Rheology of nano-confined water using novel Atomic Force Microscope

Abstract

How does water behave when it is con ned to slits and pores of the order of its own molecular size?? This question has been asked and debated for decades. The present thesis is a body of work towards the development of a novel technique to answer the above question. We use a quartz tuning fork as a force sensor as it can detect forces as weak as a pico-Newton. We attach a bre along the length of one of the arms of the tuning fork such that the sharp tip of the bre protrudes beyond the length of the arm. The other arm of the fork is xed to a piezo actuator. The bre tip is then immersed into water in a liquid cell, and is brought close to the bottom surface of the cell, which is atomically smooth freshly cleaved mica. Water is con ned between the tip and mica surface as the tip is brought within a few nanometres of the surface. With the help of the actuator piezo the tuning fork is oscillated and a shear force is applied to the con ned water column. By recording the change in amplitude and phase of the tuning fork, we get the shear response of the liquid column. Viscoelastic behaviour was observed followed by shear thinning as we approached towards the surface. It is also seen that the range at which these behaviours are observed depends upon the shear frequency and the shear rate applied. This behaviour closely resembles to liquids near their critical points. Further work is being done to verify this analogy between con ned liquid and liquid near its critical temperature. If veri ed this will consolidate all the data so far in the literature and explain the seemingly contradicting results.