Optical monitoring of micro fluidic flow conditions, employing surface plasmon resonance sensing

Abstract

Near surface flow conditions in micro / nano channels are difficult to assess by experiment. Associated flow and materials transport properties yet are poorly understood, and inconclusive. A novel optical tool for determination of the near wall transport parameters in a micro channel, utilizing the evanescent field of a surface plasmon resonance (SPR) set-up as the sensing probe, is presented. The method is based on the transient flow response due to convective diffusion, in absence of specific adsorption. An approximately step-function type temporal solute concentration variation serves as the input signal. The time varying optical response of a surface plasmon resonance sensor, acting as an integral part of a micro channel, has been taken as the output signal. It provides the flow dependent change of the solute concentration in the channel within the optical detection and near wall distance interval 0 < d < 0.5 μm. The temporal signal evolution and response time, until an initially plain aqueous solution is replaced by the solute, varies inversely with solute concentration and flow rate. In the asymptotic limits, the near wall forced convective and diffusive channel transit times, along with the associated velocities, can be extracted and separated. The validity of the scaling relation for Fickian diffusive transport has been confirmed by experiment. Furthermore, solute diffusion coefficients in the solution can be determined with high precision. Convective near wall flow reveals a distorted parabolic flow profile and indicates relaxation of the no-slip condition, and presence of slip flow. Comparison with Poiseuille flow conditions, calculated for the same channel geometry, reveals a threshold characteristic. The experimental data allow estimations of the slip length / slip velocity that varies non-linearly with the macroscopic flow rate in the channel.