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Numerical investigations on electric field characteristics with respect to capacitive detection of free-flying droplets.

Ernst A, Mutschler K, Tanguy L, Paust N, Zengerle R, Koltay P - Sensors (Basel) (2012)

Bottom Line: The simulations were realised using the computational fluid dynamic (CFD) software CFD ACE+.The sensitivity of the focused capacitor geometry was evaluated to be S(i) = 0.3 fC/nL.The simulation results are validated by experiments which exhibit good agreement.

View Article: PubMed Central - PubMed

Affiliation: Laboratory for MEMS Applications, Department of Microsystems Engineering (IMTEK), University of Freiburg, 79110 Freiburg, Germany. andreas.ernst@imtek.de

ABSTRACT
In this paper a multi-disciplinary simulation of a capacitive droplet sensor based on an open plate capacitor as transducing element is presented. The numerical simulations are based on the finite volume method (FVM), including calculations of an electric field which changes according to the presence of a liquid droplet. The volume of fluid (VOF) method is applied for the simulation of the ejection process of a liquid droplet out of a dispenser nozzle. The simulations were realised using the computational fluid dynamic (CFD) software CFD ACE+. The investigated capacitive sensing principle enables to determine the volume of a micro droplet passing the sensor capacitor due to the induced change in capacity. It could be found that single droplets in the considered volume range of 5 nL < V(drop) < 100 nL lead to a linear change of the capacity up to ΔQ < 30 fC. The sensitivity of the focused capacitor geometry was evaluated to be S(i) = 0.3 fC/nL. The simulation results are validated by experiments which exhibit good agreement.

No MeSH data available.


Simulated droplet ejection process applying the grid as described in Section 3.1. A flow (2.5 m/s) towards the nozzle is instantly stopped after 70 μs which initializes the droplet ejection.
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f4-sensors-12-10550: Simulated droplet ejection process applying the grid as described in Section 3.1. A flow (2.5 m/s) towards the nozzle is instantly stopped after 70 μs which initializes the droplet ejection.

Mentions: The numerical study of the presented capacitive measurement method requires the implementation of a droplet ejection process which generates droplets with realistic properties in terms of shape, volume and velocity. To keep the focus on the electrostatic interaction, a simple model of a droplet ejection process was realised, based on a liquid flow boundary condition driving the droplet ejection. Therefore, a liquid flow of a constant velocity (vflow) of 2.5 m/s was set as boundary condition at the top inlet of the cylindrical liquid column (cf. Section 3.1). The flow was active for 70 μs and then stopped instantly to initiate the droplet tear off. The applied parameters led to a droplet ejection process like shown in Figure 4, which reflects realistic and representative droplet properties in volume (V = 33 nL), shape and velocity (v = 1.4 m/s). The wetting conditions for the walls, surrounding the nozzle were set to a wetting angle (αwater) of 68° for pure water on the nozzle material. Furthermore, an outlet is defined at the lower end of the model to enable a balance of occurring pressure in the domain, caused by the droplet ejection. The wetting conditions at the electrodes are set to a ‘no wetting’ boundary condition (αwater = 180°), to avoid falsification of the results due to wet contamination of the electrodes.


Numerical investigations on electric field characteristics with respect to capacitive detection of free-flying droplets.

Ernst A, Mutschler K, Tanguy L, Paust N, Zengerle R, Koltay P - Sensors (Basel) (2012)

Simulated droplet ejection process applying the grid as described in Section 3.1. A flow (2.5 m/s) towards the nozzle is instantly stopped after 70 μs which initializes the droplet ejection.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3472843&req=5

f4-sensors-12-10550: Simulated droplet ejection process applying the grid as described in Section 3.1. A flow (2.5 m/s) towards the nozzle is instantly stopped after 70 μs which initializes the droplet ejection.
Mentions: The numerical study of the presented capacitive measurement method requires the implementation of a droplet ejection process which generates droplets with realistic properties in terms of shape, volume and velocity. To keep the focus on the electrostatic interaction, a simple model of a droplet ejection process was realised, based on a liquid flow boundary condition driving the droplet ejection. Therefore, a liquid flow of a constant velocity (vflow) of 2.5 m/s was set as boundary condition at the top inlet of the cylindrical liquid column (cf. Section 3.1). The flow was active for 70 μs and then stopped instantly to initiate the droplet tear off. The applied parameters led to a droplet ejection process like shown in Figure 4, which reflects realistic and representative droplet properties in volume (V = 33 nL), shape and velocity (v = 1.4 m/s). The wetting conditions for the walls, surrounding the nozzle were set to a wetting angle (αwater) of 68° for pure water on the nozzle material. Furthermore, an outlet is defined at the lower end of the model to enable a balance of occurring pressure in the domain, caused by the droplet ejection. The wetting conditions at the electrodes are set to a ‘no wetting’ boundary condition (αwater = 180°), to avoid falsification of the results due to wet contamination of the electrodes.

Bottom Line: The simulations were realised using the computational fluid dynamic (CFD) software CFD ACE+.The sensitivity of the focused capacitor geometry was evaluated to be S(i) = 0.3 fC/nL.The simulation results are validated by experiments which exhibit good agreement.

View Article: PubMed Central - PubMed

Affiliation: Laboratory for MEMS Applications, Department of Microsystems Engineering (IMTEK), University of Freiburg, 79110 Freiburg, Germany. andreas.ernst@imtek.de

ABSTRACT
In this paper a multi-disciplinary simulation of a capacitive droplet sensor based on an open plate capacitor as transducing element is presented. The numerical simulations are based on the finite volume method (FVM), including calculations of an electric field which changes according to the presence of a liquid droplet. The volume of fluid (VOF) method is applied for the simulation of the ejection process of a liquid droplet out of a dispenser nozzle. The simulations were realised using the computational fluid dynamic (CFD) software CFD ACE+. The investigated capacitive sensing principle enables to determine the volume of a micro droplet passing the sensor capacitor due to the induced change in capacity. It could be found that single droplets in the considered volume range of 5 nL < V(drop) < 100 nL lead to a linear change of the capacity up to ΔQ < 30 fC. The sensitivity of the focused capacitor geometry was evaluated to be S(i) = 0.3 fC/nL. The simulation results are validated by experiments which exhibit good agreement.

No MeSH data available.