Molecular slip at the solid-liquid interface of an acoustic-wave sensor

Molecular slip is expected to occur at the interface between the surface of a bulk-acoustic-wave quartz chemical sensor and a liquid with which it is in contact. A new property of the interface, called the interfacial slip parameter, accounts for molecular slip. It is introduced in the theory of the sensor-liquid system by generalizing the no-slip boundary condition. The interfacial slip parameter is a complex-valued quantity which is defined as the displacement of a particle of liquid in contact with the sensor surface divided by the displacement of a particle on the surface of the sensor. The theoretical expression for the impedance of the sensor is derived in terms of the interfacial slip parameter. The impedance of the sensor is measured by the network analysis method for hydrophilic and hydrophobic surfaces in contact with water-glycerol solutions. The experimental values of impedance are fitted to the theory by nonlinear regression analysis to find the interfacial slip parameter. A mechanical model of molecular slip is devised which explains the variation of the interfacial slip parameter with the viscosity of the liquid. The results indicate that there is some slip in low-viscosity liquids, near the viscosity of water, and very little slip in liquids of high viscosity, near that of glycerol.