Viscoelastic response of Liquid at Nanoconfinement
Flow properties of confined liquids play crucial roles in a wide range of areas from biology to nanofluidics. Liquids, when confined between two surfaces that are tens of nanometers apart, exhibit unique structural, dynamic, and mechanical properties, which are significantly different from those observed in bulk. In the past, viscosity measurement of confined liquids by different techniques has resulted in contradictory findings. We have developed an experimental scheme, which has two key advantages over previous techniques used to measure shear-viscosity for liquid films with thickness of few nanometers; (i) the spring measuring the viscous drag has very high stiffness (55000 N/m), and yet force sensitivity of few nN, thus reducing the thermal noise in our measurement. (ii) the force sensing spring stays out of the liquid, and hence has a high resonance frequency and quality factor, allowing us to perform off-resonance measurements with high shear frequency (5-20 kHz) and shear rates (104- 106 s-1). Using this novel shear rheometer, we investigated the role of confinement and substrate wettability on flow properties of polar (water) and non-polar (organic) liquids on several surfaces. We observed reduction in dissipation coefficient under confinement; which is modeled with Carreau-Yasuda model of shear thinning including finite slippage. We found that for purely wetting substrate the nonlinear rheological response solely originates due to nano-confinement, whereas both wettability and confinement play crucial role in case of non-wetting substrates. Finite Element Method (FEM) simulations were performed to understand the behavior of two prongs of our force sensor (tuning fork) at off resonance frequency in air and in liquid medium. Our study helps to separate out the effects of substrate wettability and confinement on shear resistance experienced by liquids at nano-confinement. The rheological response of nano-confined liquids is intriguing and we propose that it is result of criticality with respect to degree of confinement.