Gleb E. Yakubov

Surface roughness and hydrodynamic boundary conditions

We report results of investigations of a high-speed drainage of thin aqueous films squeezed between randomly nanorough surfaces. A significant decrease in the hydrodynamic resistance force as compared with that predicted by Taylors equation is observed. However, this reduction in force does not represent the slippage. The measured force is exactly the same as that between equivalent smooth surfaces obeying no-slip boundary conditions, but located at the intermediate position between peaks and valleys of asperities. The shift in hydrodynamic thickness is shown to be independent of the separation and/or shear rate. Our results disagree with previous literature data reporting very large and shear-dependent boundary slip for similar systems.

Influence of ionic strength changes on the structure of pre-adsorbed salivary films. A response of a natural multi-component layer

Salivary films coating oral surfaces are critically important for oral health. This study focuses on determining the underlying nature of this adsorbed film and how it responds to departures from physiological conditions due to changes in ionic strength. Under physiological conditions, it is found that pre-adsorbed in vitro salivary film on hydrophobic surfaces is present as a highly hydrated viscoelastic layer. We follow the evolution of this film in terms of its effective thickness, hydration and viscoelastic properties, as well as adsorbed mass of proteins, using complementary surface characterisation methods: a Surface Plasmon Resonance (SPR) and a Quartz Crystal Microbalance with Dissipation Monitoring (QCM-D). Our results support a heterogeneous model for the structure of the salivary film with an inner dense anchoring layer and an outer highly extended hydrated layer. Further swelling of the film was observed upon decreasing the salt concentration down to 1mM NaCl. However, upon exposure to deionised water, a collapse of the film occurs that was associated with the loss of water contained within the adsorbed layer. We suggest that the collapse in deionised water is driven by an onset of electrostatic attraction between different parts of the multi-component salivary film. It is anticipated that such changes could also occur when the oral cavity is exposed to food, beverage, oral care and pharmaceutical formulations where drastic changes to the structural integrity of the film is likely to have implications on oral health, sensory perception and product performance.

Dynamic effects on force measurements. I. Viscous drag on the atomic force microscope cantilever

When the atomic force microscope (AFM) is used for force measurements, the driving speed typically does not exceed a few microns per second. However, it is possible to perform the AFM force experiment at much higher speed. In this article, theoretical calculations and experimental measurements are used to show that in such a dynamic regime the AFM cantilever can be significantly deflected due to viscous drag force. This suggests that in general the force balance used in a surface force apparatus does not apply to the dynamic force measurements with an AFM. We develop a number of models that can be used to estimate the deflection caused by viscous drag on a cantilever in various experimental situations. As a result, the conditions when this effect can be minimized or even suppressed are specified. This opens up a number of new possibilities to apply the standard AFM technique for studying dynamic phenomena in a thin gap.

Forces between polystyrene surfaces in water-electrolyte solutions: Long-range attraction of two types?

A great deal of effort has recently been focused on the experimental studies of the phenomenon of long-ranged attraction between identically charged colloidal (polystyrene) particles immersed in an electrolyte. The theoretical validation suggested the need for revision of the existing and established colloidal theories assuming the observed attraction is of electrostatic origin. We, however, demonstrate that for a number of reasons (first of all hydrophobicity and roughnessof particles) the Derjaguin–Landau–Verwey–Overbeek (DLVO) behavior should not be expected for polystyrene surfaces. Indeed, the force measurements with an atomic force microscope-related set-up suggest that even within one pair of the interacting surfaces, attractive interaction of various types can be observed. There is usually a difference between the first approach and the later ones. The first approach is characterized by a short-range jump into a contact. Depending on conditions (electrolyte concentration, previous contacts of surfaces, etc.) there exists a late attraction of two types between polystyrene surfaces. The force of the first type is characterized by an abrupt jump from the maximum of a repulsive force, which is typically of longer-range than on the first approach. This is most likely due to submicroscopic bubbles trapped and/or formed due to previous contacts (and separation) of the surfaces. The attraction of the second type is weak and exponentially decaying.

Contact angles on hydrophobic microparticles at water–air and water–hexadecane interfaces

Advancing and receding contact angles on individual hydrophobic microspheres at the air-water and liquid-liquid, namely hexadecane-water, interfaces were determined. For this purpose, spherical silanated silica particles (R = 2.35 μm) and polystyrene particles of different radii (R = 1.80-4.38 μm) were attached to atomic force microscope (AFM) cantilevers. Polystyrene particles were sintered onto the AFM cantilevers to provide stability in organic solvent. Then the equilibrium position of the microsphere at the air-water interface of a drop or an air bubble was measured with an AFM-related set-up. From the equilibrium position the contact angles were calculated. Advancing and receding contact angles determined with silanated silica particles (Θa = 97° and Θr = 81°) agreed roughly with contact angles measured on similarly prepared planar surfaces (Θa = 95° and Θr = 83°). The apparent contact angles measured on polystyrene particles decreased with increasing particle size. This can be interpreted by assuming a negative line tension of the order of-0.3 μN.

Mitochondrial displacements in response to nanomechanical forces

Mechanical stress affects and regulates many aspects of the cell, including morphology, growth, differentiation, gene expression and apoptosis. In this study we show how mechanical stress perturbs the intracellular structures of the cell and induces mechanical responses. In order to correlate mechanical perturbations to cellular responses, we used a combined fluorescence‐atomic force microscope (AFM) to produce well defined nanomechanical perturbations of 10 nN while simultaneously tracking the real‐time motion of fluorescently labelled mitochondria in live cells. The spatial displacement of the organelles in response to applied loads demonstrates the highly dynamic mechanical response of mitochondria in fibroblast cells. The average displacement of all mitochondrial structures analysed showed an increase of ∼40%, post‐perturbation (∼160 nm in comparison to basal displacements of ∼110 nm). These results show that local forces can produce organelle displacements at locations far from the initial point of contact (up to ∼40 µm). In order to examine the role of the cytoskeleton in force transmission and its effect on mitochondrial displacements, both the actin and microtubule cytoskeleton were disrupted using Cytochalasin D and Nocodazole, respectively. Our results show that there is no significant change in mitochondrial displacement following indentation after such treatments. These results demonstrate the role of the cytoskeleton in force transmission through the cell and on mitochondrial displacements. In addition, it is suggested that care must be taken when performing mechanical experiments on living cells with the AFM, as these local mechanical perturbations may have significant structural and even biochemical effects on the cell. Copyright

Double-globular structure of porcine stomach mucin: a small-angle X-ray scattering study

We present evidence from small-angle X-ray scattering synchrotron experiments that porcine stomach mucin (MUC6) contains a double-globular comb structure. Analysis of the amino acid sequence of the peptide comb backbone indicates that the globular structure is determined by both the charge and hydrophobicity of the amino acids and the placement of the short hydrophilic carbohydrate side chains (approximately 2.5 nm). The double-globular structure is, thus, due to a block copolymer type hydrophobic polyampholyte charge instability in contrast to the random copolymer instabilities observed previously with synthetic polyelectrolytes (particularly polystyrene sulfonates). Careful filtering was required to exclude multimonomer aggregates from the X-ray measurements. A double Guinier analysis ( R g approximately 26 nm) and a double power law fit are consistent with two globules per chain in low salt conditions. The average radius of the globules is approximately 10 nm in salt- free condition (double Guinier fit) and the average distance of intrachain separation of the globules is 48 nm. The addition of salt causes a significant decrease in the radius of gyration (14 nm 100 mM NaCl) of the chains and is attributed to the contraction of the glycosylated peptide spacer between the two globules (the globular size continues to be approximately 10 nm and the globule separation is then 18 nm). Without salt, the scaling of the semidilute mesh size (xi) as a function of the mucin concentration (c) is xi approximately c (-0.45)compared with xi approximately c (-0.28) in high salt conditions, highlighting the globular nature of the chains. In contrast, hydrophilic flexible polyelectrolytes have a stronger concentration dependence of xi when excess salt is added.

What interactions drive the salivary mucosal pellicle formation

Graphical abstract

Understanding glycoprotein behaviours using Raman and Raman optical activity spectroscopies: characterising the entanglement induced conformational changes in oligosaccharide chains of mucin.

We illustrate the great potential of Raman and ROA spectroscopies for investigating the structure and organisation of glycoproteins and the complex matrices they can form. In combination these spectroscopic techniques are sensitive to changes in conformation revealing details of secondary and tertiary structures, probing hydrogen bonding interactions, as well as resolving side chain orientation and the absolute configuration of chiral substructures. To demonstrate this potential we have characterised the structural changes in a complex glycoprotein, mucin. Spectral changes were observed during the entanglement transition as the mucin concentration was increased. By applying two-dimensional correlation analysis (2DCos) to the ROA and Raman concentration-dependent spectral sets delicate transitions in mucin conformation could also be determined. From ~20-40 mg/ml conformational transitions assigned mainly to the sugar N-acetyl-d-galactosamine (GalNAc), which is the linking saccharide unit to the protein backbone, were monitored. Further changes in local oligosaccharide conformation above 40 mg/ml were also monitored, together with other structural transitions observed in the protein core, particularly β-structure formation. Consequently, these spectral techniques were shown to monitor the formation of transient entanglements formed by brush-brush interactions between oligosaccharide combs of mucin molecules identifying changes in both carbohydrate and protein moieties. This work clearly shows how these methods can be used to elucidate fresh insights into the complex behaviour of these large complex molecules.

Attractive Forces between Hydrophobic Solid Surfaces Measured by AFM on the First Approach in Salt Solutions and in the Presence of Dissolved Gases

Interfacial gas enrichment of dissolved gases (IGE) has been shown to cover hydrophobic solid surfaces in water. The atomic force microscopy (AFM) data has recently been supported by molecular dynamics simulation. It was demonstrated that IGE is responsible for the unexpected stability and large contact angle of gaseous nanobubbles at the hydrophobic solid-water interface. Here we provide further evidence of the significant effect of IGE on an attractive force between hydrophobic solid surfaces in water. The force in the presence of dissolved gas, i.e., in aerated and nonaerated NaCl solutions (up to 4 M), was measured by the AFM colloidal probe technique. The effect of nanobubble bridging on the attractive force was minimized or eliminated by measuring forces on the first approach of the AFM probe toward the flat hydrophobic surface and by using high salt concentrations to reduce gas solubility. Our results confirm the presence of three types of forces, two of which are long-range attractive forces of capillary bridging origin as caused by either surface nanobubbles or gap-induced cavitation. The third type is a short-range attractive force observed in the absence of interfacial nanobubbles that is attributed to the IGE in the form of a dense gas layer (DGL) at hydrophobic surfaces. Such a force was found to increase with increasing gas saturation and to decrease with decreasing gas solubility.