Understanding and manipulating fluids at the nanoscale is a matter of growing scientific and technological interest. Here we show that the viscous shear forces in nanoconfined water can be orders of magnitudes larger than in bulk water if the confining surfaces are hydrophilic, whereas they greatly decrease when the surfaces are increasingly hydrophobic. This decrease of viscous forces is quantitatively explained with a simple model that includes the slip velocity at the water surface interface. The same effect is observed in the energy dissipated by a tip vibrating in water perpendicularly to a surface. Comparison of the experimental data with the model shows that interfacial viscous forces and compressive dissipation in nanoconfined water can decrease up to two orders of magnitude due to slippage. These results offer a new understanding of interfacial fluids, which can be used to control flow at the nanoscale.

We present an Atomic Force Microscopy (AFM) based method to investigate the rheological properties of liquids confined within a nanosize gap formed by an AFM tip apex and a solid substrate. In this method, a conventional AFM cantilever is sheared parallel to a substrate surface by means of a lock-in amplifier while it is approaching and retracting from the substrate in liquid. The normal solvation forces and lateral viscoelastic shear forces experienced by the AFM tip in liquid can be simultaneously measured as a function of the tip-substrate distance with sub-nanometer vertical resolution. A new calibration method is applied to compensate for the linear drift of the piezo transducer and substrate system, leading to a more precise determination of the tip-substrate distance. By monitoring the phase lag between the driving signal and the cantilever response in liquid, the frequency dependent viscoelastic properties of the confined liquid can also be derived. Finally, we discuss the results obtained with this technique from different liquid-solid interfaces. Namely, octamethylcyclotetrasiloxane and water on mica and highly oriented pyrolytic graphite.

Obstructive sleep apnea, (OSA) is a sleep disorder traditionally associated with high blood pressure, cardiovascular disease, and/or obesity in which an obstruction of the upper respiratory airflow occurs in patients. This airflow disruption is repeated resulting in a cyclical breathing pattern that leads to frequent arousals during sleep. Epidemiological analyses of OSA have found that the disorder occurs more frequently in men than women. Yet, the severity of OSA-associated comorbidities are worse in women than in men upon initial OSA-diagnosis. The primary reasons for sex differences in OSA are thought to be associated with variations seen in normal sleep between men and women, distinctions in the clinical manifestations of sleep disturbances, and in the prevalence of risk factors for sleep disorders. However, it has also been suggested that this sex difference is due to an under-diagnosis of OSA in women during the early stages of the disease, as initial presentation of female patients with OSA has often been interpreted as depression and/or insomnia. Due to the risk factors associated with OSA, heart disease as the leading cause of death in both men and women in the United States, and the commonality of sex differences in sleep disorders, we examined the literature to determine what may contribute to these sex differences in OSA. This brief review summarizes what may be the causes of sex differences in normal sleep, sex differences that are associated with OSA, and whether this sex difference may be primarily due to an under-diagnosis of OSA in women.