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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.


Correlation of droplet volume to the corresponding maximum change in charge for the spherical droplets as well as for the dispensed droplets.
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f10-sensors-12-10550: Correlation of droplet volume to the corresponding maximum change in charge for the spherical droplets as well as for the dispensed droplets.

Mentions: It can be seen that the maximum change of the charge decreases with lower droplet volumes also in the full model simulation. A noticeable detail is the waved signal characteristics exhibited by the signals generated especially by smaller droplets (V = 22 nL and V = 17 nL, see Figure 9). Multiple signal maxima can be observed, which occur from fluctuations of the droplet shape while passing the capacitor. This effect implies the need of the identification of a value representing the realistic magnitude, equivalent to a spherical droplet of the causative volume. A good estimate for this is given by the mean value of the local minimum and its neighbouring maxima. In between the maximum lateral and longitudinal deformation of the droplet an almost spherical shape has to be attained which is represented by the said mean value. The applicability of this method was confirmed by the correlation of the calculated mean values to the results, gained from the simulation of the spherical droplets, shown in Section 4.1. The linear correlation of the droplet volumes to the signal peak values shown in Figure 10, enable to conclude that neither the influence of the capacitive coupling effect nor the deformation of the droplets caused by the ejection out of a nozzle does affect the measurement as far as the droplet volume—represented by the signal peak value—is concerned. Furthermore, the variable droplet velocity (see Table 2) seems not to affect the maximum change in charge for the considered velocity range (udrop > 1.4 m/s).


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)

Correlation of droplet volume to the corresponding maximum change in charge for the spherical droplets as well as for the dispensed droplets.
© Copyright Policy
Related In: Results  -  Collection

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

f10-sensors-12-10550: Correlation of droplet volume to the corresponding maximum change in charge for the spherical droplets as well as for the dispensed droplets.
Mentions: It can be seen that the maximum change of the charge decreases with lower droplet volumes also in the full model simulation. A noticeable detail is the waved signal characteristics exhibited by the signals generated especially by smaller droplets (V = 22 nL and V = 17 nL, see Figure 9). Multiple signal maxima can be observed, which occur from fluctuations of the droplet shape while passing the capacitor. This effect implies the need of the identification of a value representing the realistic magnitude, equivalent to a spherical droplet of the causative volume. A good estimate for this is given by the mean value of the local minimum and its neighbouring maxima. In between the maximum lateral and longitudinal deformation of the droplet an almost spherical shape has to be attained which is represented by the said mean value. The applicability of this method was confirmed by the correlation of the calculated mean values to the results, gained from the simulation of the spherical droplets, shown in Section 4.1. The linear correlation of the droplet volumes to the signal peak values shown in Figure 10, enable to conclude that neither the influence of the capacitive coupling effect nor the deformation of the droplets caused by the ejection out of a nozzle does affect the measurement as far as the droplet volume—represented by the signal peak value—is concerned. Furthermore, the variable droplet velocity (see Table 2) seems not to affect the maximum change in charge for the considered velocity range (udrop > 1.4 m/s).

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.