Investigation on surface tension of nanoscale water

Abstract

Surface tension plays an important role in nanoscale vapor-liquid phase transition such as condensation, evaporation of nano water meniscus or bubble. The measured and theoretical values due to the formation of it have been disagreed and it was been debated whether the surface tension defined in macroscopic scale is applied the same in nanoscale. However surface tension remain large unknown and imperatively need to be better understood. The capillary rise method is the oldest and most typical way to determine surface tension among several methods of surface tension measurements. We demonstrated a hybrid force measurement system to obtain the capillary force directly so that we could investigate surface tension in nanoscale. The hybrid system of a quartz tuning fork (QTF)-based, amplitude modulation atomic force microscope (AM-AFM) and a microelectromechanical system (MEMS) measures simultaneously the dynamic and static forces for nanoconfined water bridge formed between hydrophilic surfaces. This system formed a stable water bridge at a certain distance and obtained the absolute capillary force besides dynamic properties of the water meniscus. Moreover, the MEMS force sensor provides the additional force information, verifies the AFM measurement, allows accessibility of multiple driving frequencies, and expresses the stress or strain of the confined nanoscale water column. Stress and strain measurements revealed that the nanoconfined water meniscus has the Young's modulus comparable to that of soft rubber or soft tissues and the low loss tangent value close to the solid-like behavior. We calculated the capillary force on the basis of Young-Laplace equation and compared the analyzed values with the experimental findings. This numerical evaluation of Young-Laplace equation enables the surface tension in nanoscale to be investigated closely. Therefore, we found that the surface tension value of water meniscus in nanoscale significantly decreases less than 20 % of the one in bulk water. This observations may resolve the existing discrepancies between experiments and theories associated with liquid-vapor transition effects in nanoconfined water and allow a contribution to the applications such as self-assembly of bio-molecules and efficient design of nanomaterials.