Dropwise condensation yields higher heat transfer coefficients by avoiding the thermal resistance of the condensate film, seen during filmwise condensation. This work explores further enhancement of dropwise condensation heat transfer through the use of electrowetting to achieve faster droplet growth via coalescence of the condensed droplets. Electrowetting is a well understood microfluidic technique to actuate and control droplets. This work shows that AC electric fields can significantly enhance droplet growth dynamics. This enhancement is a result of coalescence triggered by various types of droplet motion (translation of droplets, oscillations of three phase line), which in turn depends on the frequency of the applied AC waveform. The applied electric field modifies droplet condensation patterns as well as the roll-off dynamics on the surface. Experiments are conducted to study early-stage droplet growth dynamics, as well as steady state condensation rates under the influence of electric fields. It is noted that this study deals with condensation of humid air, and not pure steam. Results show that increasing the voltage magnitude and frequency increases droplet growth rate and overall condensation rate. Overall, this study reports more than a 30 % enhancement in condensation rate resulting from the applied electric field, which highlights the potential of this concept for condensation heat transfer enhancement.
more »
« less
Characterization of oscillation amplitude of contact angle during AC electrowetting of water droplets
Most studies on electrowetting (EW) involve the use of AC electric fields, which cause droplets to oscillate in response to the sinusoidal waveform. Oscillation-driven mixing in droplets is the basis for multiple microfluidic applications. Presently, we study the voltage and AC frequency-dependent oscillations of electrowetted water droplets on a smooth, hydrophobic surface. We introduce a new approach towards analyzing droplet oscillations, which involves characterization of the oscillation amplitude of the contact angle (CA). An experimentally validated, fundamentals-based model to predict voltage and frequency-dependent CA oscillations is developed, which is analogous to the Lippmann’s equation for predicting voltage-dependent CAs. It is seen that this approach can help estimate the threshold voltage more accurately, than from experimental measurements of CA change. Additionally, we use a coplanar electrode configuration with high voltage and ground electrodes arranged on the substrate. This configuration eliminates measurement artefacts in the classical EW configuration associated with a wire electrode protruding into the droplet. An interesting consequence of this configuration is that the system capacitance is reduced substantially, compared to the classical configuration. The coplanar electrode configuration shows a reduced rate of CA change with voltage, thereby increasing the voltage range over which the CA can be modulated.
more » « less- Award ID(s):
- 1805179
- NSF-PAR ID:
- 10303675
- Publisher / Repository:
- IOP Publishing
- Date Published:
- Journal Name:
- Journal of Physics Communications
- Volume:
- 4
- Issue:
- 6
- ISSN:
- 2399-6528
- Page Range / eLocation ID:
- Article No. 065016
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Dropwise condensation heat transfer is significantly higher than filmwise condensation heat transfer due to the absence of the thermal resistance associated with the condensed water film. This study uses electrowetting to enhance coalescence and roll-off of condensed droplets, with the objective of enhancing the condensation rate. Coalescence enhancement is achieved by electric field-driven droplet motion such as translation of droplets, and oscillations of the three-phase line. Experiments are conducted to study early-stage droplet growth dynamics, and steady state condensation under electrowetting fields. Results show that droplet growth and roll-off increases with the voltage and frequency of the applied AC field. AC electric fields are seen to be more effective than DC electric fields. The overall condensation rate depends on the roll-off size of droplets, frequency of roll-off events, and on the interactions of the rolled-off droplets with the remainder of the droplets. All these phenomena can be altered by the applied electric field. An analytical heat transfer model is developed which uses the measured droplet size distribution to estimate the surface heat flux. Overall, this study reports that electric fields can enhance the condensation rate by more than 30 %.more » « less
-
null (Ed.)This paper presents a reverse electrowetting-on-dielectric (REWOD) energy harvester integrated with rectifier, boost converter, and charge amplifier that is, without bias voltage, capable of powering wearable sensors for monitoring human health in real-time. REWOD has been demonstrated to effectively generate electrical current at a low frequency range (< 3 Hz), which is the frequency range for various human activities such as walking, running, etc. However, the current generated from the REWOD without external bias source is insufficient to power such motion sensors. In this work, to eventually implement a fully self-powered motion sensor, we demonstrate a novel bias-free REWOD AC generation and then rectify, boost, and amplify the signal using commercial components. The unconditioned REWOD output of 95–240 mV AC is generated using a 50 μL droplet of 0.5M NaCl electrolyte and 2.5 mm of electrode displacement from an oscillation frequency range of 1–3 Hz. A seven-stage rectifier using Schottky diodes having a forward voltage drop of 135–240 mV and a forward current of 1 mA converts the generated AC signal to DC voltage. ∼3 V DC is measured at the boost converter output, proving the system could function as a self-powered motion sensor. Additionally, a linear relationship of output DC voltage with respect to frequency and displacement demonstrates the potential of this REWOD energy harvester to function as a self-powered wearable motion sensor.more » « less
-
This paper presents a reverse electrowetting-on-dielectric (REWOD) energy harvester integrated with rectifier, boost converter, and charge amplifier that is, without bias voltage, capable of powering wearable sensors for monitoring human health in real-time. REWOD has been demonstrated to effectively generate electrical current at a low frequency range (<3 Hz), which is the frequency range for various human activities such as walking, running, etc. However, the current generated from the REWOD without external bias source is insufficient to power such motion sensors. In this work, to eventually implement a fully self-powered motion sensor, we demonstrate a novel bias-free REWOD AC generation and then rectify, boost, and amplify the signal using commercial components. The unconditioned REWOD output of 95-240 mV AC is generated using a 50 μL droplet of 0.5M NaCl electrolyte and 2.5 mm of electrode displacement from an oscillation frequency range of 1-3 Hz. A seven-stage rectifier using Schottky diodes having a forward voltage drop of 135-240 mV and a forward current of 1 mA converts the generated AC signal to DC voltage. ~3 V DC is measured at the boost converter output, proving the system could function as a self-powered motion sensor. Additionally, a linear relationship of output DC voltage with respect to frequency and displacement demonstrates the potential of this REWOD energy harvester to function as a self-powered wearable motion sensor.more » « less
-
Contact charge electrophoresis (CCEP) uses steady electric fields to drive the continuous, oscillatory motion of conductive particles and droplets between two or more electrodes. These rapid oscillations can be rectified to direct the motion of objects within microfluidic environments using low-power, dc voltage. Here, we compare high precision experimental measurements of CCEP within a microfluidic system to equally detailed theoretical predictions on the motion of a conductive particle between parallel electrodes. We use a simple, capillary microfluidic platform that combines high-speed imaging with precision electrical measurements to enable the synchronized acquisition of both the particle location and the electric current due to particle motion. The experimental results are compared to those of a theoretical model, which relies on a Stokesian dynamics approach to accurately describe both the electrostatic and hydrodynamic problems governing particle motion. We find remarkable agreement between theory and experiment, suggesting that particle motion can be accurately captured by a combination of classical electrostatics and low-Reynolds number hydrodynamics. Building on this agreement, we offer new insight into the charge transfer process that occurs when the particle nears contact with an electrode surface. In particular, we find that the particle does not make mechanical contact with the electrode but rather that charge transfer occurs at finite surface separations of >0.1 μm by means of an electric discharge through a thin lubricating film. We discuss the implications of these findings on the charging of the particle and its subsequent dynamics.more » « less
