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

Abstract

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.

Effect of line tension on axisymmetric nanoscale capillary bridges at the liquid-vapor equilibrium

“…Since the cosine is always positive, the substrate must be hydorophilic to form a catenary capillary bridge in Eq. (24). When, the line tension is positive, the cosine of Young's contact angle θ Y can be larger than 1 (cos θ Y > 1) as shown in Fig.…”

“…Recently, there appear the normal capillary force measurements of nanoscale bridges [16,24], which are interpreted by the size-or curvature-dependent liquid-vapor surface tension of nanoscale liquids using Tolman's formula [55,56]. As shown in Fig.…”

“…Then, the capillary bridge can easily form, which is the reason why the capillary condensation is so ubiquitous. Therefore, various theoretical [1,2,18,19], numerical [20][21][22], and experimental [16,23,24] studies have been conducted by many engineers and scientists for many years.…”

“…After the invention of various force spectrosopy techniques such as the surface force apparatus (SFA) and the atomic force microscopes (AFM), the problem of capillary bridge, in particular, the normal capillary force by the bridge have been attracted renewed interest recently [23,25,26]. Since the nanoscale liquid bridges can easily form even in the low relative humidity, it is * Permanent address Tokyo City University, Setagaya-ku, Tokyo 158-8557, Japan; iwamatm@tcu.ac.jp (corresponding author) also a convenient tool to study the surface properties such as the size-dependence of surface tension of the nanoscale liquid bridges [16,24]. It can also serve as a testing ground of various statistical mechanical theory of nanoscale liquids [27][28][29][30][31].…”

“…However, the classical capillary theory using the macroscopic concepts such as the surface tension [17,33] and the line tension [33,34] is still useful [29,35,36]. In particular, the analytical or semi-analytical results obtained from the classical capillary theory have been useful to understand the physics of capillary phenomena and to analyze experimental results directly [16,24].…”