X-ray Surface Forces Apparatus Research Articles

Cohesion Gain Induced by Nanosilica Consolidants for Monumental Stone Restoration.

Mineral nanoparticle suspensions with consolidating properties have been successfully applied in the restoration of weathered architectural surfaces. However, the design of these consolidants is usually stone-specific and based on trial and error, which prevents their robust operation for a wide range of highly heterogeneous monumental stone materials. In this work, we develop a facile and versatile method to systematically study the consolidating mechanisms in action using a surface forces apparatus (SFA) with real-time force sensing and an X-ray surface forces apparatus (X-SFA). We directly assess the mechanical tensile strength of nanosilica-treated single mineral contacts and show a sharp increase in their cohesion. The smallest used nanoparticles provide an order of magnitude stronger contacts. We further resolve the microstructures and forces acting during evaporation-driven, capillary-force-induced nanoparticle aggregation processes, highlighting the importance of the interactions between the nanoparticles and the confining mineral walls. Our novel SFA-based approach offers insight into nano- and microscale mechanisms of consolidating silica treatments, and it can aid the design of nanomaterials used in stone consolidation.

Structure and Dynamics of Confined Liquids: Challenges and Perspectives for the X-ray Surface Forces Apparatus.

The molecular-scale structure and dynamics of confined liquids has increasingly gained relevance for applications in nanotechnology. Thus, a detailed knowledge of the structure of confined liquids on molecular length scales is of great interest for fundamental and applied sciences. To study confined structures under dynamic conditions, we constructed an in situ X-ray surface forces apparatus (X-SFA). This novel device can create a precisely controlled slit-pore confinement down to dimensions on the 10 nm scale by using a cylinder-on-flat geometry for the first time. Complementary structural information can be obtained by simultaneous force measurements and X-ray scattering experiments. The in-plane structure of liquids parallel to the slit pore and density profiles perpendicular to the confining interfaces are studied by X-ray scattering and reflectivity. The normal load between the opposing interfaces can be modulated to study the structural dynamics of confined liquids. The confinement gap distance is tracked simultaneously with nanometer precision by analyzing optical interference fringes of equal chromatic order. Relaxation processes can be studied by driving the system out of equilibrium by shear stress or compression/decompression cycles of the slit pore. The capability of the new device is demonstrated on the liquid crystal 4′-octyl-4-cyano-biphenyl (8CB) in its smectic A (SmA) mesophase. Its molecular-scale structure and orientation confined in 100 nm to 1.7 μm slit pores was studied under static and dynamic nonequilibrium conditions.

Understanding nanorheology and surface forces of confined thin films

Understanding the nanorheology and associated intermolecular/surface forces of fluids in confined geometries or porous media is of both fundamental and practical importance, providing significant insights into various applications such as lubrication and micro/nanoelectromechanical systems. In this work, we briefly reviewed the fundamentals of nanoreheolgy, advances in experimental techniques and theoretical simulation methods, as well as important progress in the nanorheology of confined thin films. The advent of advanced experimental techniques such as surface forces apparatus (SFA), X-ray surface forces apparatus (XSFA) and atomic force microscope (AFM) and computational methods such as molecular dynamics simulations provides powerful tools to study a wide range of rheological phenomena at molecular level and nano scale. One of the most challenging issues unresolved is to elucidate the relationship between the rheological properties and structural evolution of the confined fluid films and particles suspensions. Some of the emerging research areas in the nanorheology field include, but are not limited to, the development of more advanced characterization techniques, design of multifunctional rheological fluids, bio-related nanorheology, and polymer brushes.

An x-ray confinement cell for studies of complex fluids under shear and confinement

An x-ray confinement cell is described in this article. Advantages over the previous generation x-ray surface forces apparatus include a stiffer shearing mechanism and improved ease of operation. This completely new device has also been optimized for x-ray diffraction studies of shearing complex fluids under confining gaps ranging from several hundred angstroms to millimeters.

Mechanical and Structural Properties of BaCrO4 Nanorod Films under Confinement and Shear

AbstractUsing an X‐ray surface forces apparatus (X‐SFA) we have investigated the effects of normal load (stress) and shear on the ordering and tribological properties of 10 nm × 30 nm surfactant‐coated BaCrO4 nanorods in isooctane, confined within submicrometer films. The film structure and corresponding friction forces were monitored as a function of time and shearing distance at different gap sizes and loads. The X‐ray diffraction patterns indicate a cubic phase of nanorods coexisting with a surfactant phase that depends on the load, film thickness, shear rate, and shearing time. Atomic force microscopy and birefringence measurements performed on each surface after a shearing experiment showed ordered domains of nanorods over length scales of several tens of micrometers.

The x-ray surface forces apparatus for simultaneous x-ray diffraction and direct normal and lateral force measurements

We describe the experimental setup and principles of operation of the second-generation x-ray surface forces apparatus that allows for the first time simultaneous x-ray scattering and direct force measurements.

Direct observation of shear-induced orientational phase coexistence in a lyotropic system using a modified x-ray surface forces apparatus.

The second generation x-ray surface forces apparatus (XSFA-II) allows for the first time simultaneous in situ small-angle x-ray scattering and surface force measurements. We have used the XSFA-II to monitor shear-induced orientational transitions in a lyotropic model lubricant system. Upon applying small shear amplitudes (approximately 20 micrometer) to a relatively thick (approximately 800 micrometer) film, we observed evidence for the formation of an orientational boundary layer at the shearing surface. Time-resolved x-ray diffraction revealed the gradual transition to shear-favored orientation by growth of the boundary layer.

In situ X-ray diffraction studies of a multilayered membrane fluid under confinement and shear

The structure of a fluid membrane system composed of surfactant-co-surfactant-oil-water mixtures has been investigated under confinement and shear conditions. Small angle x-ray scattering (SAXS) was employed with a second generation x-ray surface forces apparatus (XSFA-II) to study the time evolution of the orientational structure of the lamellar fluid under oscillatory shear. In a regime of relatively big confinement gap (∼800 μm) and small shear amplitude (∼40 μm), direct evidence of an “orientational phase separation” behavior, where a surface boundary layer adopts different orientation and separates from the bulk region, was observed for the first time. Under continuous shearing, the surface boundary layer grows in thickness and aligns towards a shear-favored (low friction) state while the bulk orientation remains unchanged. To further investigate the effects of surface confinement, we spatially mapped, in ∼1 μm sections, the orientation structure of the lamellar fluid sample confined between two glass surfaces using a micro-focused x-ray beam produced by a linear Bragg–Fresnel lens at the Advanced Photon Source. The data confirmed the expected trend that the smectic domains align progressively better with respect to the surface as they approach the surface.

Structure under confinement in a smectic-A and lyotropic surfactant hexagonal phase

Simple and complex fluids undergo significant changes in their structural and rheological behaviour as they are progressively confined between narrowing walls. The understanding of these new fluid properties is of fundamental interest in applications ranging from thin film lubrication in micromachines to catalysis. The X-Ray surface forces apparatus (XSFA) is capable of non-destructive imaging of the structure of confined complex fluid systems, on length scales ranging from nanometers to several tens of microns. In this article, recent work is presented on studies of confined complex fluids with the XSFA. Confinement can significantly align the liquid crystalline smectic phase. The degree of orientation depends critically on the compliance of the confining surface: “soft surfaces” exhibit a critical gap for alignment of 3.4 μm, while “hard surfaces” do not exhibit gap dependent alignment. Shear-induced orientation has been shown to dominate over confinement induced alignment with the systems studied. In the final part of the paper, we discuss an important new development where it is demonstrated that the XSFA may be used as an alignment tool for lyotropic liquid crystals.

Alignment of Complex Fluids under Confinement and Flow

We report on a new application of the X-ray Surface Forces Apparatus [1-4] (XSFA) to the study of the alignment of complex fluid thin films under confinement and/or flow using synchrotron X-ray scattering. The smectic A liquid crystal 8CB (4-cyano-4'-octylbiphenyl) and the hexagonal phases of a zwitterionic polyisoprene melt and of a lyotropic liquid crystal composed of sodium dodecyl sulfate (SDS), pentanol and water were investigated in the regime where the distances between the confining surfaces range from 0.4 to tens of microns. In this paper, we demonstrate that this new technique allows preparation of highly-oriented complex fluid samples suitable for high resolution X-ray diffraction and scattering studies. In addition, we have directly measured forces across thin films of 8CB using the conventional Surface Forces Apparatus (SFA). The force measurements complement well the direct X-ray imaging and provide indirect information on the sample alignment in films a few tens of Angstroms thick.

Study of flow in a smectic liquid crystal in the X-ray Surface Forces Apparatus

We describe the newly invented X-ray Surface Forces Apparatus (X-SFA) which allows the simultaneous measurement of forces and collective structures of confined complex fluids under static and flow conditions. The structure of the smectic liquid crystal 8CB (4-cyano-4{prime}-octylbiphenyl) confined between two mica surfaces with separation ranging from 4000 to 20,000 {angstrom} was measured. At small gaps and no shear, the smectic layers take on distinct stable orientations, including the bulk forbidden {open_quotes}b{close_quotes} orientation, which persist under low shear ({dot {gamma}} {<=} 30 s{sup -1}). However, at higher shear rates ({dot {gamma}} = 360 s{sup -1}), the shear acts to dramatically order and align the smectic layers into a single {open_quotes}a{close_quotes} orientation.

The X-Ray Surface Forces Apparatus: Structure of a Thin Smectic Liquid Crystal Film Under Confinement

An x-ray surface forces apparatus for simultaneously measuring forces and structures of confined complex fluids under static and flow conditions is described. This apparatus, combined with an intense synchrotron x-ray source, allows investigation of molecular orientations within a thin liquid crystal film confined between two shearing mica surfaces 3900 angstroms apart. The layer-forming smectic liquid crystal 8CB (4-cyano-4'-octylbiphenyl) adopted a series of distinct planar layer orientations, including the bulk flow-forbidden b orientation.

The Application of the X-Ray Surface Forces Apparatus (XSFA) to Studies of Confined Complex Fluid Systems

ABSTRACTWe report here on the application of the new technique of the X-Ray Surface Forces Apparatus (XSFA) to the study of the smectic liquid crystal 8CB (4-cyano-4′-octylbiphenyl) and a zwitterionic polyisoprene melt. The XSFA allows one to study the structure of fluid films under confinement and flow using intense synchrotron x-ray radiation. The above systems were investigated with the distances between the confining surfaces ranging from 0.4μm to a few tens of microns. Two different kinds of confining surfaces were used leading to different structural behavior of the samples as a function of the confining gap.