Phase field simulation of liquid filling on grooved surfaces for complete, partial, and pseudo-partial wetting cases.

We develop and harness a phase field simulation method to study liquid filling on grooved surfaces. We consider both short-range and long-range liquid-solid interactions, with the latter including purely attractive and repulsive interactions as well as those with short-range attraction and long-range repulsion. This allows us to capture complete, partial, and pseudo-partial wetting states, demonstrating complex disjoining pressure profiles over the full range of possible contact angles as previously proposed in the literature. Applying the simulation method to study liquid filling on grooved surfaces, we compare the filling transition for the three different classes of wetting states as we vary the pressure difference between the liquid and gas phases. The filling and emptying transitions are reversible for the complete wetting case, while significant hysteresis is observed for the partial and pseudo-partial cases. In agreement with previous studies, we also show that the critical pressure for the filling transition follows the Kelvin equation for the complete and partial wetting scenarios. Finally, we find the filling transition can display a number of distinct morphological pathways for the pseudo-partial wetting cases, as we demonstrate here for varying groove dimensions.

Solid with infused reactive liquid (SWIRL): A novel liquid-based separation approach for effective CO2 capture.

Economical CO2 capture demands low-energy separation strategies. We use a liquid-infused surface (LIS) approach to immobilize reactive liquids, such as amines, on a textured and thermally conductive solid substrate with high surface-area to volume ratio (A/V) continuum geometry. The infused, micrometer-thick liquid retains that high A/V and directly contacts the gas phase, alleviating mass transport resistance typically encountered in mesoporous solid adsorbents. We name this LIS class "solid with infused reactive liquid" (SWIRL). SWIRL-amine requires no water dilution or costly mixing unlike the current liquid-based commercial approach. SWIRL-tetraethylenepentamine (TEPA) shows stable, high capture capacities at power plant CO2 concentrations near flue gas temperatures, preventing energy-intensive temperature swings needed for other approaches. Water vapor increases CO2 capacity of SWIRL-TEPA without compromising stability.

Predicting Hemiwicking Dynamics on Textured Substrates.

The ability to predict liquid transport rates on textured surfaces is key to the design and optimization of devices and processes such as oil recovery, coatings, reaction-separation, high-throughput screening, and thermal management. In this work we develop a fully analytical model to predict the propagation coefficients for liquids hemiwicking through micropillar arrays. This is carried out by balancing the capillary driving force and a viscous resistive force and solving the Navier-Stokes equation for representative channels. The model is validated against a large data set of experimental hemiwicking coefficients harvested from the literature and measured in-house using high-speed imaging. The theoretical predictions show excellent agreement with the measured values and improved accuracy compared to previously proposed models. Furthermore, using lattice Boltzmann (LB) simulations, we demonstrate that the present model is applicable over a broad range of geometries. The scaling of velocity with texture geometry, implicit in our model, is compared against experimental data, where good agreement is observed for most practical systems. The analytical expression presented here offers a tool for developing design guidelines for surface chemistry and microstructure selection for liquid propagation on textured surfaces.

Experimental Correlation Between Interfacial Water Structure and Mineral Reactivity.

We present an experimental demonstration of the effect of solvent structure on the interfacial reactivity of the silica/water interface using in situ vibrational Sum-frequency Generation (vSFG) spectroscopy. The response of the molecular arrangement of the interfacial solvent to the presence of cations is pH dependent with the highest sensitivity at neutral pH, relevant to geochemical and biological environments. The pH-dependent changes in vSFG spectra are in excellent correlation with the enhancement of quartz dissolution in salt water, which was hypothesized by Dove et al. to be due to changes of the interfacial solvent structure at the silica surface. vSFG provides mechanistic insights into silica dissolution and sheds light on the role of ions in altering interfacial solvent ordering, which has implications in fields ranging from protein-water interactions to oil recovery.

Interfacial structure and melting temperature of alcohol and alkane molecules in contact with polystyrene films.

Infrared-visible sum-frequency-generation spectroscopy (SFG) is used to investigate the interfacial structure of hexadecanol (C16H33OH) and heneicosane (C21H44) in contact with polystyrene films (PS) spin coated on a sapphire substrate. The interfacial structure of hexadecanol is very different from heneicosane in contact with PS. In the crystalline state, the hexadecanol molecules are oriented with the C-C-C axis parallel to the surface plane in contact with PS. For the crystalline heneicosane/PS interface, the SFG spectra are very similar to those observed for molecules oriented with the symmetry axis of the methyl groups parallel to the surface normal. The structure of both hexadecanol (or heneicosane) and the phenyl groups changes sharply at the melting temperature of hexadecanol (or heneicosane). Upon heating the hexadecanol/PS sample above the glass transition temperature (T(g)) of PS, the hexadecanol molecules penetrate through the PS film and adsorb on the sapphire substrate. The adsorbed hexadecanol molecules are oriented with the symmetry axis of the methyl groups parallel to the surface normal. The structure of the PS molecules at the sapphire interface is different because the PS phenyl groups are now in contact with the hydrophobic tails of the hexadecanol molecules, rather than the hydrophilic sapphire substrate. The adsorbed hexadecanol molecules do not disorder at the bulk melting temperature of hexadecanol. In comparison, no adsorption of heneicosane molecules next to sapphire interface upon annealing was observed. The differences between the adsorption of hexadecanol and heneicosane can be explained by the preferential interactions between the hydroxyl groups of the alcohol and hydrophilic sapphire substrate.

Sum frequency generation studies of surfaces of high-surface-area powdered materials.

We report the first observation of sum frequency generation (SFG) photons on high-surface-area powders, critically important materials in heterogeneous catalysis. We utilize SFG in total internal reflection (TIR) geometry and show that the TIR-SFG approach markedly reduces the destructive interference associated with nonlinear optical spectroscopy of small particle surfaces, making SFG studies of high-surface-area powders possible. The index of refraction of materials and their distance from the TIR-SFG prism are key parameters in generating and detecting the sum frequency signal. We find that TIR-SFG is highly sensitive to capillary condensation. To demonstrate the capability of the TIR-SFG technique, we measure the thermodynamics of methanol adsorption and desorption on high-surface-area SiO2.

Molecular structure of an alkyl-side-chain polymer-water interface: origins of contact angle hysteresis.

A new and direct approach to verify surface heterogeneity as the microscopic origin of contact-angle hysteresis is demonstrated. IR-visible sum-frequency-generation spectroscopy (SFG) was used to selectively probe the molecules at the interface of an alkyl-side-chain polymer [poly(vinyl n-octadecyl carbamate-co-vinyl acetate)] with water. The spectra indicate that in contact with water, the polymer surface is heterogeneous (having areas of differing surface energies). This evidence of surface heterogeneity supports the hysteresis observed in the advancing and receding contact angles of the polymer surface with water. The same measurements made for the chemically and structurally similar surface of an octadecyltrichlorosilane self-assembled monolayer indicates a homogeneous surface at the water interface. In this case, contact-angle hysteresis measurements implicate surface roughness as the cause of hysteresis. Atomic force microscopy measurements of roughness for these surfaces further support our conclusions. The polymer-water interface was probed using SFG at above-ambient temperatures, and an order-to-disorder transition (ODT) of alkyl side chains at the interface was observed, which closely follows the melting of crystalline side chains in the bulk. This transition explains the increased wettability of the polymer, by water, when the temperature is raised above the bulk melting temperature. Furthermore, the irreversibility of this ODT suggests that the disordered polymer-water interface is the thermodynamic equilibrium state, whereas the before-heating structure of this interface is a kinetically hindered metastable state.

Infrared-visible sum frequency generation spectroscopic study of molecular orientation at polystyrene/comb-polymer interfaces.

Surface-sensitive infrared-visible sum frequency generation spectroscopy (SFG) in total internal reflection geometry has been used to study the structure of poly(vinyl n-octadecyl carbamate-co-vinyl acetate) (PVNODC) or poly(octadecyl acrylate) (PA-18) in contact with a deuterated or hydrogenated polystyrene (dPS or hPS) layer. SFG spectra from the PVNODC (or PA-18)/hPS interface show methyl and methylene peaks corresponding to PVNODC (or PA-18) and phenyl peaks corresponding to the PS. Analysis suggests that the methyl groups are tilted at angles less than 30 degrees with respect to the surface normal. The presence of a strong methylene peak suggests the PVNODC alkyl side chains contain more gauche defects at the PS/PVNODC interface in comparison to PVNODC (or PA-18)/air interfaces. On heating, the SFG intensity from the PS/PA-18 interface drops sharply near the bulk melting temperature (T(m)) of PA-18. Interestingly, a similar drop in SFG signal is also observed for the PS phenyl groups. This demonstrates the ability of the phenyl group at the PS/PA-18 interface to rearrange itself upon the solid-to-liquid transition of the PA-18 alkyl side chain, at a temperature well below the bulk PS glass transition temperature. For PS/PVNODC interfaces, the drop in SFG intensity is gradual and in agreement with the three broad transitions of PVNODC observed in the bulk.