Nano-newton drag sensor based on flexible micro-pillars

Abstract

A new simple and low-cost sensor concept to measure drag forces on fixed particles in the near-wall region with detectable forces down to pico-newton with a novel microoptomechanical system (MOMS) based on a flexible micro-pillar is presented. The cylindrical pillar with a diameter of a few microns is manufactured from an elastomer such that it is very flexible and easily deflected by the fluid forces or supplementary forces acting on flow obstacles attached to the tip. The pillar-tip bending is detected optically using a highly magnifying optical system. A feasibility study was carried out in a plate–cone rheometer with an air bubble of diameter 140 µm attached to the pillar's tip to show that the micro-pillar sensor is capable of detecting net particle drag forces. The Reynolds number based on the bubble diameter Rebubble was varied in the range of 0.1–15. The calibration of the pure sensor structure shows linear behaviour of the pillar. The experimental results of the bubble drag in plane shear flow showed good agreement with theoretical predictions. The optics and the sensor used in this experiment allow a reliable detection of forces down to a minimum of about 5–10 nN. Using microscopic optics even smaller forces are detectable. The sensor geometry can be varied in a wide range, making it possible to measure forces of order down to nano-newtons on particles possessing a geometric length of a few µm to several hundred µm. Therefore, the method fits well into existing drag measurement concepts closing a gap between optical tweezers and recent atomic force cantilevers. Furthermore, the new sensor concept possesses very low intrusive interference. It has to be emphasized that unlike cantilever methods, the new micro-pillar sensor allows measurements of the two wall-parallel drag components. Using arrays of sensors, drag forces on multiple particles in a plane can be measured.