Recent advances in technology, in particular soft robotics and micro-electronics, have renewed the interested in the impact of viscoelastic boundaries and active boundary modulation on hydrodynamic drag and boundary layer turbulence. Viscoelastic boundary materials, such as those found in dolphin skin, are known to have the potential to reduce boundary drag, by delaying the transition from laminar to turbulent flow in the boundary layer around the body and minimizing boundary layer turbulence. The possible mechanisms to reduce boundary layer turbulence include counteracting boundary layer coherent structures or impacting momentum transfer near the boundary. Actuating a deformable membrane in a channel flow allows the investigation of the impact of boundary actuation on boundary layer turbulence for a range of actuation parameters and flow channel speeds. We developed a deformable boundary and tested the system in channel flow, in direct contact with the water, actuating at various wave patterns and frequencies. The impact on boundary layer velocity was investigated with Particle Image Velocimetry, as well as numerical simulations (see companion paper). Boundary actuation is shown to impact the boundary layer velocity profile and near boundary momentum transfer. We characterize the parameter space most likely to reduce boundary layer turbulence in a natural environment, which could lead to more energy-efficient platforms and underwater vehicles.
Soft pressure sensors have a wide range of applications, such as aerodynamic control of cars and unmanned aerial vehicles, navigation of underwater vehicles, and wearable electronics. Existing soft pressure sensors are typically based on capacitive or resistive principles. However, these sensors, made of multiple layers of different materials, tend to delaminate under negative pressures and thus cause sensor failure. In this work, we present the fabrication method for soft capacitive pressure sensors that can reliably detect both positive and negative pressures. The pressure sensor is comprised of one layer of Ecoflex-0030 substrate with cavity channels embedded inside, and two layers of polydimethylsiloxane (PDMS), with two layers of patterned PEDOT:PSS films serving as the electrodes of the sensor. The PEDOT:PSS films are screen printed orthogonally on both sides of the Ecoflex-0030 substrate, and each side is encapsulated by another PDMS layer, which is much stiffer than the Ecoflex-0030 substrate. More importantly, the cavity channels in the Ecoflex-0030 substrate greatly enhance the substrate deformation, hence the capacitive sensor would exhibit remarkable relative change in capacitance when a pressure is applied. Secondly, the encapsulation of PDMS on the Ecoflex substrate protects the electrodes and effectively avoids the delamination problem under negative pressure. In particular, we report the detailed characterization of sensitivity and repeatability of the fabricated pressure sensor for positive and negative pressures of up to 50 kPa. Furthermore, a 12×12 pressure sensor array is fabricated to demonstrate the capability of mapping pressure distributions created by both compressive loads and vacuum suction.