[Show abstract][Hide abstract] ABSTRACT: We report on adsorption of lysozyme (LYS), ovalbumin (OVA) or ovotransferrin (OVT) on particles of a synthetic smectite (synthetic layered aluminosilicate). In our approach we used atomic force microscopy (AFM) and quartz crystal microbalance (QCM) to study the protein-smectite systems in water solutions at pH ranging from 4 to 9. The AFM provided insights into the adhesion forces of protein molecules to the smectite particles, while the QCM measurements yielded information about the amounts of the adsorbed proteins, changes in their structure and conditions of desorption. The binding of the proteins to the smectite surface was driven mainly by electrostatic interactions and hence properties of the adsorbed layers were controlled by pH. At high pH values a change in orientation of the adsorbed LYS molecules and a collapse or desorption of OVA layer were observed. Lowering pH to the value 4 caused LYS to desorb and swelling the adsorbed OVA. The stability of OVT-smectite complexes was found the lowest. OVT revealed a tendency to desorb from the smectite surface at all investigated pH. The minimum desorption rate was observed at pH close to the isoelectric point of the protein, which suggests that non-specific interactions between OVT and smectite particles significantly contribute to the stability of these complexes.
[Show abstract][Hide abstract] ABSTRACT: The hydrodynamic drainage force between a spherical silica particle and a soft, elastic polydimethylsiloxane surface was measured using the colloidal probe technique. The experimental force curves were compared to finite element simulations and an analytical model. The hydrodynamic repulsion decreased when the particle approached the soft surface as compared to a hard substrate. In contrast, when the particle was pulled away from the surface again, the attractive hydrodynamic force was increased.
Langmuir : the ACS journal of surfaces and colloids. 09/2014;
[Show abstract][Hide abstract] ABSTRACT: Spatular terminals on gecko's toe pads play an important role in directional adhesion and friction required for reversible attachment. Inspired by gecko's toe pad design, we study friction of polydimethylsiloxane (PDMS) micropillars terminated with asymmetric (spatular shaped) overhangs. Friction forces in the direction of and against the spatular end were evaluated and compared with friction forces on symmetric T-shaped pillars and pillars without overhangs. The shape of the friction curves and the values of the friction forces on spatula-terminated pillars were orientation-dependent. Kinetic friction forces were enhanced when shearing against the spatular end, while static friction was stronger in the direction towards spatular end. The overall friction force was higher in the direction against spatula end. The maximum value was limited by the mechanical stability of the overhangs during shear. The aspect ratio of the pillar had a strong influence on the magnitude of the friction force and its contribution surpassed and masked that of spatular tip for aspect ratios >2.
[Show abstract][Hide abstract] ABSTRACT: To analyze the photothermal ablation of polymers, we designed a temperature measurement setup based on spectral pyrometry. The setup allows to acquire 2D temperature distributions with 1 μm size and 1 μs time resolution and therefore the determination of the center temperature of a laser heating process. Finite element simulations were used to verify and understand the heat conversion and heat flow in the process. With this setup, the photothermal ablation of polystyrene, poly(α-methylstyrene), a polyimide and a triazene polymer was investigated. The thermal stability, the glass transition temperature Tg and the viscosity above Tg were governing the ablation process. Thermal decomposition for the applied laser pulse of about 10 μs started at temperatures similar to the start of decomposition in thermogravimetry. Furthermore, for polystyrene and poly(α-methylstyrene), both with a Tg in the range between room and decomposition temperature, ablation already occurred at temperatures well below the decomposition temperature, only at 30–40 K above Tg. The mechanism was photomechanical, i.e. a stress due to the thermal expansion of the polymer was responsible for ablation. Low molecular weight polymers showed differences in photomechanical ablation, corresponding to their lower Tg and lower viscosity above the glass transition. However, the difference in ablated volume was only significant at higher temperatures in the temperature regime for thermal decomposition at quasi-equilibrium time scales.
[Show abstract][Hide abstract] ABSTRACT: We demonstrate that disk-shaped steel meshes coated with a superamphiphobic layer are able to float not only on water but also on organic liquids. A coated disk-shaped steel mesh of 1 cm radius has a loading capacity of 17 mN in water and still remarkable 9 mN in n-hexadecane. Experimentally measured supporting forces and loading capacities agree well with theoretical predictions. Inspired by the giant water lily, pan-shaped "oil lilies" with even higher loading capacity and artificial oil striders carrying more than ten times their own weight are designed. Even after the artificial devices are fully immersed into different liquids, they show self-draining properties due to capillary forces.
Langmuir : the ACS journal of surfaces and colloids. 08/2014;
[Show abstract][Hide abstract] ABSTRACT: We investigated the equilibrium chain-exchange kinetics of amphiphilic diblock copolymer micelles, using a new method based on fluorescence correlation spectroscopy. The micelles were formed from polystyrene-block-poly[oligo(ethylene glycol) methyl ether methacrylate] (PS–POEGMA) in different solvents and studied at various temperatures. This linear-brush copolymer was chosen as a model system, forming micelles with short and bulky corona. Depending on the applied solvent, fast exchange could be observed even at temperatures well below the nominal glass transition of the core-forming PS block. The effect is caused by swelling of the core and allows extensive tuning of the chain-exchange rate by adding to the system minor amounts of good or bad solvent for the core block.
[Show abstract][Hide abstract] ABSTRACT: The kinetics of crystallization (heterogeneous vs homogeneous nucleation) and the relation to the local segmental dynamics are studied in a series of poly(ethylene oxide)-b-poly(ε-caprolactone) (PEO-b-PCL) diblock copolymers confined within self-ordered nanoporous alumina (AAO) by X-ray scattering, polarizing optical microscopy, differential scanning calorimetry and dielectric spectroscopy. In the bulk and for the more asymmetric copolymer, the minority phase (PEO) nucleates solely via homogeneous nucleation. When the same diblock copolymers reside inside AAO, nucleation of PEO is completely suppressed. The majority block (PCL) is also affected by confinement and crystallizes at lower temperatures via homogeneous nucleation. These findings can be discussed in terms of the proposed temperature vs curvature “phase diagram”. In this diagram the melt and glassy states are separated by the two nucleation regimes, heterogeneous and homogeneous at high and low temperatures, respectively. Homogeneous nucleation is controlled by the faster part of the distribution of PCL segmental relaxation which under confinement speeds-up.
[Show abstract][Hide abstract] ABSTRACT: Thiophene-based redox-active self-assembled monolayers (SAMs) were prepared on gold substrates. The alkanethiol derivatives of 1TPh-OC12SH and ETPh-OC12SH contain thiophene (1T) and 3,4-ethylenedioxythiophene (ET) units, respectively, with unprotected (nonsubstituted) thiophene α-carbons. PhETPh-OC12SH contains the ET unit, and all thiophene carbons are protected. Using these thiophene alkanethiol derivatives, we characterized the effect of thiophene carbon protection on the redox behavior of the thiophene SAMs by cyclic voltammetry. The formation of SAMs was confirmed by X-ray photoelectron spectroscopy and reflective IR. The IR peaks in the fingerprint region were assigned with the help of DFT calculations. Although 1TPh-OC12SH and ETPh-OC12SH SAMs lost their electrochemical activity during the first anodic scan, PhETPh-OC12SH SAMs are stable and maintain their electrochemical activity for at least 1200 redox cycles.
[Show abstract][Hide abstract] ABSTRACT: Water and oil repellent coatings-so-called superamphiphobic coatings-greatly reduce the interaction between a liquid and a solid. So far, only flat or weakly curved superhydrophobic and superamphiphobic surfaces have been designed. This raises the question of whether highly curved structures or microspheres are feasible. Therefore, we coated microspheres with a superamphiphobic layer and measured the force between the spheres and a liquid. A qualitatively different dependence of the adhesion force on the applied load for superamphiphobic and smooth spheres is detected. Furthermore, we demonstrate both experimentally and theoretically that superamphiphobicity fails below a critical particle radius, depending on topological details and type of liquid. Therefore, this study sets a fundamental physical limit to the application of superamphiphobic layers for small objects with high curvature.
[Show abstract][Hide abstract] ABSTRACT: Super liquid-repellent layers need to have a high impalement pressure and high contact angles, in particular a high apparent receding contact angle. Here, we demonstrate that to achieve both, the features constituting the layer should be as small as possible. Therefore, two models for super liquid-repellent layers are theoretically analyzed: A superhydrophobic layer consisting of an array of cylindrical micropillars and a superamphiphobic layer of an array of pillars of spheres. For the cylindrical micropillars a simple expression for the apparent receding contact angle is derived. It is based on a force balance rather than a thermodynamic approach. The model is supported by confocal microscope images of a water drop on an array of hydrophobic cylindrical pillars. The ratio of the width of a pillar w to the center-to-center spacing a is a primary factor in controlling the receding angle. Keeping the ratio w/a constant, the absolute size of surface features should be as small as possible, to maximize the impalement pressure.
Advances in Colloid and Interface Science. 01/2014;
[Show abstract][Hide abstract] ABSTRACT: The Bjerrum length is approached in a low polarity solvent by encapsulating, both, a borate anion and a phosphonium cation in a rigid lipophilic dendrimer shell. In addition the cation size is varied by 34-fold. We thus obtain superweak ions with potential applications in catalytic processes.
[Show abstract][Hide abstract] ABSTRACT: One way of measuring adhesion forces in fine powders is to place the particles on a surface, retract the surface with a high acceleration, and observe their detachment due to their inertia. To induce detachment of micrometer-sized particles, an acceleration in the order of 500 000 g is required. We developed a device in which such high acceleration is provided by a Hopkinson bar and measured via laser vibrometry. Using a Hopkinson bar, the fundamental limit of mechanically possible accelerations is reached, since higher values cause material failure. Particle detachment is detected by optical video microscopy. With subsequent automated data evaluation a statistical distribution of adhesion forces is obtained. To validate the method, adhesion forces for ensembles of single polystyrene and silica particles on a polystyrene coated steel surface were measured under ambient conditions. We were able to investigate more than 150 individual particles in one experiment and obtained adhesion values of particles in a diameter range of 3 - 13 µm. Measured adhesion forces of small particles agreed with values from colloidal probe measurements and theoretical predictions. However, we observe a stronger increase of adhesion for particles with a diameter larger than roughly 7 - 10 µm. We suggest that this discrepancy is caused by surface roughness and heterogeneity. Large particles adjust and find a stable position on the surface due to their inertia while small particles tend to remain at the position of first contact. The new device will be applicable to study a broad variety of different particle - surface - combinations on a routine basis, including strongly cohesive powders like pharmaceutical drugs for treatment of lung diseases.
[Show abstract][Hide abstract] ABSTRACT: We report the use of Y(3+)-substituted TiO2 (0.5%Y-TiO2) in solid-state mesoscopic solar cells, consisting of CH3NH3PbI3 as the light harvester and spiro-OMeTAD as the hole transport material. A power conversion efficiency of 11.2% under simulated AM 1.5 full sun illumination was measured. A 15% improvement in the short-circuit current density was obtained compared with pure TiO2, due to the effect of Y(3+) on the dimensions of perovskite nanoparticles formed on the semiconductor surface, showing that the surface modification of the semiconductor is an effective way to improve the light harvesters' morphology and electron transfer properties in the solid-state mesoscopic solar cells.
[Show abstract][Hide abstract] ABSTRACT: Herein we present a detailed study of the structure-function relationship in the organic photovoltaic (OPV) blend film composed of N,N'-bis(1-ethylpropyl)-perylene-3,4,9,10-tetracarboxylic diimide (EP-PDI) and the low energy gap copolymer of poly[4,8-bis-substituted-benzo[1,2-b:4,5-b']dithiophene-2,6-diyl-alt-4-substituted-thieno[3,4-b]thiophene-2,6-diyl] (PBDTTT-E-O). The hierarchical organization in the photoactive layers and in extruded fibres of PBDTTT-E-O:EP-PDI was studied by fluorescence optical microscopy, atomic force microscopy and wide angle X-Ray scattering (WAXS). WAXS revealed a nanophase separated structure where PBDTTT-E-O domains of 4.3 nm in size coexist with EP-PDI domains of 20 nm size. Thermal annealing results in an increase of the PBDTTT-E-O domains but it does not affect the size of the EP-PDI domains. Only the length of the EP-PDI columns in each domain is increased by thermal treatment. The photophysical characterization of the PBDTTT-E-O:EP-PDI layers and the electrical characterization of the corresponding OPV and unipolar carrier devices were performed. The quenching of the EP-PDI excimer luminescence is correlated with the photocurrent generation efficiency of the OPV devices. At high annealing temperatures the EP-PDI columnar length becomes larger than the previously reported diffusion length of the PDI excimer and fewer excimers dissociate at the EP-PDI/polymer interfaces, leading to reduced photocurrent generation. The charge transport properties of the PBDTTT-E-O:EP-PDI blend film were studied as a function of the active layer microstructure that was tuned by thermal treatment. Thermal processing increases electron mobility, but the poor connectivity of the EP-PDI domains keeps hole mobility six-times higher. In respect to the as-spun OPV device, a three-fold increase is found in the power conversion efficiency of the device annealed at 100C. The high surface roughness of the PBDTTT-E-O:EP-PDI photoactive layer impedes the efficient extraction of charges and a thin and smooth perylene-3,4,9,10-tetra¬carboxylic bis-benz¬imidazole overlayer is required for increasing the device performance to a PCE~1.7%. The inversion in the polarity of the device contacts resulted in an inverted device with PCE~1.9%.We provide rational guidelines for the accurate tuning of the layer microstructure in PDI-based photoactive layers of efficient OPV devices. Local disorder in the EP-PDI aggregates is essential i) for the optimum electron transport that is ensured by the efficient connectivity of the EP-PDI columns in adjacent EP-PDI domains and ii) for preventing the stabilization of the neutral photoexcitations in the EP-PDI domains in the form of slowly diffusive excimers. The high photocurrent generation efficiency achieved suggests the EP-PDI excimers are formed faster than the activation of triplet states and photocurrent losses are minimized.
[Show abstract][Hide abstract] ABSTRACT: We employ spontaneous Brillouin light scattering spectroscopy and detailed theoretical calculations to reveal and identify elastic excitations inside the band gap of hypersonic hybrid superlattices. Surface and cavity modes, their strength and anticrossing are unambiguously documented and fully controlled by layer thickness, elasticity, and sequence design. This new soft matter based superlattice platform allows facile engineering of the density of states and opens new pathways to tunable phoxonic crystals.
[Show abstract][Hide abstract] ABSTRACT: A combination of energy-filtered transmission electron microscopy (EFTEM) and thermally stimulated current (TSC) was used in order to investigate the effect of thermal annealing on the performance of an organic solar cell based on P3HT and PCBM as well-studied reference system. By probing specific elements EFTEM allows spectroscopic imaging with enhanced resolution compared to standard TEM techniques. Here, we applied EFTEM to cross sections of pristine and thermally annealed organic solar cells to probe the sulfur concentration as measure for the P3HT distribution within the photoactive layer, and thus resolve its phase separation. Thermal annealing for 10 min at 130 °C resulted in a reordering of P3HT and PCBM into better defined domains. The effect of the morphological changes on the presence of trap states was studied by TSC measurements. The TSC spectra recorded for the pristine and the thermally annealed solar cells showed three peak contributions respectively that could be assigned to the neat materials P3HT and PCBM as well as the blend. The pristine solar cell revealed a significantly lower density of trap states in the P3HT phase compared to the annealed solar cell. In combination with our EFTEM measurements we were able to attribute this finding to the increased number of P3HT rich domains present in the annealed device. Annealing of P3HT:PCBM solar cells has a beneficial impact not only on the local molecular order, but in particular on providing percolation paths for both charge carrier types.
The Journal of Physical Chemistry C 10/2013; · 4.84 Impact Factor