Energy dissipation of nanoconfined hydration layer: long-range hydration on the hydrophilic solid surface

Probing the shear viscoelasticity of a nanoscale ionic liquid meniscus

Ionic liquids (ILs) are emerging as novel solvents that exhibit peculiar mechanical properties in the form of thin films on metal surfaces under normal pressure. However, the mechanical properties of ILs in the form of nano-meniscus have not been analyzed yet. Here, we investigate the shear viscoelasticity of a single IL meniscus at the nanoscale. To characterize the shear rheological properties of ILs, we employ a quartz tuning fork-based atomic force microscope, conduct dynamic force spectroscopy, and analyse shear properties using the non-Newtonian-Maxwell model. The elastic response of the IL nanomeniscus is found to be about 25 times higher than that of the bulk IL bridge, whereas the viscous responses are similar. In addition, by conducting shear velocity-dependent measurements, we find that the IL meniscus shows nonlinear rheological behaviours. Interestingly, we observe that the relaxation time of the IL increases at a tip-substrate distance of about 60 nm.

Atomic Force Microscopy to Identify Dehydration Temperatures for Small Volumes of Active Pharmaceutical Ingredients

The environmental conditions associated with changing the hydration state of active pharmaceutical ingredients (API) are crucial to understanding their stability, bioperformance, and manufacturability. Identifying the dehydration event using < 1μg of material is an increasingly important challenge. Atomic Force Microscopy indentation mapping is implemented at controlled temperatures between 25-100°C, for nanoscale volumes of hydrated APIs exhibiting distinct dehydration behavior and anhydrous APIs as controls. For caffeine hydrate and azithromycin dihydrate, the relative mechanical modulus increases ~10-fold at dehydration temperatures. These are confirmed by conventional macroscopic measurements including Variable Temperature Powder X-ray Diffraction, Thermogravimetric Analysis, and Differential Scanning Calorimetry. Conversely, no such mechanical transition is observed for anhydrous ibuprofen or a proprietary anhydrous compound. AFM-based mechanical mapping is therefore demonstrated for small-volume determination of temperature-induced solid-state dehydration events, which may enable spatially or temporally mapping for future studies of dehydration mechanisms and kinetics, as a function of commercially relevant nanoscale heterogeneities.

Molecular features of hydration layers probed by atomic force microscopy

Structurally-ordered layers of water are universally formed on a solid surface in aqueous solution or under ambient conditions. Although such hydration layers are commonly probed via atomic force microscopy (AFM), the current understanding on how the hydration layers manifest themselves in an AFM experiment is far from complete. By using molecular dynamics simulation, we investigate the hydration layers on a hydrophilic or hydrophobic surface probed by a nanoscale tip. We study the density and molecular orientation of water, the free energy, and the force on the tip by varying the tip-surface distance. The force-distance curve oscillates due to the transition between the mono-, bi-, and tri-layers of water confined between the tip and the surface. If both the tip and the surface are hydrophobic, water confined between the tip and the surface evaporates due to the dewetting transition, giving a hydrophobic force without oscillation. The periodicity of oscillation in the force differs from the structural periodicity of water. With a close proximity of the tip, the molecular dipoles align parallel to the surface, regardless of whether the tip and the surface are hydrophilic or hydrophobic.

Adhesive force measurement of steady-state water nano-meniscus: Effective surface tension at nanoscale

When the surface of water is curved at nanoscale as a bubble, droplet and meniscus, its surface tension is expected to be smaller than that of planar interface, which still awaits experimental studies. Here, we report static and dynamic force spectroscopy that measures the capillary force of a single nanoscale water meniscus at constant curvature condition. Based on the Young-Laplace equation, the results are used to obtain the effective surface tension (ST) of the meniscus, which decreases to less than 20% of the bulk value at the radius-of-curvature (ROC) below 25 nm, while indicating the bulk behaviour above ~130 nm ROC. Interestingly, such a possibility provides a qualitative resolution of the unsettled discrepancies between experiments and theories in the thermodynamic activation processes for the mentioned three types of nano-curvatured water. Our results may not only lead to development of microscopic theories of ST as well as further experimental investigations, but also help better understanding of the ST-induced nanoscale dynamics such as cluster growth or protein folding, and the ST-controlled design of nano-biomaterials using the nano-meniscus.

Anomalous Behavior of Ultra-Low-Amplitude Capillary Waves. A Glimpse of the Viscoelastic Properties of Interfacial Water?

We investigate, both theoretically and by a differential interferometric technique, the behavior of large-wavelength capillary waves (of the order of 10^-4 m) selectively excited at the surface of drops and bubbles with typical eigenfrequencies of the order of 10² Hz. The resonance peaks of gas bubbles or hydrocarbon drops in water (radius less than 1 mm) highlight anomalously small dissipation in the region of ultralow (sub-nanometric) oscillation amplitudes, reaching a plateau at higher amplitudes. This is in sharp contrast to the usual oscillating systems, where an anomalous behavior holds at large amplitudes alone. Dissipation is strongly dependent on the excited vibrational modes and, in spite of remarkable numerical differences, water-vapor and water-hydrocarbon interfaces exhibit the same overall trend. A phenomenological model was developed, based on the assumption that water possesses a threshold viscoelasticity, above which it behaves like a regular viscous fluid. The well-known Deborah number was then estimated within the anomalous region and found to lie in the range of viscoelastic fluids. In agreement with previous studies of nanohydrodynamics (e.g., atomic force microscopy measurements with sub-nanometric tip motions), the present one lends support to the idea that every self-aggregating fluid exhibits yield stress behavior, including classical Newtonian fluids like water. The essential requirement is that the applied perturbation lie below a critical threshold, above which viscous behavior is recovered. Our differential interferometric technique seems particularly suitable for this type of studies, as it allows measurement of long-wavelength capillary waves with sub-nanometric resolution on the oscillation amplitudes.

Young's modulus of nanoconfined liquids?

In material science, bioengineering, and biology, thin liquid films and soft matter membranes play an important role in micro-lubrication, ion transport, and fundamental biological processes. Various attempts have been made to characterize the elastic properties, such as Young's modulus, of such films using Hertz theory by incorporating convoluted mathematical corrections. We propose a simple way to extract tip-size independent elastic properties based on stiffness and force measurement through a spherical tip on a flat surface. Using our model, the Young's modulus of nanoconfined, molecularly-thin, layers of a model liquid TEHOS (tetrakis 2-ethylhexoxy silane) and water were determined using a small-amplitude AFM. This AFM can simultaneously measure the stiffness and forces of nanoscale films. While the stiffness scales linearly with the tip radius, the measured Young's modulus essentially remains constant over an order of magnitude variation in the tip radius. The values obtained for the elastic modulus of TEHOS and water films on the basis of our method are significantly lower than the confining surfaces' elastic moduli, in contrast with the uncorrected Hertz model, suggesting that our method can serve as a simple way to compare elastic properties of nanoscale thin films as well as to characterize a variety of soft films. In addition, our results show that the elastic properties (elastic modulus) of nanoconfined liquid films remain fairly independent of increasing confinement.

Probing nonlinear rheology layer-by-layer in interfacial hydration water

Viscoelastic fluids exhibit rheological nonlinearity at a high shear rate. Although typical nonlinear effects, shear thinning and shear thickening, have been usually understood by variation of intrinsic quantities such as viscosity, one still requires a better understanding of the microscopic origins, currently under debate, especially on the shear-thickening mechanism. We present accurate measurements of shear stress in the bound hydration water layer using noncontact dynamic force microscopy. We find shear thickening occurs above ∼ 10(6) s(-1) shear rate beyond 0.3-nm layer thickness, which is attributed to the nonviscous, elasticity-associated fluidic instability via fluctuation correlation. Such a nonlinear fluidic transition is observed due to the long relaxation time (∼ 10(-6) s) of water available in the nanoconfined hydration layer, which indicates the onset of elastic turbulence at nanoscale, elucidating the interplay between relaxation and shear motion, which also indicates the onset of elastic turbulence at nanoscale above a universal shear velocity of ∼ 1 mm/s. This extensive layer-by-layer control paves the way for fundamental studies of nonlinear nanorheology and nanoscale hydrodynamics, as well as provides novel insights on viscoelastic dynamics of interfacial water.

Nanoscale Trapping and Squeeze-Out of Confined Alkane Monolayers

We present combined force curve and conduction atomic force microscopy (AFM) data for the linear alkanes CnH2n+2 (n = 10, 12, 14, 16) confined between a gold-coated AFM tip and a graphite surface. Solvation layering is observed in the force curves for all liquids, and conduction AFM is used to study in detail the removal of the confined (mono)layer closest to the graphite surface. The squeeze-out behavior of the monolayer can be very different depending upon the temperature. Below the monolayer melting transition temperatures the molecules are in an ordered state on the graphite surface, and fast and complete removal of the confined molecules is observed. However, above the melting transition temperature the molecules are in a disordered state, and even at large applied pressure a few liquid molecules are trapped within the tip-sample contact zone. These findings are similar to a previous study for branched alkanes [ Gosvami Phys. Rev. Lett. 2008, 100, 076101 ], but the observation for the linear alkane homologue series demonstrates clearly the dependence of the squeeze-out and trapping on the state of the confined material.

Shear-stress function approach of hydration layer based on the Green-Kubo formula

We present the analytic expression of the stress correlation (SC) function for the ubiquitous hydration water layer (HWL) using the Green-Kubo equation and the shear modulus of HWL. The SC function is then experimentally obtained by measuring the viscoelastic properties of HWL using shear-mode dynamic force spectroscopy. Interestingly, the SC changes sign from positive to negative as the HWL thickness increases, where the shear stresses acting on the HWLs bound to two nearby surfaces are out of phase. We also suggest that the repulsive hydration force originates from the SC of HWL. Our results provide the first demonstration of the microscopic understanding of the HWL viscoelasticity and may allow a deeper insight on the HWL dynamics as well as the complex liquids.