Joan E. Curry

Thermodynamics of a fluid confined to a slit pore with structured walls

In this article we extend our previous thermodynamic analysis of films confined to slit pores with smooth walls (i.e., plane–parallel solid surfaces without molecular structure) to the situation in which the walls themselves possess structure. Structured‐wall models are frequently employed to interpret experiments performed with the surface forces apparatus (SFA), in which thin films (1–10 molecular diameters thick) are subjected to shear stress by moving the walls laterally over one another at constant temperature, chemical potential, and normal stress or load. The periodic structure of the walls is reflected in a periodic variation of the shear stress with the lateral alignment (i.e., shear strain) of the walls. We demonstrate by means of a solvable two‐dimensional model that the molecular length scale imposed by the structure of the walls precludes the derivation of a simple mechanical expression for the grand potential analogous to that which holds in the smooth‐wall case. This conclusion is borne out...

Transient coexisting nanophases in ultrathin films confined between corrugated walls

Grand‐canonical Monte Carlo and microcanonical molecular dynamics methods have been used to simulate an ultrathin monatomic film confined to a slit‐pore [i.e., between solid surfaces (walls)]. Both walls comprise atoms rigidly fixed in the face centered cubic (100) configuration; one wall is smooth on a nanoscale and the other is corrugated (i.e., scored with regularly spaced rectilinear grooves one to several nanometers wide). Properties of the film have been computed as a function of the lateral alignment (registry), with the temperature, chemical potential, and distance between the walls kept constant. Changing the registry carries the film through a succession of equilibrium states, ranging from all solid at one extreme to all fluid at the other. Over a range of intermediate registries the film consists of fluid and solid portions in equilibrium, that is fluid‐filled nanocapillaries separated by solid strips. The range of registries over which such fluid–solid equilibria exist depends upon the width of the grooves and the frequency of the corrugation. For grooves of width comparable to the range of the interatomic potential, fluid and solid phases cease to coexist. In the limit of very wide grooves the character of the film is similar to that of the film confined by strictly smooth walls. The rich phase behavior of the confined film due to the coupling between molecular (registry) and nano (corrugation) scales has obvious implications for boundary lubrication.

Binary mixtures of simple fluids in structured slit micropores

The grand canonical Monte Carlo method is used to study a binary mixture of Lennard-Jones atoms confined to an atomically structured slit micropore which is in thermodynamic equilibrium with its bulk phase counterpart. In one example, the mixture consists of atoms of two distinct sizes, but with the same minimum depth in potential energy. In another example a binary mixture of different size atoms is again considered, but in the latter case the larger atom has a deeper potential energy minimum. Three mechanisms are found which influence selective adsorption of a mixture species: (i) liquid-like fluid layering, (ii) inplane solid-like ordering and (iii) molecular sieving. The large atoms are completely eliminated from the pore when the wall separation is physically too small for the large atoms to fit, or when both species physically fit in the pore and the small component epitaxially aligns with the surface or freezes. Complete elimination of the small species is not observed. A significant excess of larg...

Structure of a model lubricant in a mica slit pore

Grand canonical ensemble Monte Carlo computer simulations are used to study a monolayer octamethylcyclotetrasiloxane (OMCTS) film confined between micalike surfaces to better understand the relationship between fluid properties and the atomic structure of the surfaces. OMCTS packs as a well ordered hexagonally close-packed film for all relative alignments of the surfaces. The orientation of the fluid lattice is guided by the mica surfaces. The angle between the symmetry planes of the fluid and the hexagonal oxygen rings on the mica surfaces is approximately 30°. The coupling between OMCTS and the confining mica surfaces is shown to be intermediate in that (1) the film sustains a shear stress and (2) the hexagonal symmetry of the film becomes distorted when the surfaces are out of registry but the film does not melt. Incrementally shifting one of the surfaces laterally leads to periodic distortions in the fluid lattice that are caused by the fluid being stretched to accommodate variations in the pore space...

Evaporative deposition patterns of bacteria from a sessile drop: effect of changes in surface wettability due to exposure to a laboratory atmosphere.

Evaporative deposition from a sessile drop is a simple and appealing way to deposit materials on a surface. In this work, we deposit living, motile colloidal particles (bacteria) on mica from drops of aqueous solution. We show for the first time that it is possible to produce a continuous variation in the deposition pattern from ring deposits to cellular pattern deposits by incremental changes in surface wettability which we achieve by timed exposure of the mica surface to the atmosphere. We show that it is possible to change the contact angle of the drop from less than 5 degrees to near 20 degrees by choice of atmospheric exposure time. This controls the extent of drop spreading, which in turn determines the architecture of the deposition pattern.

Intercalation of Trichloroethene by Sediment-Associated Clay Minerals

The objective of this research was to examine the potential for intercalation of trichloroethene (TCE) by clay minerals associated with aquifer sediments. Sediment samples were collected from a field site in Tucson, AZ. Two widely used Montmorillonite specimen clays were employed as controls. X-ray diffraction, conducted with a controlled-environment chamber, was used to characterize smectite interlayer d-spacing for three treatments (bulk air-dry sample, sample mixed with synthetic groundwater, sample mixed with TCE-saturated synthetic groundwater). The results show that the d-spacing measured for the samples treated with TCE-saturated synthetic groundwater are larger (~26%) than those of the untreated samples for all field samples as well as the specimen clays. These results indicate that TCE was intercalated by the clay minerals, which may have contributed to the extensive elution tailing observed in prior miscible-displacement experiments conducted with this sediment.

Computer simulation of anisotropic diffusion in monolayer films in mica slit pores

Molecular dynamics computer simulations are used to study diffusion of monolayer octamethylcyclotetrasiloxane and cyclohexane films confined between atomically structured uncharged mica surfaces. Diffusion parallel to the walls is found to be anisotropic due to the influence of the atomically structured surfaces. If the surfaces are aligned perfectly the fluid occupies isolated regions of the pore space and diffusion is the same in all lateral directions and is a minimum. If one of the surfaces is shifted laterally in the x-direction by one-half unit cell diffusion is enhanced in the x-direction along conduits formed by the overlapping potential energy fields of the surfaces. This work augments earlier work by the authors and confirms that diffusion of a confined simple fluid in a nanoscale slit-pore is tunable in terms of direction and magnitude through control of the relative alignment of the surfaces.

Nanophase coexistence and sieving in binary mixtures confined between corrugated walls

The grand canonical Monte Carlo method is used to study a binary mixture of Lennard‐Jones atoms confined to a corrugated slit micropore which is in thermodynamic equilibrium with its bulk phase counterpart. The micropore has atomically structured walls; one of the which possesses nanoscale structure in the form of rectilinear grooves (corrugation). The grooved surface divides the confined fluid film into two strip shaped regions, that inside and that outside the grooves. Transverse solidlike order in the film gives rise to shear stress. Transverse order coupled with packing restrictions give rise to a difference between the pore and bulk fluid mixture compositions. Solidlike order may appear within the grooves only, outside the grooves only, or in both regions simultaneously. As the relative alignment of the walls is shifted the pore fluid undergoes freeze–thaw cycles in one or both regions with associated changes in the shear stress and pore fluid composition. The degree of transverse order in the film i...

Structure in confined fluids: Phase separation of binary simple liquid mixtures

One- to five-layer cyclohexane and octamethyltetracyclosiloxane(OMCTS) films confined between mica-like surfaces are studied to elucidate changes in the lattice type and composition of the films. Grand canonical ensemble Monte Carlo computer simulations are used to study the laterally confined film. In contrast to previous studies, solid-like order is induced primarily by the strong fluid-solid interaction and is largely a function of pore width. Solid-like order within the layers causes the composition of the pore fluid to shift from the bulk composition, favoring either cyclohexane or OMCTS, depending on the pore width. A shift in the relative alignment of the surfaces perturbs the solid-like fluid structure but does not cause the sudden shear melting transition associated with epitaxial alignment of the fluid atoms with the surface.

Fate of nickel ion in (II-III) hydroxysulphate green rust synthesized by precipitation and coprecipitation

In order to investigate the efficiency of sulfate green rust (GR2) to remove Ni from solution, GR2 samples were synthesized under controlled laboratory conditions. Some GR2 samples were synthesized from Fe(II) and Fe(III) sulfate salts by precipitation. Other samples were prepared by coprecipitation, of Ni(II), Fe(II) and Fe(III) sulfate salts, i.e., in the presence of Ni. In another sample, Ni(II) sulfate salt was added to pre-formed GR2. After an initial X-ray diffraction (XRD) characterization all samples were exposed to ambient air in order to understand the role of Ni in the transformation of the GR2 samples. XRD was repeated after 45 days. The results showed that Nious GR2 prepared by coprecipitation is isomorphous to Ni-free GR2, i.e. Ni is incorporated into the crystalline structure. Fe(II) was not replaced by Ni(II) in the crystalline structure of GR2 formed prior to exposure to solution-phase Ni. This suggests Ni was adsorbed to the GR2 surface. Sulfate green rust is more efficient in removing Ni from the environment by coprecipitation.