Manoj K. Chaudhury

The role of van der Waals forces and hydrogen bonds in “hydrophobic interactions” between biopolymers and low energy surfaces

Abstract The thermodynamic nature of interfaces and of adhesion is reexamined in the light of the Lifshitz theory of the forces acting across condensed phases. A new term is proposed, γLW, which consists of the sum of the terms heretofore ascribed to London, Debye, and Keesom forces, LW referring to Lifshitz-van der Waals. This term and a second term γSR account for the entirety of two-phase interactions in nonionic systems; SR refers to short range forces. This new analysis of forces is of value in explaining some important biological and other phenomena. The rather strong attachment of hydrophilic proteins, e.g., human serum albumin (HSA) and human immunoglobulin G (IgG), to low energy surfaces, e.g., polytetrafluoroethylene (PTFE) and polystyrene (PST), while immersed in H2O, cannot be ascribed solely to Lifshitz-van der Waals forces (LW). For instance, it can be shown that the LW interaction would give rise to a repulsion between HSA and PTFE. The short range (SR) interactions, e.g., between H2O and HSA, are due to H-bonds, which cannot directly account for interactions with PTFE. However, the combined SR interfacial tensions between the H-bonding liquid, the biopolymer, and the low energy surface still result in a strong attraction between PTFE and HSA, immersed in H2O. This is analogous to the behavior of a liquid-air interface (where the fact that the direct interaction between a given solute and air is zero does not preclude the solute from being preferentially attracted to the interface). This SR attraction (minus the LW repulsion) between HSA and PTFE, in H2O, is of the same order of magnitude as the adsorption energy derived from the Langmuir isotherm obtained for this system. Analogous results are found with IgG and PTFE, and also with HSA and IgG, with PST. Desorption patterns (obtained by changing the γLW and γSR of the liquid medium) allow an insight into the degree of local dehydration (or “denaturation”) of adsorbed proteins under various conditions. It is suggested that the term interfacial forces more aptly describes the underlying mechanism than “hydrophobic interactions.”

Macroscopic Evidence of the Effect of Interfacial Slippage on Adhesion

The adhesion strengths of a viscoelastic adhesive were measured on various substrates that were prepared by grafting silanes bearing organic functional groups to silicon wafers. Conventional theories predict that adhesion should be proportional to the surface free energy of the substrate; but adhesion on a fluorocarbon surface was significantly greater than on some of the hydrocarbon surfaces, although the fluorocarbon surface has the lowest surface free energy. This result could be explained by invoking a model of adhesion based on the slippage of the adhesive at the interface.

Estimation of the polar parameters of the surface tension of liquids by contact angle measurements on gels

Abstract In a previous paper it was shown that negative interfacial tensions between predominantly monopolar surfaces (i.e., surfaces with mainly H-acceptor properties) and polar liquids are real phenomena. Such negative interfacial tensions do however decay rapidly. For miscible liquids, the decay of the interface is, in general, so rapid that it practically excludes measurement of interfacial tension. However, if one liquid is present in the form of a gel, and if the other liquid is placed as a drop upon the gel, there is often enough time to measure contact angles. This may be done at various concentrations of the liquid encased in the gel, and an extrapolation made to zero concentration of the gelling agent. With this method we found the existence of negative interfacial tensions at liquid/liquid interfaces.

Correlation Between Surface Free Energy and Surface Constitution

Self-assembled monolayers (SAMs) of alkylsiloxanes on elastomeric PDMS (polydimethylsiloxane) were used as model systems to study interactions between surfaces. Surface free energies (γsv) of these chemically modified surfaces were estimated by measuring the deformations that resulted from the contact between small semispherical lenses and flat sheets of the elastomer under controlled loads. The measured surface free energies correlated with the surface chemical compositions of the SAMs and were commensurate with the values estimated from the measurements of contact angles. This study provides direct experimental evidence for the validity of estimates of the surface free energies of low-energy solids obtained from contact angles.

Determination of contact angles and pore sizes of porous media by column and thin layer wicking

A simple method is described for the determination of contact angles (0) on powdered materials such as clay particles. This is done by depositing the particles from a liquid suspension onto glass slides, by sedimentation, followed by drying. The dried thin-layer plates are then subjected to wicking in a number of liquids using the Washhurn equation to determine cos . However, one other unknown in the Washburn equation, i.e. the average interstitial pore radius R, must first be determined. This is done by wicking with low-energy spreading liquids, such as alkanes. It could be shown with spherical monosized polymer particles, as well as with clay particles, that spreading liquids pre-wet the surfaces of the particles over which they subsequently spread. Thus, it can be demonstrated that spreading coefficients, in the sense of Harkins, play no role in this type of spreading and cos 0 equals unity in the Washburn equation for all values of γ1, for all spreading liquids (L). Results were obtained by thin layer...

Biologically inspired crack trapping for enhanced adhesion

We present a synthetic adaptation of the fibrillar adhesion surfaces found in nature. The structure consists of protruding fibrils topped by a thin plate and shows an experimentally measured enhancement in adhesion energy of up to a factor of 9 over a flat control. Additionally, this structure solves the robustness problems of previous mimic structures and has preferred contact properties (i.e., a large surface area and a highly compliant structure). We show that this geometry enhances adhesion because of its ability to trap interfacial cracks in highly compliant contact regimes between successive fibril detachments. This results in the requirement that the externally supplied energy release rate for interfacial separation be greater than the intrinsic work of adhesion, in a manner analogous to lattice trapping of cracks in crystalline solids.

Roles of discontinuities in bio-inspired adhesive pads

Morphological intricacies of the biological attachment pads generate considerable interest owing to their remarkable ability to control adhesion to various surfaces. Motivated by the adhesive microstructures of insects, we examine the behaviour of adhesion and crack propagation in patterned adhesive films. These films are made of silicone elastomers that were patterned with lateral, longitudinal or crosswise incisions from which a thin silanized glass plate was removed in a displacement-controlled peel experiment. The behaviours of crack propagation on these patterned adhesive films are controlled by simple incision patterns, their depths and spacing. With the crosswise incisions, significant enhancement (×10–20) of fracture energy has been achieved. These findings point towards an important mechanism by which of biological organisms might enhance adhesion, and provide a simple design principle for manipulating the interfacial fracture in a variety of artificial attachment devices.

Interfacial interaction between low-energy surfaces

Abstract This review is concerned primarily with the correlation between the interfacial interactions and the constitutive properties of low-energy organic surfaces. It starts with a discussion on the estimation of the surface free energy of organic solids from contact angles, followed by a review of the surface energetics and adhesion. The experimental measurements of surface free energy, in most cases, are themselves dependent upon the specific models of interfacial energetics and therefore are indirect. A direct method of estimating adhesion and surface free energy is based on contact mechanics, which measures the deformation produced on contacting elastic semispheres under the influence of surface forces and external loads, Since the equilibrium is described by the balance of the elastic and surface forces of the system, the load-deformation data can be translated directly to estimate the adhesion and surface free energies. In most cases however, the contact deformations obtained from the loading and unloading cycles exhibit hysteresis, which are sensitive to the structure and chemical compositions of the interfaces. For non-hysteretic systems, the surface free energies obtained from these contact deformations compare well with the values obtained from contact angles. The application of this method to the studies of dispersion and hydrogen-bonding interaction is reviewed.

The influence of elastic modulus and thickness on the release of the soft-fouling green alga Ulva linza (syn. Enteromorpha linza) from poly(dimethylsiloxane) (PDMS) model networks

The effect of modulus and film thickness on the release of adhered spores and sporelings (young plants) of the green fouling alga Ulva (syn. Enteromorpha) was investigated. PDMS elastomers of constant thickness (100 μm) but different elastic moduli were prepared by varying cross-link density with functional silicone oligomers with degrees of polymerization ranging from 18–830. This provided a 50-fold range of modulus values between 0.2 and 9.4 MPa. Three PDMS coatings of different thicknesses were tested at constant elastic modulus (0.8 MPa). The data revealed no significant increase in percentage spore removal except at the lowest modulus of 0.2 MPa although sporelings released more readily at all but the highest modulus. The influence of coating thickness was also greater for the release of sporelings compared to spores. The release data are discussed in the light of fracture mechanics models that have been applied to hard fouling. New concepts appertaining to the release of soft fouling organisms are proposed, which take into account the deformation in the adhesive base of the adherand and deformation of the PDMS film.

Adhesive contact of cylindrical lens and a flat sheet

Methods are developed to estimate the adhesion and surface free energies of compliant materials from the contact deformations of cylindrical lenses with flat sheets. Some important differences are found between the cylindrical contact studied here and the widely studied geometry of spherical contact. For example, while the pull‐off force is completely independent of the elastic constants (K) of the materials for spherical contacts, the pull‐off force for cylindrical contact is proportional to K1/3. Furthermore, for cylindrical contacts the contact width at separation reaches to a value of 39% of the width (a0) at zero load, whereas the corresponding value is 0.63a0 for spherical contact. The feasibility of using cylindrical contacts to estimate the surface and adhesive energies of polymers was investigated using elastomeric polydimethylsiloxane (PDMS) as a model system. PDMS was used in two ways: (1) unmodified and (2) with its surface hydrolyzed with dilute hydrochloric acid. Significant hysteresis of ad...