Abstract
We introduce a “Rheo-chip” prototypical rheometer which is able to characterise model fluids under oscillatory flow at frequencies f up to 80 Hz and nominal strain up to 350, with sample consumption of less than 1 mL, and with minimum inertial effects. Experiments carried out with deionized (DI) water demonstrate that the amplitude of the measured pressure drop falls below the Newtonian prediction at 3 Hz. By introducing a simple model which assumes a linear dependence between the back force and the dead volume within the fluid chambers, the frequency response of both and of the phase delay could be modeled more efficiently. Such effects need to be taken into account when using this type of technology for characterising the frequency response of non-Newtonian fluids.
Highlights
Characterising the mechanical response of low-viscosity (μ = 10−3 –0.1 Pa·s) complex fluids at high frequencies is of great utility for understanding a wide number of industrial processes
We present a microfluidic prototype which aims at characterising low-viscosity complex fluids under oscillatory shear flow over a range of f = 0.05–80 Hz, and nominal strain 20 ≤ γ ≤350
Compared to conventional rheometers, such novel technique enables exploring a wider range of both frequency and strain, with minimum inertial effects, while sample consumption is reduced and interfacial effects are avoided
Summary
Several authors have studied the high frequency behavour of microfluidic devices obtained by combining piezo actuators and membranes with straight channels [17], fluid chambers [19] or cross-slot flow geometries [20]. Each one of the two fluid chambers comprises sidewise extra inlets and outlets which are used to completely fill the chambers with sample before the experiments are run (see the caption of Figure 1) Both the pressure sensors were located at a distance L P = 25 mm from the fluid chambers, which excludes entry effects on the measured ∆P (L P /h >>1). The amplifier, the strain gauges from the piezos and the pressure sensors were connected to a NI-cRIO controller (National Instruments, Austin, TX, USA) It generates signals with a maximum amplitude of 1 V and a sampling rate much larger than the frequency, and acquires the actuators’ displacement and the pressure drop signals.
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