Crossed-cylinder Geometry Research Articles

Direct force measurements between sub-micron rod-shaped colloids by AFM

The colloidal probe (CP) technique is an essential tool for quantitative direct force measurements by atomic force microscopy (AFM). By attaching a colloidal particle to the free end of an AFM-cantilever, not only a defined interaction geometry can be accomplished, but also a nearly arbitrary surface chemistry becomes possible. Commonly, the CP-technique is utilized for spherical particles in the sphere/sphere or sphere/plane interaction geometry. Here, the CP-technique has been extended to rod-shaped colloids with diameters well below one micrometer, thus preparing ‘rod probes’ based on a procedure similar to that known from DNA combing, allowing for a controlled alignment between these rods and the rod probe. In combination with AFM-imaging by the rod on the probe, sufficiently orthogonally aligned pairs of rods can be identified, and direct force measurements in the crossed-cylinder geometry, similar to the surface force apparatus (SFA), can be accomplished. A proof-of-concept of direct force measurements between silica rods with diameters of 270 ± 50 nm has been presented. The acquired interaction force profiles have been quantitatively evaluated within the framework of the full solutions of the Poisson-Boltzmann equation, including charge regulation.

Surface forces generated by the action of electric fields across liquid films.

We explore the force generation and surface interactions arising when electric fields are applied across fluid films. Using a surface force balance (SFB) we measure directly the force between two electrodes in crossed-cylinder geometry across dielectric and electrolytic fluids. In the case of dielectric films the field between the electrodes exerts a force which can be well explained using classic expressions and with no fitting parameters. However when the electrodes are separated by a film of electrolyte, an alternating electric field induces a force which diverges substantially from the calculated static response of the electrolyte. The magnitude of the force is larger than predicted, and the interaction can switch from attractive to repulsive. Furthermore, the approach to steady state in electrolyte takes place over 102-103 s which is very slow compared to both the charging and viscous timescales of the system. The non-trivial electrolyte response in AC electric fields, measured here directly, is likely to underlie several recent reports of unexpected and bifurcating forces driving colloids in AC fields. Our measurements suggest ways to control colloidal and soft matter using electric fields, as well as providing a direct measure of the length- and time-scales relevant in AC electrochemical and electrokinetic systems.

Long-range interaction forces between 1,3,5-cyclohexanetrisamide fibers in crossed-cylinder geometry

Abstract We report on direct force measurements between single melt-electrospun 1,3,5-cyclohexanetrisamide (CTA) fibers in crossed-cylinder geometry. The two CTA compounds selected for this study differ in their peripheral substituents: aliphatic (-C 10 H 21 ) and fluorinated chains (-CH 2 C 6 F 13 ), respectively. Melt-electrospinning of the CTAs results in smooth and circular fibers with diameters of about 5 μm. Individual segments of these fibers were attached to tipless atomic force microscope (AFM) cantilevers and used to measure long-range interaction forces versus a second fiber from the same compound in crossed-cylinder geometry. This geometry is well-known from the surface force apparatus and allows for the normalization of forces according to the Derjaguin approximation. From symmetrical measurements, i.e. measurements between fibers from the same type of CTA, we quantify the diffuse layer properties in aqueous electrolyte solutions within the framework of the Derjaguin, Landau, Verwey and Overbeek (DLVO) theory. Apparent diffuse layer potentials resulting from the fits to the full solutions of the Poisson–Boltzmann equation show that the fiber surfaces bear a negative surface charge. Most likely, the origin of charging is the adsorption of hydroxyl ions as residual charges from the electrospining process would be compensated upon the immersion in the electrolyte solutions. Such ion adsorption processes are well-known for other hydrophobic surfaces such as aliphatic and fluorinated self-assembled monolayers (SAMs) (terminating with CH 3 and CF 3 groups). The apparent diffuse layer potentials for CTA fibers are comparable to the values reported for these SAMs.

Roles of chemical pattern period and film thickness in directed self-assembly of diblock copolymers

We develop a new numerical self-consistent field theory (SCFT) scheme for examining thin film nanostructures of cylinder-forming AB diblock copolymers on a chemically patterned substrate. Using this, we make a systematic analysis to achieve a fundamental understanding of the model system, and the conditions to create various novel film morphologies are scrutinized by varying the pattern period and film thickness. At a fixed pattern period, eight candidate phases which are divided into two groups according to the film thickness are found, and then their free energies are compared to identify the stable morphology. Among the thinner film morphologies, a crossed cylinder geometry is found to be stable with a comfortable margin. While for the thicker film morphologies, the stability of the phases depends strongly on both the pattern period and film thickness. In addition, we identify an unstable phase capable of thinning the films and consequently providing instability for the ordering of the thicker film nanostructures. A qualitative agreement is found between our theory and previously reported experimental results.

Effect of salt on the compression of polyelectrolyte brushes in a theta solvent

Classical strong-stretching theory (SST) predicts that, as opposing polyelectrolyte brushes are compressed together in a salt-free theta solvent, they contract so as to maintain a finite polymer-free gap, which offers a potential explanation for the ultra-low frictional forces observed in experiments despite the application of large normal forces. However, the SST ignores chain fluctuations, which would tend to close the gap resulting in physical contact and in turn significant friction. In a preceding study, we examined the effect of fluctuations using self-consistent field theory (SCFT) and illustrated that high normal forces can still be applied before the gap is destroyed. We now look at the effect of adding salt. It is found to reduce the long-range interaction between the brushes but has little effect on the short-range part, provided the concentration does not enter the salted-brush regime. Consequently, the maximum normal force between two planar brushes at the point of contact is remarkably unaffected by salt. For the crossed-cylinder geometry commonly used in experiments, however, there is a gradual reduction because in this case the long-range part of the interaction contributes to the maximum normal force.

A novel modular fretting wear test rig

A novel modular experimental apparatus was designed and developed to measure and visualize fretting wear and friction for Hertzian circular and elliptical contacts and flat on flat contacts. The experimental apparatus utilizes a magnetostrictive actuator to reciprocate a flat, ball, or cylinder between two fixed specimens. Two stationary flat or cylindrical specimens mounted on a rotary table clamp the reciprocating specimen from the top and bottom to generate the fretting contact. The two stationary test specimens installed on the rotary table perpendicular to the moving specimen form a crossed cylinder geometry which creates a well-defined circular contact. An elliptical contact with different aspect ratios can be obtained by varying the angle between the fixed and the moving specimens. Dead weights placed on top of the upper stationary specimen provide the normal load. A force sensor located in line between the actuator output shaft and the specimen is used to measure friction. The test rig's modular design allows it to be configured for Hertzian circular (ball-on-flat, crossed cylinder), elliptical (crossed cylinder), and conformal (flat-on-flat) contacts. In the ball on flat configuration a steel flat or sapphire window is used in contact with the reciprocating ball. When the sapphire window is used a microscope and high speed camera is employed for in situ visualization and recording of the contact.

A novel AFM based method for force measurements between individual hair strands

Interactions between hairs and other natural fibers are of broad interest for both applications and fundamental understanding of biological interfaces. We present a novel method, that allows force measurements between individual hair strands. Hair fragments can be laser-cut without altering their surface chemistry. Subsequently, they are glued onto Atomic force microscopy (AFM) cantilevers. This allows carrying out measurements between the hair fragment and surface immobilized hair in a well-defined crossed-cylinder geometry. Both force–distance and friction measurements are feasible. Measurements in air with controlled humidity and in aqueous environment show clear differences which can be explained by the dominating role of capillary interactions in air. Friction is found to be anisotropic, reflecting the fine structure of hair cuticula. While the investigations are focused on the particular example of human hair, we expect that the approach can be extended to other animal/plant fibers and thus offers perspectives for broad spectrum systems.

On adhesion of work material in metal forming

The transfer of work material to the tool surface limits the tool life in many forming operations. Using a dedicated load-scanning test equipment with crossed-cylinder geometry, dry forming of austenitic stainless steel was simulated by provoking adhesion to the TiN-coated tool specimen. High-resolution electron microscopy combined with analytical techniques was used to examine the interface between tool and work material. The decisive mechanism for adhesion and transfer of steel to the TiN surface is suggested. The oxide layer on the steel surface, especially the Fe-oxide, initiates the metal transfer. The interfacial oxide acts as a glue between stainless steel and TiN and increases the adhesive forces. Obviously, the adhesion and internal strength of the oxide layer is far stronger than anticipated. It may even be stronger than the bonding to the austenitic steel itself. A consequence of these findings is that the development of galling resistance-forming tool materials and coatings for austenitic stainless steels should not only aim to improve the bulk tool material, but also to reduce the adhesion strength between the tool surface and the oxide layer on the work material.

Hydrodynamic slippage inferred from thin film drainage measurements in a solution of nonadsorbing polymer

Thin film drainage measurements are presented for submicron films of an “ideal elastic” or Boger fluid, which is a high molecular weight polymer solution in a high viscosity solvent. The measurements are made in a surface force apparatus, with the fluid being squeezed between two mica surfaces in a crossed cylinder geometry and the film thickness measured as a function of time to study its drainage behavior. No equilibrium surface forces are detected in this system, indicating that the polymer is nonadsorbing. The effect of fluid elasticity is predicted to make drainage more rapid in a Boger fluid than for the equivalent Newtonian fluid. Qualitatively this is what is observed for films less than 600 nm thick, but the drainage is even more rapid than predicted for the elastic fluid. To account for this, it is suggested that slippage is occurring at the fluid–solid interfaces, and the data is analyzed in terms of a simple slip model. The slip length required to fit the data is in the range 30–50 nm.

The effect of number of cycles on the critical transition boundary between fretting fatigue and fretting wear

The object of the present study was to investigate the influence of number of cycles on the critical amplitudes of tangential force and displacement, identifying the transition from a mixed stick-slip regime (fretting fatigue) to a gross slip regime (fretting wear) over a wide range of test conditions. Fretting experiments were conducted on three metal specimen combinations: copper/copper; stainless steel/stainless steel; copper/stainless steel. All experiments took place in air, at ambient temperature, using a crossed-cylinder geometry. Normal loads of 3.4 and 11.4 N were applied with frequencies ranging from 10 to 800 Hz. In most cases, n = 1.2×10 4 and n = 6×10 6 were adopted as the representative lower and higher number of cycles, respectively. At different numbers of cycles, critical amplitudes of tangential force and displacement were measured. The scars fretted under separately selected conditions were examined by scanning electron microscopy. It was found that the critical amplitudes of both tangential force and displacement dropped with increasing number of cycles for all test combinations, but there was an upper limit above which the drop of critical transition values no longer occurred with further increases in the number of cycles. The micromorphology of fretting scars (in a mixed stick-slip regime) revealed that the stick zone has shrunk after a larger number of cycles under the conditions of constant amplitude of tangential force and displacement, and that the damage mechanisms vary for different combinations, although they are all characterized by a central stick zone surrounded by a slip annulus. It was suggested that the decrease of critical amplitudes after a larger number of cycles results from the shrinkage of the stick area, which may be a complex process related to plastic deformation, strain hardening, and the change of stress distribution on the contact surfaces.

Effect of layer spacing on wear of Ni/Cu multilayer alloys

Alloys consisting of multiple alternate layers of nickel and copper have been prepared by electrodeposition and studied in sliding wear. Most recently a multilayer alloy with a layer spacing of 3.8 nm was studied, to complement earlier studies of multilayer alloys with layer spacings of 10 and 100 nm. In order to determine wear characteristics, the alloys were tested in unlubricated sliding against type 52100 steel in a crossed-cylinder geometry at various loads. It was found that the most wear-resistant alloy was the one having the smallest layer spacing, 3.8 nm. Further, all the multilayer alloys showed less wear than either copper or nickel deposited from the same solution and tested under the same conditions. The multilayer structure in these alloys is thought to provide internal barriers to wear which occurred by plastic deformation processes. The data suggest that the individual layer spacing determines the level of stress necessary to initiate severe wear.

Sliding wear studies of ni-cu composition-modulated coatings on steel

Sliding wear measurements have been conducted on Ni-Cu composition-modulated coatings on steel. The electrodeposited coatings consisted of alternate layers of nickel and copper with equal layer spacings of either 10 or 100 nm. Pure nickel and pure copper coatings were also prepared in a similar manner and studied. The coated cylinders were slid against AISI type 52100 steel in a crossed cylinder geometry under clean conditions without lubrication at various loads and sliding distances. It was found that the most wearresistant coating was the composition-modulated alloy having the smallest layer spacing, 10 nm. This effect was most pronounced at low loads. Both composition-modulated coatings showed less wear than coatings of pure copper and nickel deposited from the same solution. The layer microstructure in these composition-modulated coatings may provide internal barriers to wear damage, thus leading to the increases in wear resistance. Results from examination of the worn specimens and collections of wear debris are also described.

Contact conditions and surface degradation mechanisms in low amplitude fretting

The characteristic contact conditions and mechanisms of surface degradation in low amplitude (1–16 μm) fretting have been investigated. A crossed cylinder geometry set-up was used with pure niobium as test material. Fretting experiments, varying displacement amplitude, normal force, frequency of vibration and number of cycles, have been performed. Dynamic tangential force measurements and metallographic examination revealed the existence of three different fretting regimes: stick, mixed stick and slip, and gross slip. In the study, the influence of normal force and frequency on the regime boundaries was determined. Niobium was selected as test material since it is a pure metal with strongly strain-rate-dependent mechanical properties and, therefore, is expected to be sensitive to frequency-related effects in fretting. The size of the fretting wear scar was found to increase rapidly with increasing amplitude. The wear rates were significantly higher in the gross slip regime in which material removal predominantly occurred by delamination and crushing of surface oxides. Surface degradation and wear were also observed under partial stick contact conditions. In this case, wear was mainly due to the generation and intersection of surface cracks. The extensive crack formation is caused by the cyclic shearing of material in the near-surface region during fretting. The frequency of vibration is found to influence both the contact conditions and the rate of surface damage. The limiting displacement amplitude to yield partial stick conditions is reduced and the size of the wear scar is increased with increasing frequency (in the range 10–20 000 Hz). This is an effect of the increase in both the interfacial strain rate and the temperature with increasing frequency.

Friction, metallic transfer and debris analysis of sliding surfaces

The variation in the coefficient of friction with sliding distance for different sliding conditions for both similar and dissimilar metals in a crossed-cylinder geometry is presented. Metallic transfer due to adhesion at the early stage of sliding controls the coefficient of friction of a soft metal sliding on a hard metal. Debris generated in the wear process are formed of the soft metal only. For a hard metal sliding on a soft metal friction is influenced by the ploughing action. There is mutual transfer of metals between the sliding surfaces and the generated debris in this case are formed of both metals. These observations are confirmed by scanning electron microscopy and electron probe microanalysis.

Surface Topography and Functional Requirements for Dry Sliding Surfaces

Friction and wear of dry sliding surfaces were examined as a function of the initial surface topography, relative hardness and the geometrical arrangement using crossed-cylinder geometry. The initial surface topography was found to have no effect on the initial and steady-state friction coefficients and the wear rates of the sliding members. Also, the initial friction coefficient was independent of the geometrical arrangement. However, the steady state friction coefficients and the wear rates were strongly dependent on the relative hardness and the geometrical arrangement of the sliding pair. When the hard surface was stationary and the soft surface was moving both surfaces were polished. By contrast, when the soft surface was stationary and the hard surface was moving, polishing did not occur. The surfaces of identical materials were always rough and had much higher coefficients of friction due to plowing. Entrapped wear debris increased the friction coefficient which in turn increased the wear rate. Based on these observations a model for steady state friction is presented and the implications of these findings to dry sliding surfaces are discussed.