Surface Forces Apparatus Research Articles

Examining the Origins of the Hydration Force Between Lipid Bilayers Using All-Atom Simulations

Using 237 all-atom double bilayer simulations, we examined the thermodynamic and structural changes that occur as a phosphatidylcholine lipid bilayer stack is dehydrated. The simulated system represents a micropatch of lipid multilayer systems that are studied experimentally using surface force apparatus, atomic force microscopy and osmotic pressure studies. In these experiments, the hydration level of the system is varied, changing the separation between the bilayers, in order to understand the forces that the bilayers feel as they are brought together. These studies have found a curious, strongly repulsive force when the bilayers are very close to each other, which has been termed the "hydration force," though the origins of this force are not clearly understood. We computationally reproduce this repulsive, relatively free energy change as bilayers come together and make qualitative conclusions as to the enthalpic and entropic origins of the free energy change. This analysis is supported by data showing structural changes in the waters, lipids and salts that have also been seen in experimental work. Increases in solvent ordering as the bilayers are dehydrated are found to be essential in causing the repulsion as the bilayers come together.

Intrinsic slip on hydrophobic self-assembled monolayer coatings

The slippage of liquids over solid surfaces is receiving a growing interest for its potential applications to microfluidic devices. Hydrophobic coatings of smooth surfaces have been reported able to induce a significant slip length. A considerable debate is found in the literature, and a substantial scatter in the measured slip lengths, ranging from zero to few microns, exists. Here we discuss a detailed molecular dynamics simulation of liquid water flowing over a self-assembled-monolayer of octadecyltrichlorosilane. The data provide a value of the slip length in the range of 0.2–0.6 nm, smaller than the most recent value of 20 nm obtained by means of an accurately controlled experiment with the surface force apparatus. The discrepancy between experiments and simulations is discussed, commenting on the possible effect of surface imperfections.

Measuring Flow Profiles and Slip by Single Molecule Tracking Experiments in Thin Liquid Films

A method for analyzing single molecule tracking data is presented, which allows the measurement of flow profiles in shear flows of ultrathin liquid films. The results show that the velocity profile detected by single molecule tracking is in general different from the analytical solution of the Navier−Stokes equation due to the diffusion of the probe molecules. However, the theory presented allows the extraction of even slip boundary phenomena, which for the first time provides access to boundary conditions on a molecular scale. A verification of the theory and its capabilities is presented by means of tracking single polystyrene particles in a 500 nm thin water film confined in a surface forces apparatus.

Nanotribology of Symmetric and Asymmetric Liquid Lubricants

When liquid molecules are confined in a narrow gap between smooth surfaces, their dynamic properties are completely different from those of the bulk. The molecular motions are highly restricted and the system exhibits solid-like responses when sheared slowly. This solidification behavior is very dependent on the molecular geometry (shape) of liquids because the solidification is induced by the packing of molecules into ordered structures in confinement. This paper reviews the measurements of confined structures and friction of symmetric and asymmetric liquid lubricants using the surface forces apparatus. The results show subtle and complex friction mechanisms at the molecular scale.

SNAREs: Could They be the Answer to an Energy Landscape Riddle in Exocytosis?

During exocytosis, chemical transmitters stored in secretory vesicles can be released upon fusion of these intracellular organelles to the plasma membrane. In this process, SNARE proteins that form a ternary core complex play a central role. This complex could provide the means for generation/storage of energy necessary for driving the fusion of vesicular and plasma membranes. Recently, the amount of energy for (dis)assembly of the ternary complex has been measured using various experimental approaches, including atomic force microscopy, the surface force apparatus, and isothermal titration calorimetry. The obtained measurements are in good agreement with the calculated energy required for membrane fusion achieved by theoretical modeling approaches. Whether the energy expenditure to form the ternary SNARE complex can be utilized towards membrane fusion and/or docking/tethering of vesicles to the plasma membrane still remains one of the key contemporary issues in biophysics and neuroscience.

Double Packaged System for Localized Drug Delivery for Ovarian Cancer

This research aims at developing a drug delivery system that will provide a plethora of benefits such as cost effectiveness, reduction of toxicity and a control over the release of chemotherapeutic drugs in Ovarian Cancer patients. We have designed a model drug delivery system consisting of non-ionic surfactant vesicles (niosomes) packaged within a biodegradable, temperature and pH sensitive hydrogel (chitosan) network. Optimization of the release rates were accomplished by altering the condition of its two components, chitosan and niosome. Two ovarian cancer drugs Paclitaxel and Carboplatin were used for encapsulation. It was found that medium molecular weight chitosan with a crosslinker:chitosan ratio of 4:1 which corresponded to a pH of 7.4 resulted in the finest controlled release. Surface characteristics, such as the interaction between the niosomes and chitosan were determined using Surface Forces Apparatus. The system was also tested in-vivo in mice models. Xenogen was used to study the release of drugs in-vivo. Our results will help in the development of a low cost and improved method for drug delivery with application to intracavitary ovarian cancer treatment and other cancer types.

Interactions between polymer brushes and a polymer solution: mesoscale modelling of the structural and frictional properties

We report mesoscopic simulations of polymer brushes in the presence of free polymer chains. We have used the dissipative particle dynamics method to model entangled polymer chains in a good solvent. We have studied the structure of the grafted and free chains under shear, as well as the dynamics of the detechment of polymer chains from the grafted surface. A comparison between entangled and nonentangled systems is presented. Entanglements are modeled using a bond repulsion potential. The rheology has been studied at different polymer grafting densities, in the presence of a varying number of free chains. We have shown that the detachment of grafted chains creates a central zone of free chains which exhibits a bulk like behaviour. The viscosity of this region is significantly increased compared to that of a polymer brush with the same grafting density and no detached chains. The presence of free polymer chains dramatically changes the friction coefficient. We have established that a fraction of detached chains of only 15% induces a decrease in the friction of 50%. This behaviour is correlated to the decrease of the interpenetration between the two opposing brushes. The detachment of chains from the surface, that may well occur at high sliding velocity in the surface force apparatus, is likely to have a significant effect on the observed experimental friction.

Adhesion Mechanisms of the Mussel Foot Proteins mfp-1 and mfp-3

Mussels adhere to a variety of surfaces by depositing a highly specific ensemble of 3,4-dihydroxyphenyl-L-alanine (DOPA) containing proteins. The adhesive properties of Mytilus edulis foot proteins mfp-1 and mfp-3 on mica (a common alumino-silicate clay mineral) and TiO2 surfaces were directly measured at the nano-scale by using a surface forces apparatus (SFA). The adhesion energy between mfp-3 and mica was on the order of W=3×10-4 J/m2 which corresponds to an approximate force per plaque of ∼100 gm - more than enough to hold a mussel in place if no peeling occurs.

Resonance shear measurement of nanoconfined ionic liquids

Two types of imidazolium-based ionic liquid (IL), 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([C(4)mim][NTF(2)]) and 1-butyl-3-methylimidazolium tetrafluoroborate ([C(4)mim][BF(4)]), confined between silica surfaces were investigated by surface force apparatus (SFA)-based resonance shear measurements together with surface force measurements. The surface force profiles in the ILs showed oscillatory solvation forces below the characteristic surface separations: 10.0 nm for [C(4)mim][NTf(2)] and 6.9 nm for [C(4)mim][BF(4)]. The more pronounced solvation force found in [C(4)mim][NTf(2)] suggests that the crystal-forming ability of the IL contributes to the stronger layering of the ILs adjacent to the surface. The resonance shear measurement and the physical model analysis revealed that the viscosities of the confined ILs were 1-3 orders of magnitude higher than that of the bulk IL. This paper also focused on the correlation between the resonance shear behaviour and the lubrication property of the ILs, and the suspension rheology in the ILs. An understanding of the solid-IL interface and of ILs confined in nanospace will facilitate the further development of novel applications employing ILs.

Viscosity and interfacial properties in a mussel-inspired adhesive coacervate.

The chemistry of mussel adhesion has commanded the focus of much recent research activity on wet adhesion. By comparison, the equally critical adhesive processing by marine organisms has been little examined. Using a mussel-inspired coacervate formed by mixing a recombinant mussel adhesive protein (fp-151-RGD) with hyaluronic acid (HA), we have examined the nanostructure, viscosity, friction, and interfacial energy of fluid-fluid phase-separated coacervates using the surface forces apparatus and microscopic techniques. At mixing ratios of fp-151-RGD:HA resulting in marginal coacervation, the coacervates showed shear-thickening viscosity and no structure by cryo-transmission electron microscopy (cryo-TEM). However, at the mixing ratio producing maximum coacervation, the coacervate showed shear-thinning viscosity and a transition to a bicontinuous phase by cryo-TEM. The shear-thinning viscosity, high friction coefficient (>1.2), and low interfacial energy (<1 mJ m(-2)) observed at the optimal mixing ratio for coacervation are promising delivery, spreading and adhesion properties for future wet adhesive and coating technologies.

Instability of Confined Water Films between Elastic Surfaces

We investigated the dynamics of nanometer thin water films at controlled ambient humidity adsorbed onto two atomically smooth mica sheets upon rapidly bringing the surfaces into contact. Using a surface forces apparatus (SFA) in imaging mode, we found that the water films break up into a distribution of drops with a typical thickness of a few nanometers and a characteristic lateral size and spacing of several micrometers. Whereas the characteristic length is found to be independent of the ambient humidity, the characteristic time of the breakup decreases from approximately 1 to 0.01 s with increasing humidity. The existence of characteristic length and time scales shows that this breakup is controlled by an instability rather than a conventional nucleation and growth mechanism for SFA experiments. These findings cannot be explained by a dispersion-driven instability mechanism. In contrast, a model involving the elastic energies for the deformation of both the mica sheets and the underlying glue layer correctly reproduces the scaling of the characteristic length and time with humidity.

Effect of Surfactant Oligomerization Degree on Lubricant Properties of Mixed Surfactant-Diblock Copolymer Films

In this study, we investigate the coadsorption and the behavior under shear of mixtures of an anionic–neutral hydrophilic–hydrophilic diblock copolymer and oligomeric cationic surfactants on mica surfaces. Three different ammonium quaternary oligomeric surfactants are studied: a monomer, a gemini, and a trimer. Using both an Atomic Force Microscope (AFM) and a Surface Force Apparatus modified for nanotribology studies (SFA-N), we compare the morphology, the interactions between two coated surfaces, and the frictional properties of the resulting film for the three surfactants investigated. While the cohesion of the trimeric and gemini self-assembled layers is markedly improved by the presence of the copolymer, a less significant effect is observed for the monomeric surfactant. Several shear-induced dynamic transitions between states of low- and high-friction forces are observed in all cases. The nature and the characteristic times of these transitions are strongly influenced by the oligomerization degree of the assembled surfactant.

Density Effects on Collapse, Compression, and Adhesion of Thermoresponsive Polymer Brushes

We probe, using the surface forces apparatus, the thermal response of poly(N-isopropylacrylamide) (PNIPAM) brushes of various grafting densities, grown from plasma-activated mica by means of surface-initiated polymerization. We thus show that dense thermoresponsive brushes collapse gradually as temperature is increased and that grafting density greatly affects their ability to swell: the swelling ratio of the brushes, which characterizes the thickness variation between the swollen and the collapsed state, is found to decrease from approximately 7 to approximately 3 as the number of grafted chains per unit area increases. Such a result, obtained with an unprecedented resolution in grafting density, provides qualitative support to calculations by Mendez et al. [Macromolecules 2005, 38, 174]. We further show that, in contrast to swelling, adhesion between two PNIPAM brushes appears to be rather insensitive to their molecular structure.

Liquid- to Solid-Like Failure Mechanism of Thin Polymer Films at Micro- and Nanoscales

A surface forces apparatus and various optical visualization techniques were used to study the transition mechanisms between liquid-like snapping and solid-like failure at nano- and microscales. Transient deformations and flows of polymer necks were studied over a large range of pulling forces, pulling rates, temperatures, and viscosities. A continuous transition is suggested—from simple neck-thinning and snapping, through viscous fingering, to sharp solid-like cracking— between these two limiting modes of failure if one chooses to vary the system properties and experimental conditions in a systematic way. The Saffman−Taylor fingering equation was found to remain valid down to nanoscopic dimensions, and a modified version of it is suggested to be suitable to elastic, solid-like materials.

Structural aging and stiction dynamics in confined liquid films

The static friction (stiction) of the molecularly thin films of an irregularly shaped molecule 1,3-dimethylbutyl octyl ether (DBOE) confined between mica surfaces was investigated using the surface forces apparatus. Stop-start experiments were carried out and the stiction spike was measured as a function of surface stopping (aging) time t and applied pressure P. The results show two relaxation processes, one on stopping and one on starting, where each process has a fast and a slow time constant. For stopping mode, there is no stiction spike when t is shorter than a characteristic nucleation time, tau(n) (fast time constant). When t exceeds tau(n), stiction spike appears whose height increases logarithmically with t. With regard to starting, the relaxation behavior was evaluated by a double exponential fit of the slipping regime (force decay) of the spike and two time constants (tau(1) and tau(2)) were obtained. The fast time constant on starting tau(1) is almost equal to that on stopping tau(n). To the best of our knowledge, this is the first direct observation of the agreement of the time constant on stopping and that on starting, indicative of a reversible structural transition (solid-liquid transition) in the stop-start stiction dynamics. The two fast time constants exhibit exponential dependence on P, which implies a glasslike nature of the transition. Comparison with the stick-slip friction reveals that the solid-liquid transition involved in stiction and that in stick-slip dynamics is different for DBOE; first-order-like discontinuous transition is suggested for stick-slip friction. Origins of the different solid-liquid transition dynamics in stiction and in stick-slip friction are discussed by comparing with the dynamics of other confined liquid systems.

Gecko adhesion pad: a smart surface?

Recently, it has been shown that humidity can increase the adhesion of the spatula padsthat form the outermost (adhesive) surface of the tokay gecko feet by 50% relative to themain adhesion mechanism (i.e. van der Waals adhesive forces), although the mechanismby which the enhancement is realized is still not well understood. A change inthe surface hydrophobicity of a gecko setal array is observed when the array,which supports the spatulae, is exposed to a water drop for more than 20 min,suggesting a change in the hydrophilic–lyophilic balance (HLB), and thereforeof the conformation of the surface proteins. A surface force apparatus (SFA)was used to quantify these changes, i.e. in the adhesion and friction forces, whileshearing the setal array against a silica surface under (i) dry conditions, (ii) 100%humidity and (iii) when fully immersed in water. The adhesion increased in thehumid environment but greatly diminished in water. Although the adhesion forceschanged significantly, the friction forces remained unaffected, indicating that thefriction between these highly textured surfaces is ‘load-controlled’ rather than‘adhesion-controlled’. These results demonstrate that the gecko adhesive pads havethe ability to exploit environmental conditions to maximize their adhesion andstabilize their friction forces. Future designs of synthetic dry adhesives inspired bythe gecko can potentially include similar ‘smart’ surfaces that adapt to theirenvironment.

Delamination and Renovation of a Molecular Surfactant−Polymer Boundary Lubricant Film

We have studied the behavior under compression and shear of two molecularly smooth mica surfaces immersed in aqueous solutions of a dimeric cationic surfactant and an oppositely charged polyelectrolyte-neutral diblock copolymer by using a surface force apparatus-nanotribometer, SFA-N. The surfactant and copolymer coadsorb as a mix molecular boundary lubricant film on the negatively charged mica surfaces leading to low friction in a sliding mechanical contact. However, under fritting conditions, shearing can induce different dynamic transitions of the confined films. Transitions from the initial low friction steady state to new steady states of low or high friction can be induced when the sliding velocity is increased above certain values. These dynamical transitions occur together with thickness reduction of the confined film. A reverse transition to the low friction steady state accompanied by the renovation of the film can be triggered by reintroducing some matter in the contact, via increasing the amplitude of the fritting cycles.

Anisotropic Reversible Aggregation of Latex Nanoparticles Suspended in a Lyotropic Nematic Liquid Crystal: Effect of Gradients of Biaxial Order

We studied the anisotropic aggregation of spherical latex particles dispersed in a lyotropic liquid crystal presenting three nematic phases: calamitic, biaxial, and discotic. We observed that in the nematic calamitic phase aggregates of latex particles are formed, which become larger and anisotropic in the vicinity of the transition to the discotic phase, due to a coalescence process. Such aggregates are weakly anisotropic and up to 50 microm long and tend to align parallel to the director field. At the transition to the discotic phase, the aggregates dissociated and re-formed when the system was brought back to the calamitic phase. This shows that the aggregation is due to attractive and repulsive forces generated by the particular structure of the nematic phase. The surface-induced positional order was investigated by surface force apparatus experiments with the lyotropic system confined between mica surfaces, revealing the existence of a presmectic wetting layer around the surfaces and oscillating forces of increasing amplitude as the confinement thickness was decreased. We discuss the possible mechanisms responsible for the reversible aggregation of latex particles, and we propose that capillary condensation of the NC phase, induced by the confinement between the particles, could reduce or remove the gradient of order parameter, driving the transition of aggregates from solidlike to liquidlike and gaslike.

Fine calibration of the residual dissipation in a surface forces apparatus using a capacitive sensor

The design of a dynamic surface force apparatus using a capacitive sensor has been useful in the past to study the nanorheological behavior of thin liquid films confined between a sphere and a plane. This allows a precise measurement of the hydrodynamical boundary condition of liquids down to some molecular sizes. However for thick liquid films, the viscous force is decreased and the dissipation due to the flow of air in the capacitive sensor becomes of the same order of magnitude than the force due to the liquid flow. In the present paper we present an original technique to characterize this residual force due to the capacitive sensor and we show that a precise subtraction of this force extends the precision of the device to the case of thick liquid films of low viscosity. This correction should be of major importance to extend the capability of surface forces apparatus to measure very large boundary slippage of simple liquids.

Formation of Supported Bilayers on Silica Substrates

We have investigated the formation of phospholipid bilayers of the neutral (zwitterionic) lipid dimyristoyl-phosphatidylcholine (DMPC) on various glass surfaces from vesicles in various aqueous solutions and temperatures using a number of complementary techniques: the surface forces apparatus (SFA), quartz crystal microbalance (QCM), fluorescence recovery after photobleaching (FRAP), fluorescence microscopy, and streaming potential (SP) measurements. The process involves five stages: vesicle adhesion to the substrate surfaces via electrostatic and van der Waals forces, steric interactions with neighboring vesicles, rupture, spreading via hydrophobic fusion of bilayer edges, and ejection of excess lipid, trapped water, and ions into the solution. The forces between DMPC bilayers and silica were measured in the SFA in phosphate buffered saline (PBS), and the adhesion energy was found to be 0.5-1 mJ/m(2) depending on the method of bilayer preparation. This value is stronger than the expected adhesion predicted by van der Waals interactions. Theoretical analysis of the bilayer-silica interaction shows that the strong attraction is likely due to an attractive electrostatic interaction between the uncharged bilayer and negatively charged silica owing to the surfaces interacting at "constant potential." However, the bilayer-silica interaction in distilled water was found to be repulsive at all distances, which is attributed to the surfaces interacting at "constant charge." These results are consistent with QCM measurements that show vesicles readily forming bilayers on silica in high salt but only weakly adhering in low salt conditions. We conclude that the electrostatic interaction is the most important interaction in determining the adhesion between neutral bilayers and charged hydrophilic surfaces. SP and FRAP experiments gave insights into the bilayer formation process as well as information on the surface coverage, lateral diffusion of the lipid molecules, and surface potential of the bilayers during the spreading process.