Digital microfluidics combines the advantages of a low consumption of reagents with a high flexibility of processing fluid samples. For applications in life sciences not only the processing but also the characterization of fluids is crucial. In this contribution, a microfluidic platform, combining the actuation principle of electrowetting on dielectrics for droplet manipulations and the sensor principle of impedance spectroscopy for the characterization of the fluid composition and condition, is presented. The fabrication process of the microfluidic platform comprises physical vapor deposition and structuring of the metal electrodes onto a substrate, the deposition of a dielectric isolator and a hydrophobic top coating. The key advantage of this microfluidic chip is the common electric nature of the sensor and the actuation principle. This allows for fabricating digital microfluidic devices with a minimal number of process steps. Multiple measurements on fluids of different composition (including rigid particles) and of different conditions (temperature, sedimentation) were performed and process parameters were monitored online. These sample applications demonstrate the versatile applications of this combined technology.

Increasing demands for online monitoring of liquids have not only resuted in many new devices relying on well-established sensing parameters like shear viscosity but also initiated research on alternative parameters. Recently, the longitudinal viscosity has been evaluated as a promising candidate because the devices arising enable the bulk of the liquid to be probed rather than a thin surface layer. We report on a multi-purpose sensor which allows simultaneous measurement of the sound velocity and longitudinal viscosity of liquids. The device embodiment features a cube-shaped chamber containing the sample liquid, where one boundary surface carries a flush-mounted PZT transducer. In operation, the transducer induces standing, resonant pressure waves in the liquid under test. We studied the influences of sound velocity and longitudinal viscosity on the generated pressure waves by means of the Navier–Stokes equation for adiabatic compressible liquids and exploited both parameters as the basic sensing mechanism. Furthermore, a three-port network model describing the interaction of the transducer and sample liquid was developed in order to be applied for extracting the parameters of interest from the raw measurement data. Finally, we demonstrate the device and method by carrying out and discussing test measurements on glycerol–water solutions.

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Ultrasonic sensors can be used to determine physical fluid parameters like viscosity, density, and speed of sound. In this contribution, we present the concept for an integrated sensor utilizing pressure waves to sense the characteristic acoustic impedance of a fluid. We note that the basic setup generally allows to determine the longitudinal viscosity and the speed of sound if it is operated in a resonant mode as will be discussed elsewhere. In this contribution, we particularly focus on a modified setup where interferences are suppressed by introducing a wedge reflector. This enables sensing of the liquid's characteristic acoustic impedance, which can serve as parameter in condition monitoring applications. We present a device model, experimental results and their evaluation.

We present a robust viscosity measurement system based on a torsionally oscillating pipe. The sensitive surface of the sensor performs periodic movements in the fluid to be sensed, generating a shear wave that penetrates the fluid. Due to this interaction, the resonance characteristic of the structure is affected, in particular the quality factor decreases with increasing viscosity. The pipe is mounted at its center where it features a nodal point of the preferred resonant mode, reducing temperature issues while simultaneously enabling high quality factors. A mathematical model is presented illustrating how different parameters influence the sensitivity of the sensor. Long-term measurements were performed to demonstrate the time stability of the sensor setup.