Flow of Newtonian fluids with different polarity in micro scale

Abstract

Due to the great development of medicine, micro-electro mechanical systems (MEMS) and seepage theory of unconventional oil and gas, fluid flow especially liquid flow in micro scale is getting more and more attention. In micro scale, flow is affected by many factors which are negligible in macro scale, thus shows different characteristics compared to conventional theory, such as Navier-Stokes equations. Aiming to study the flow characteristics of deionized water and isopropanol in micro scale, a visual experimental device is established. The direct objective is to measure flow rate of experimental fluids under different pressure gradient, in microtubes with different radius. The pressure is measured directly by pressure transducer. Flow rate is measured indirectly by measuring the displacement of the meniscus in metering tube in a measureable time interval. The tubes are made of fused silica. We choose deionized water and isopropanol as experimental fluids because they both have hydroxide radical. The polarity of water and isopropanol is 10.2 and 4, respectively. The pressure gradient ranges 0.04–24.8 MPa/m. The radius of tubes range 1–7.5 μm. The length of experimental tubes ranges 8–49 cm. Uncertainty of all experiments is less than 3%. The results showed that fluid flow in microtubes with radius smaller than 7.5 μm did not conform to Navier-Stokes equations. Experimental flow rate is lower than theoretical value calculated by Poiseuile equation. The flow rate deviation ranges 7.53%–70.75%. Flow rate-pressure gradient curves showed nonlinear characteristics in all experiments. Based on the boundary layer theory of laminar flow, the thickness of boundary layer is calculated. Relationship between boundary layer thickness and its influencing factors is investigated. In this study, thickness of boundary layer is positively correlated with polarity and viscosity, while it is negatively correlated with pressure gradient and radius of the tube. There was a power function relationship between the thickness of boundary layer and pressure gradient. Coefficients and the powers are regressed for all eight experiments, and Poiseuille equation is corrected based on the thickness function of boundary layer. The fitting result is quite good. Polarity is an important factor affecting fluid flow in micro scale. In order to compare the flow characteristics of fluids with different polarity, the impact of viscosity is eliminated. The comparison shows that the stronger the polarity, the thicker the corrected thickness of boundary layer. In conclusion, fluid flow in microtubes is affected by pressure gradient, radius, viscosity and polarity of fluid and solid wall molecules. Under low pressure gradient, micro flow shows nonlinear characteristics, which declines as pressure gradient increases since the efficient flow space is enlarged continuously. When the pressure gradient is high, the thickness of boundary layer is constant, and the nonlinear flow converses to linear flow. The reason is that thickness of boundary layer decreases with pressure gradient. As for fluid solid interaction, it is the inter-attractive between –OH in fluid molecules and polarized silica wall. As a result, a stagnant fluid layer is formed near the wall. For fluid with greater polarity, the effect of fluid-solid interaction on micro flow is greater, and the normalized thickness of boundary layer is larger, the upper limit of nonlinear flow is higher as well.