Mechanical properties of nanoconfined water layers are still poorly understood and continue to create controversy, despite their importance for biology and nanotechnology. We report on dynamic nanomechanical measurements of water films compressed to a few single molecular layers. We show that the mechanical properties of nanoconfined water layers change significantly with their dynamic state. In particular, we observed a sharp transition from viscous to elastic response even at extremely slow compression rates, indicating that mechanical relaxation times increase dramatically once water is compressed to less than 3-4 molecular layers. DOI: 10.1103/PhysRevLett.105.106101 PACS numbers: 68.08.Àp, 07.79.Lh, 61.30.Hn, 62.10.+s Water is the fundamental solvent of all living organisms [1] and plays a crucial role in macromolecular structure formation. In nanotribology and nanofluidics [2], the behavior of molecularly thin water films is also crucial. Yet the influence of the nanomechanical dynamics of water is largely unexplored. Nanomechanical measurements of water have produced contradictory results, including the properties of water close to hydrophobic surfaces [3], the evidence (or lack thereof) of a sharp increase in viscosity upon confinement [4][5][6][7][8], and the question of the no-slip boundary condition [9].Atomic force microscopy (AFM) and surface force apparatus measurements suggest that water layers confined between hydrophilic surfaces assume spontaneous order [4-6,10,11] and exhibit sharp increases in effective viscosity, relaxation times, and elasticity [4][5][6]. However, other measurements indicate that water under similar circumstances shows little change in effective viscosity [7]. It is also not clear if layering influences only the elastic response of the liquid or both the viscous and elastic response [5,[12][13][14][15]. Recent measurements have shown that nanoconfined liquids can exhibit sharp changes in viscoelastic properties in response to mild changes in their dynamical state [12,16,17]. To resolve these issues, it is imperative to carefully measure the elastic and viscous response of nanoconfined water layers under different dynamic conditions.We used a small-amplitude (A ¼ 0:6-1:1 A) AFM technique [18], developed in our lab, to perform linear viscoelastic measurements of molecularly confined ultrapure water layers at extremely slow loading rates (Fig. 1). Although we used ultrapure water, there could be a substantial amount of ions in solution originating from the freshly cleaved mica surface [19]. Measurements were performed far below the resonance to ascertain wellbehaved phase behavior of the cantilever motion. This ensured that phase changes corresponded to the dissipative behavior of the liquid and not the complicated phase behavior of the cantilever. The results were independent of cantilever frequency in the range of 400-970 Hz, as long as we avoided instrumental resonances (see also [12]). The load rate was controlled by the approach speed (2-14 A=s) of an atomically flat m...

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The dynamic drying process is studied in spatially heterogeneous film-forming latex suspensions across a wide range of dispersion concentrations using optical imaging techniques. Systematic changes in latex suspension concentration are found to affect lateral drying heterogeneity and surface topology. A nonmonotonic decay in contact angle is observed at the edges of drying droplets by continuously monitoring evaporation dynamics, which is quantitatively characterized by the peak strain and peak formation time. An analytical model is developed to explain the nonmonotonic contact-angle decay by considering a transient dilational stress imposed on a viscoelastic solid model for the particle network. Importantly, the latex concentration dependence of this phenomenon provides evidence for a smooth transition from fluid-line pinning to fluid-line recession behavior during drying, leading to ringlike to volcanolike deposition patterns, respectively. Using experimental data for drying heterogeneity, we quantitatively explore the influence of Marangoni flow and capillary pressure on drying behavior. Moreover, our results show that latex concentration and particle packing can also be strategically used to reduce contact-line friction, thereby affecting fluid-line recession. Taken together, these results show that studying latex suspensions in seemingly simple droplet geometries provides insight into the emergent spatially heterogeneous viscoelastic properties during film formation.

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