studied with respect to their behavior under shear excitation with frequencies in the MHz range. At high protein concentration and neutral pH, viscoelastic behavior was found in the sense that the storage modulus, G', was nonzero. Fits of the frequency dependence of G'(ω) and G''(ω) (G'' being the loss modulus) using the Maxwell-model produced good agreement with the experimental data. The fit parameters were the relaxation time, τ, and the shear modulus at the inverse relaxation time, G* (at the "cross-over frequency" ω = 1/τ). The influence of two different pharmaceutical excipients (histidine and citrate) was studied at variable concentrations of the antibody and variable pH. In cases, where viscoelasticity was observed, G* was in the range of a few kPa, consistent with entropy-driven interactions. τ was small at low pH, where the antibody carries a positive charge. τ increased with increasing pH. The relaxation time τ was found to be correlated with other parameters quantifying protein-protein interactions, namely the steady shear viscosity (η), the second osmotic virial coefficient as determined with both self-interaction chromatography (B) and static light scattering (B), and the diffusion interaction parameter as determined with dynamic light scattering (k). While B and k describe protein-protein interactions in diluted samples, the QCM can be applied to concentrated solutions, thereby being sensitive to higher-order protein-protein interactions.
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http://dx.doi.org/10.1039/c7cp05711cDOI Listing
December 2017

Probing the electrical impedance of thin films on a quartz crystal microbalance (QCM), making use of frequency shifts and piezoelectric stiffening.

Rev Sci Instrum 2016 Nov;87(11):115002

Institute of Physical Chemistry, Clausthal University of Technology, Clausthal-Zellerfeld 38678, Germany.

Using a temperature-responsive polymer film as an example, it was shown that a conventional quartz crystal microbalance (QCM) can probe a sample's electrical properties in addition to its thickness and softness. The film's electrical impedance was accessed by alternating between the driving voltage being applied to the front electrode and the back electrode. The opposing electrode was grounded in both cases. In the first configuration, the electrical properties of the sample do have an influence on the resonance frequency because of piezoelectric stiffening. In the second, they do not. Using this scheme, it was monitored how the electrical impedance of a film composed of a mixture of poly-N-isopropylacrylamide and polyvinylalcohol changes when the film swells and deswells.
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http://dx.doi.org/10.1063/1.4966247DOI Listing
November 2016

Lipid phase behavior studied with a quartz crystal microbalance: A technique for biophysical studies with applications in screening.

J Chem Phys 2016 Nov;145(20):204904

Institute of Functional Interfaces (IFG), Karlsruhe Institute of Technology (KIT), Hermann-von Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany.

Quartz crystal microbalance (QCM) is emerging as a versatile tool for studying lipid phase behavior. The technique is attractive for fundamental biophysical studies as well applications because of its simplicity, flexibility, and ability to work with very small amounts of material crucial for biomedical studies. Further progress hinges on the understanding of the mechanism, by which a surface-acoustic technique such as QCM, senses lipid phase changes. Here, we use a custom-built instrument with improved sensitivity to investigate phase behavior in solid-supported lipid systems of different geometries (adsorbed liposomes and bilayers). We show that we can detect a model anesthetic (ethanol) through its effect on the lipid phase behavior. Further, through the analysis of the overtone dependence of the phase transition parameters, we show that hydrodynamic effects are important in the case of adsorbed liposomes, and viscoelasticity is significant in supported bilayers, while layer thickness changes make up the strongest contribution in both systems.
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http://dx.doi.org/10.1063/1.4968215DOI Listing
November 2016

Colloidal Stability and Magnetic Field-Induced Ordering of Magnetorheological Fluids Studied with a Quartz Crystal Microbalance.

Sensors (Basel) 2015 Dec 4;15(12):30443-56. Epub 2015 Dec 4.

Institute of Physical and Information Technologies, CSIC, C/Serrano, 144, Madrid 28006, Spain.

This work proposes the use of quartz crystal microbalances (QCMs) as a method to analyze and characterize magnetorheological (MR) fluids. QCM devices are sensitive to changes in mass, surface interactions, and viscoelastic properties of the medium contacting its surface. These features make the QCM suitable to study MR fluids and their response to variable environmental conditions. MR fluids change their structure and viscoelastic properties under the action of an external magnetic field, this change being determined by the particle volume fraction, the magnetic field strength, and the presence of thixotropic agents among other factors. In this work, the measurement of the resonance parameters (resonance frequency and dissipation factor) of a QCM are used to analyze the behavior of MR fluids in static conditions (that is, in the absence of external mechanical stresses). The influence of sedimentation under gravity and the application of magnetic fields on the shifts of resonance frequency and dissipation factor were measured and discussed in the frame of the coupled resonance produced by particles touching the QCM surface. Furthermore, the MR-fluid/QCM system has a great potential for the study of high-frequency contact mechanics because the translational and rotational stiffness of the link between the surface and the particles can be tuned by the magnetic field.
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http://dx.doi.org/10.3390/s151229808DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4721728PMC
December 2015

Coupled resonances allow studying the aging of adhesive contacts between a QCM surface and single, micrometer-sized particles.

Nanotechnology 2015 Dec 10;26(48):484001. Epub 2015 Nov 10.

Institute of Physical Chemistry, Clausthal University of Technology, D-38678 Clausthal-Zellerfeld, Germany.

Interparticle contacts and contacts between particles and surfaces are known to change over time. The contact area, the contact stiffness, and the contact strength usually increase as the contact ages. Contact aging is mostly driven by capillary forces, but also by plastic deformation. Making use of acoustic resonators, we have studied the stiffness of contacts between the surface of a quartz crystal microbalance (QCM) and individual, micrometer-sized particles adsorbed to the resonator surface. Studying single particles avoids ensemble-averaging. Central to the analysis is the coupled resonance, occurring when a surface-attached particle together with the link forms a resonator of its own. If the frequency of this second resonator comes close to one of the crystal's overtones, plots of shifts in resonance bandwidth versus overtone order display a resonance curve. This secondary resonance is caused by the coupling between the particle's resonance and the main resonance. One can read the frequency of the coupled resonance from this plot. Similarly, resonance curves are observed in plots of frequency and bandwidth versus time, if the contact stiffness varies smoothly with time. Because the coupled resonance is a characteristic feature, it is easily identified even in cases where frequency shifts of some other origin are superimposed onto the data. For the cases studied here, the links stiffened while they dried. Interestingly, the efficiency of coupling between the particle resonance and the main resonance decreased at the same time. This can be explained with an increase in the link's bending stiffness. The analysis highlights that a QCM experiment amounts to vibrational spectroscopy on surface-attached particles. Among the application examples is the adsorption and drying of a lycopodium spore. Clearly, the technique is also applicable to problems of bioadhesion.
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http://dx.doi.org/10.1088/0957-4484/26/48/484001DOI Listing
December 2015

Frequency Shifts of a Quartz Crystal Microbalance Calculated with the Frequency-Domain Lattice-Boltzmann Method: Application to Coupled Liquid Mass.

Anal Chem 2015 Jul 10;87(14):7476-84. Epub 2015 Jul 10.

‡Institute of Applied Mechanics, Clausthal University of Technology, 38678 Clausthal-Zellerfeld, Germany.

In recent years the quartz crystal microbalance (QCM) has seen an impressive evolution from a film-thickness monitor to a surface-analytical instrument with capabilities much beyond gravimetry. In particular, the instrument has often been applied to adsorbates from a liquid phase and, also, to samples with structure in the surface plane. In order to quantitatively predict frequency shifts induced by such samples from a model, one needs to compute the in-phase component of the area-averaged periodic tangential stress at the resonator surface. A method is described which performs this task, making use of a variant of the Lattice-Boltzmann (LB) method. The algorithm differs from the conventional LB method in that it deals with oscillatory flows and only covers linear hydrodynamics. The adsorption of small particles (mimicking proteins) was chosen as an example to test the performance. These samples are acoustically thin, which simplifies the calculations. Also, the material's finite compliance can be neglected in this limit. The simulations predict the amount of solvent trapped between neighboring particles, which contributes to the adsorbate's apparent mass. The unknown amount of hydrodynamically coupled liquid is a serious problem in the interpretation of QCM experiments. On an experimental level, the amount of trapped solvent can be estimated from the comparison of the optical layer thickness (determined with ellipsometry) and the acoustic layer thickness (determined with a QCM). Since the amount of trapped liquid decreases when neighboring particles aggregate into clusters, this analysis can lead to a statement on the degree of clustering. The LB-based simulations show, though, that the relation between the cluster geometry and the amount of trapped solvent is highly nontrivial. The details of the geometry do matter. The LB-based algorithm can calculate the amount of trapped solvent for user-specified particle shapes, orientations, interparticle distances, and also distributions thereof. It is an essential step in the quantitative interpretation of QCM results obtained on thin samples with in-plane structure.
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http://dx.doi.org/10.1021/acs.analchem.5b01912DOI Listing
July 2015

Stiffness of sphere-plate contacts at MHz frequencies: dependence on normal load, oscillation amplitude, and ambient medium.

Beilstein J Nanotechnol 2015 30;6:845-56. Epub 2015 Mar 30.

Clausthal University of Technology, Institute of Physical Chemistry, Arnold-Sommerfeld-Straße 4, 38678 Clausthal-Zellerfeld, Germany.

The stiffness of micron-sized sphere-plate contacts was studied by employing high frequency, tangential excitation of variable amplitude (0-20 nm). The contacts were established between glass spheres and the surface of a quartz crystal microbalance (QCM), where the resonator surface had been coated with either sputtered SiO2 or a spin-cast layer of poly(methyl methacrylate) (PMMA). The results from experiments undertaken in the dry state and in water are compared. Building on the shifts in the resonance frequency and resonance bandwidth, the instrument determines the real and the imaginary part of the contact stiffness, where the imaginary part quantifies dissipative processes. The method is closely analogous to related procedures in AFM-based metrology. The real part of the contact stiffness as a function of normal load can be fitted with the Johnson-Kendall-Roberts (JKR) model. The contact stiffness was found to increase in the presence of liquid water. This finding is tentatively explained by the rocking motion of the spheres, which couples to a squeeze flow of the water close to the contact. The loss tangent of the contact stiffness is on the order of 0.1, where the energy losses are associated with interfacial processes. At high amplitudes partial slip was found to occur. The apparent contact stiffness at large amplitude depends linearly on the amplitude, as predicted by the Cattaneo-Mindlin model. This finding is remarkable insofar, as the Cattaneo-Mindlin model assumes Coulomb friction inside the sliding region. Coulomb friction is typically viewed as a macroscopic concept, related to surface roughness. An alternative model (formulated by Savkoor), which assumes a constant frictional stress in the sliding zone independent of the normal pressure, is inconsistent with the experimental data. The apparent friction coefficients slightly increase with normal force, which can be explained by nanoroughness. In other words, contact splitting (i.e., a transport of shear stress across many small contacts, rather than a few large ones) can be exploited to reduce partial slip.
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http://dx.doi.org/10.3762/bjnano.6.87DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4419684PMC
May 2015

Coarsening of the pore network in drying latex films upon interparticle aggregation.

Langmuir 2014 Aug 29;30(31):9384-9. Epub 2014 Jul 29.

Institute of Physical Chemistry, Clausthal University of Technology , 38678 Clausthal-Zellerfeld, Germany.

The lateral drying front observed during film formation from latex dispersions with a Tg of the polymer around room temperature is composed of three three distinct lines. The lines are characterized by a decrease in turbidity, a renewed sharp increase in turbidity, and a more gradual decrease in turbidity at the end of what can be called a "halo". Microcracks with herringbone morphology develop at the first line, where the turbidity decreases. If macrocracks are present, these nucleate close to the end of the halo. At the line, where the turbidity sharply increases, one also observes an increase in stress birefringence. The substructure of the drying front is characteristically different from the structures described previously for films drying from hard particles. In particular, the renewed increase in turbidity cannot be explained as pore-opening, but rather is the consequence of a coarsening of the pore network after the particles jump into contact. A capillary instability sets in, by which the small pores collapse under the polymer/water interfacial energy, while the larger pores expand correspondingly. The instability (related to the Rayleigh instability of liquid jets) makes the films appear turbid. Also, the induced mechanical heterogeneity prevents straight macrocracks from penetrating into the halo because crack deflection and crack branching would result, which is energetically unfavorable.
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http://dx.doi.org/10.1021/la501354kDOI Listing
August 2014

Steady flows above a quartz crystal resonator driven at elevated amplitude.

Phys Rev E Stat Nonlin Soft Matter Phys 2014 Apr 23;89(4):043016. Epub 2014 Apr 23.

Institute of Physical Chemistry, Arnold-Sommerfeld-Strasse 4, D-38678 Clausthal-Zellerfeld, Germany.

A steady flow of liquid was observed above the surface of a quartz crystal microbalance under conditions where the oscillation amplitude exceeded 10 nm. The streaming flow occurs parallel to the displacement vector and is directed towards the center of the plate. It is expected to have applications in acoustic sensing, in microfluidics, and in micromechanics in a wider sense. The flow is caused by the nonlinear term in the Navier-Stokes equation, which can produce a nonzero time-averaged force from a periodic velocity field. Central to the explanation are the flexural admixtures to the resonator's mode of vibration. Unlike pressure-driven flows, the acoustically driven steady flow attains its maximum velocity at a distance of a few hundred nanometers from the surface. It is therefore efficient in breaking bonds between adsorbed particles and the resonator surface. As a side aspect, the flow pattern amounts to a diagnostic tool, which gives access to the pattern of vibration. In particular, it leads to an estimate of the magnitude of the flexural admixtures to the thickness-shear vibration.
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http://dx.doi.org/10.1103/PhysRevE.89.043016DOI Listing
April 2014

Partial slip in mesoscale contacts: dependence on contact size.

Phys Rev E Stat Nonlin Soft Matter Phys 2013 Sep 23;88(3):032408. Epub 2013 Sep 23.

Institute of Physical Chemistry, Clausthal University of Technology, 38678 Clausthal-Zellerfeld, Germany.

Using acoustic resonators, we have studied the occurrence and the magnitude of partial slip between glass spheres and polymer surfaces. The measurement relies on the shifts of resonance frequency and bandwidth, Δf and ΔΓ, induced by the contact as well as the dependence of Δf and ΔΓ on the amplitude of oscillation. One often finds a decrease of Δf at elevated amplitudes, which goes back to partial slip (also "microslip"). Building on two different models of partial slip, we derive the frequency-amplitude relation from the force-displacement relation. In accordance with both models, the bandwidth is found to increase with amplitude in the partial slip regime. For the highest amplitudes and largest spheres investigated, one observes a decrease of bandwidth with amplitude, which is interpreted as a transition to gross slip. Deviating from both models of partial slip, Δf is sometimes found to be independent of amplitude in the low-amplitude range. Constant Δf implies linear force-displacement relations. The critical amplitude for the onset of partial slip depends on the contact radius, where partial slip is more pronounced for larger contacts. This finding can be explained by a smooth stress profile at the edge of the contact with no singularity. The stress at the edge might be lowered by nanoscale roughness, by capillary forces, or by the inability of the two surfaces to reestablish a sticking contact at the turning point of the oscillation.
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http://dx.doi.org/10.1103/PhysRevE.88.032408DOI Listing
September 2013

Correlation between particle deformation kinetics and polymer interdiffusion kinetics in drying latex films.

Langmuir 2013 Sep 29;29(36):11317-21. Epub 2013 Aug 29.

Institute of Physical Chemistry, Clausthal University of Technology , D-38678 Clausthal-Zellerfeld, Germany.

Using an experimental setup which determines the turbidity of the sample and the efficiency of Förster resonance energy transfer (FRET) at the same time, we have correlated the particle deformation kinetics in a drying latex film, quantified by light scattering with the kinetics of polymer interdiffusion. Interdiffusion was quantified making use of energy transfer (FRET) between donor molecules and acceptor molecules, bound to polymer chains on different particles. When the chains cross the interparticle boundaries, the rate of energy transfer increases. The latex was prepared by miniemulsion polymerization. The amount of emulsifier employed during polymerization had a pronounced effect on the relative timing of interdiffusion and particle deformation. Increasing the amount of emulsifier delayed the onset of interdiffusion relative to the time when the film became transparent. This is mostly the consequence of a size effect, as opposed to surfactant acting as a barrier for transport.
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http://dx.doi.org/10.1021/la402121jDOI Listing
September 2013

Attractive forces on hard and soft colloidal objects located close to the surface of an acoustic-thickness shear resonator.

Phys Rev E Stat Nonlin Soft Matter Phys 2013 Jul 2;88(1):013001. Epub 2013 Jul 2.

Institute of Physical Chemistry, Clausthal University of Technology, 38678 Clausthal-Zellerfeld, Germany.

Colloidal particles located close to the surface of an acoustic thickness shear resonator feel an attractive steady force, which is induced by the high-frequency tangential motion of the resonator surface. The range of the force is about half the penetration depth of the transverse viscous wave, that is, half of the thickness of the Stokes boundary layer. For an oscillation amplitude of 10 nm and a particle radius of 100 nm, the depth of attractive potential well corresponds to about 3 times the thermal energy, k(B)T. The force therefore suffices to overcome Brownian motion.
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http://dx.doi.org/10.1103/PhysRevE.88.013001DOI Listing
July 2013

Addition of halloysite nanotubes prevents cracking in drying latex films.

Langmuir 2012 Jun 4;28(23):8674-80. Epub 2012 Jun 4.

Key Laboratory for Magnetism and Magnetic Materials of Ministry of Education, Lanzhou University, Lanzhou 730000, People's Republic of China.

Investigating the process of film drying from aqueous dispersions containing a polymer latex as well as halloysite nanotubes (HNTs), we found that composite films could be formed without cracking under conditions where films of the pure polymer would always crack. Scanning electron micrographs showed that the HNTs were well dispersed and, further, that the distribution of fiber orientations was close to isotropic. The pendulum hardness of films formed from acrylate dispersions strongly increased upon addition of the inorganic phase. The pencil hardness, on the other hand, was poor, which presumably goes back to insufficient coupling between the organic and the inorganic phase. All films were white in appearance. For fiber concentrations higher than 10 vol %, the final films were porous.
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http://dx.doi.org/10.1021/la3011597DOI Listing
June 2012

Probing colloid-substratum contact stiffness by acoustic sensing in a liquid phase.

Anal Chem 2012 May 3;84(10):4504-12. Epub 2012 May 3.

Department of Biomedical Engineering, University Medical Center Groningen and University of Groningen, Groningen, The Netherlands.

In a quartz crystal microbalance, particles adhering to a sensor crystal are perturbed around their equilibrium positions via thickness-shear vibrations at the crystal's fundamental frequency and overtones. The amount of adsorbed molecular mass is measured as a shift in resonance frequency. In inertial loading, frequency shifts are negative and proportional to the adsorbed mass, in contrast with "elastic loading", where particles adhere via small contact points. Elastic loading in air yields positive frequency shifts according to a coupled resonance model. We explore here the novel application of a coupled resonance model for colloidal particle adhesion in a liquid phase theoretically and demonstrate its applicability experimentally. Particles with different radii and in the absence and presence of ligand-receptor binding showed evidence of coupled resonance. By plotting the frequency shifts versus the quartz crystal microbalance with dissipation overtone number, frequencies of zero-crossing could be inferred, indicative of adhesive bond stiffness. As a novelty of the model, it points to a circular relation between bandwidth versus frequency shift, with radii indicative of bond stiffness. The model indicates that bond stiffness for bare silica particles adhering on a crystal surface is determined by attractive Lifshitz-van der Waals and ionic-strength-dependent, repulsive electrostatic forces. In the presence of ligand-receptor interactions, softer interfaces develop that yield stiffer bonds due to increased contact areas. In analogy with molecular vibrations, the radii of adhering particles strongly affect the resonance frequencies, while bond stiffness depends on environmental parameters to a larger degree than for molecular adsorption.
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http://dx.doi.org/10.1021/ac300366sDOI Listing
May 2012

Adsorbed liposome deformation studied with quartz crystal microbalance.

J Chem Phys 2012 Feb;136(8):084702

CIC biomaGUNE, Paseo Miramón182, San Sebastián 20009, Spain.

Deformation of surface-adsorbed liposomes is an important parameter that governs the kinetics of their transformations, but one that is very difficult to measure in the case of nm-size liposomes. We investigate the deformation of dimyristoyl phosphatidyl choline liposomes by quartz crystal microbalance (QCM) as a function of temperature and show that it follows the dependence of this lipid's bending modulus on temperature, as expected from theoretical considerations. To corroborate our approach, we model QCM response from adsorbed liposomes by explicitly considering their shape and mechanical properties.
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http://dx.doi.org/10.1063/1.3687351DOI Listing
February 2012

Hearing what you cannot see and visualizing what you hear: interpreting quartz crystal microbalance data from solvated interfaces.

Anal Chem 2011 Dec 17;83(23):8838-48. Epub 2011 Oct 17.

Biosurfaces Unit, CIC biomaGUNE, Paseo Miramon 182, 20009 Donostia-San Sebastian, Spain.

Over the last 2 decades, the quartz crystal microbalance (QCM or QCM-D) has emerged as a versatile tool for investigating soft and solvated interfaces between solid surfaces and bulk liquids because it can provide a wealth of information about key structural and functional parameters of these interfaces. In this Feature, we offer QCM users a set of guidelines for interpretation and quantitative analysis of QCM data based on a synthesis of well-established concepts rooted in rheological research of the last century and of new results obtained in the last several years.
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http://dx.doi.org/10.1021/ac201778hDOI Listing
December 2011

Hemispherical nanobubbles reduce interfacial slippage in simple liquids.

Phys Chem Chem Phys 2011 Oct 15;13(40):18015-22. Epub 2011 Sep 15.

Institute of Physical Chemistry, Clausthal University of Technology, Arnold-Sommerfeld-Str. 4, D-38678 Clausthal-Zellerfeld, Germany.

Using an electrochemical quartz crystal microbalance (EQCM), we have produced bubbles of nanoscopic size at the front electrode of an acoustic shear wave resonator. Nanobubbles are usually expected to increase the resonance frequency because they have a low density and, also, because a liquid slides easily at a liquid-air interface. However, the bubble-induced frequency shift in many cases was negative, which implies positive hydrodynamic thickness and reduced slippage. The explanation is based on Laplace pressure. Due to the bubbles' inherent stiffness, the space in-between neighboring bubbles may turn into an assembly of pockets which move with the underlying substrate in the same way as a solid film. If, first, the bubbles are so small that the Laplace pressure can overcome the viscous drag, and, second, the contact angle is in the range of 90°, the latter effect dominates. This interpretation was corroborated by a calculation using the finite element method (FEM). The argument as such is not limited to acoustic shear waves: hemispherical nanobubbles increase the surface drag in stationary flows in the same way.
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http://dx.doi.org/10.1039/c1cp21548eDOI Listing
October 2011

QCM study of the adsorption of polyelectrolyte covered mesoporous TiO2 nanocontainers on SAM modified Au surfaces.

J Colloid Interface Sci 2011 Oct 22;362(1):180-7. Epub 2011 Jun 22.

Institute of Technical and Macromolecular Chemistry, University of Paderborn, Warburger Str. 100, 33098 Paderborn, Germany.

Mesoporous TiO(2) nanocontainers (NCs) covered with polyelectrolyte multilayers were adsorbed on self-assembled monolayer (SAM) modified gold substrates at different values of pH and ionic strength. The adsorption process was followed in situ by means of a quartz crystal microbalance (QCM) and the morphology of the adsorbate was investigated by means of FE-SEM images taken of the substrates after each adsorption process. Deposition could be achieved if either the particles and the surface had opposite charge, or if the salt concentration was sufficiently high, reducing the repulsion between the spheres and the surface. In the latter case the adsorption kinetics could be explained in the context of the DLVO-theory. Using conditions of like charges, one has a means to control the speed of deposition by means of ionic strength. However, interparticle aggregation and cluster deposition on the surface were observed at high ionic strength. Such conditions have to be avoided to obtain a uniform deposition of separated nanocontainers on the surface.
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http://dx.doi.org/10.1016/j.jcis.2011.06.018DOI Listing
October 2011

Self-stratification during film formation from latex blends driven by differences in collective diffusivity.

Langmuir 2010 Aug;26(16):13162-7

Institute of Physical Chemistry, Clausthal University of Technology, Arnold-Sommerfeld-Str. 4, D-38678 Clausthal-Zellerfeld, Germany.

Coatings with vertical gradients in composition were produced by drying an aqueous polymer dispersion containing both charged and neutral particles. After drying, the neutral component was enriched at the film/air interface. The spontaneous vertical segregation between the two types of particles goes back to a difference in collective diffusivity. As the film dries, a layer enriched in polymer develops at the top. Due to their mutual repulsion, charged spheres escape from this layer more quickly than their neutral counterparts. Provided that the total time of drying is between the times of diffusion for the two types of particles (approximately H(0)(2)/D(c) with H(0) the initial film thickness and D(c) the collective diffusivity of the respective species), a concentration gradient persists after the film has turned dry. This effect can be used to create a functionally graded material (FGM) in a single coating step.
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http://dx.doi.org/10.1021/la101697rDOI Listing
August 2010

A novel method to measure diffusion coefficients in porous metal-organic frameworks.

Phys Chem Chem Phys 2010 Jul 7;12(28):8092-7. Epub 2010 Jun 7.

Ruhr-Universität Bochum, Lehrstuhl für Physikalische Chemie 1, 44780 Bochum, Germany.

We present a novel method to determine diffusion constants of small molecules within highly porous metal-organic frameworks (MOFs). The method is based on the recently proposed liquid-phase epitaxy (LPE) process to grow MOF thin films (SURMOFs) on appropriately functionalized substrates, in particular on organic surfaces exposed by thiolate-based self-assembled monolayers (SAMs). By applying the LPE-method to SAM-coated quartz crystals, the time-dependence of the mass-uptake of the MOF when exposing it to a gas is measured by a quartz-crystal microbalance (QCM). The homogenous nature of the SURMOFs together with their well-defined thickness allow to analyze the QCM-data using Fickian diffusion to yield the diffusion constant. We demonstrate the potential of this method for the case of pyridine diffusion within HKUST-1 (Cu(3)(BTC)(2)) MOF, for which the diffusion coefficient at room temperature is found to amount to 1.5 x 10(-19) m(2) s(-1). Assuming a Fickian diffusion and a hopping mechanism, we yield a binding energy of 0.78 eV of the pyridine to the Cu(2+) sites within the HKUST-1 MOF, a value in good agreement with the results of precise ab initio quantum chemistry calculations.
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http://dx.doi.org/10.1039/b927601gDOI Listing
July 2010