Ultrasonic viscometer with integrated depth measurement

We report on a microfabricated silicon ultrasonic sensor with integrated holes and four sets of piezoresistive strain gauge for viscosity measurement. The silicon beam is actuated at its λ/2 longitudinal resonance frequency, 95 kHz, by a PZT plate bonded at its longitudinal mode node. The modes excited by the actuator are a combination of surface motion generating the shear strain in liquids and an out-plane motion drive bulk mode into the liquid. Two rows of 700 μm pores are used to create a depth dependent varying liquid sampling volume. The viscosity is measured by changes in the transducer strain signals upon immersion in liquid. Steps in the strain signal are obtained by increasing the sensor-liquid contact area in a non-continuous steps, which can provide the immersion depth and liquid viscosity information simultaneously. The microprobe was characterized in various bulk viscosities reference standards. From the vibration amplitude measured by the strain gauges, we were able to quantify the damping in the fluid that is proportional to the viscosity.