Manipulating liquid is of great significance in fields from life sciences to industrial applications. Owing to its advantages in manipulating liquids with high precision and flexibility, electrowetting on dielectric (EWOD) has been widely used in various applications. Despite this, its efficient operation generally needs electrode arrays and sophisticated circuit control. Here, we develop a largely unexplored triboelectric wetting (TEW) phenomenon that can directly exploit the triboelectric charges to achieve the programmed and precise water droplet control. This key feature lies in the rational design of a chemical molecular layer that can generate and store triboelectric charges through agile triboelectrification. The TEW eliminates the requirement of the electric circuit design and additional source input and allows for manipulating liquids of various compositions, volumes, and arrays on various substrates in a controllable manner. This previously unexplored wetting mechanism and control strategy will find diverse applications ranging from controllable chemical reactions to surface defogging.

Triboelectric wetting for continuous droplet transport

Statistical modeling of electrowetting-induced droplet coalescence for condensation applications

Condensation is significantly enhanced by condensing vapor as droplets (instead of a film), which rapidly shed-off. Electrowetting (EW)-induced coalescence and shedding of droplets have been recently shown to accelerate condensation. This work studies the influence of AC electrowetting fields on short-duration droplet shedding on hydrophobic surfaces. Experiments involve tracking the shedding of an ensemble of water droplets under the influence of EW fields, with three parameters being varied (voltage, AC frequency, and device geometry). Significant physical insights into EW-induced droplet shedding are obtained. First, EW enables almost complete removal of water (dry area fraction ∼98%) in very short time durations (∼ 1 s). Second, while the dry area fraction does depend on the applied voltage, significant water shedding can be achieved without needing to apply voltages significantly higher than the threshold voltage. Third, the frequency of the AC waveform does not influence the dry area fraction (for voltages above the threshold voltage); however the time constant associated with droplet shedding strongly depends on the AC frequency. Fourth, the orientation of the device influences water removal due to electrostatic pinning of droplets. Importantly, the measured water removal fluxes immediately after the application of an EW field are two orders of magnitude higher than those measured over a long-duration condensation experiment; this highlights the benefits of intermittent EW fields as opposed to continuous EW fields. Overall, these results suggest that EW on hydrophobic surfaces offers benefits comparable to those offered by superhydrophobic surfaces.

AC electrowetting promoted droplet shedding on hydrophobic surfaces

https://www.jfda-online.com/cgi/viewcontent.cgi?article=1239&context=journal

Electrowetting-on-dielectric (EWOD): Current perspectives and applications in ensuring food safety

Electrically driven dynamic spreading of drops on soft solids is of fundamental importance in a plethora of applications ranging from bio-medical diagnostics to liquid lenses and optoelectronics. However, strategies reported in this regard are challenged by the fact that the spreading gets significantly arrested due to viscoelastic dissipation at the three phase contact line. Circumventing these limits, here we bring out a possibility of substantial augmentation in the rate of electro-spreading on a soft matrix by deploying nano-scale fluidic suspensions of optimal volume fraction. We attribute these findings to a consequent increment in the electrical stresses toward combating the viscoelastic dissipation in the interfacial layer. We also present a simple scaling theory that unveils the manner in which the nano-suspension alters the spreading dynamics of a droplet, effectively by changing the final equilibrium contact angle. These findings open up new possibilities of using nano-fluids of optimal concentration toward modulating the dynamic spreading of a drop on a deformable substrate, a paradigm hitherto remaining unexplored.

Nano-particles in optimal concentration facilitate electrically driven dynamic spreading of a drop on a soft viscoelastic solid

Small-scale electrodes and gaps subjected to repeated short bursts of AC voltage were used to improve droplet coalescence and growth for water harvesting by actively bashing smaller droplets together to form larger ones. Several different electrode patterns were tested under the same conditions. The results indicate that condensation on a cooled flat surface was increased using electrowetting (EW) by accelerating the slow coalescence process where smaller droplets join to form larger droplets and leave behind a dry surface for new droplets to form. A pattern consisting of 100-μm wide interdigitated electrodes separated by 100-μm gaps showed the fastest growth in droplet size. The largest droplets formed with such a pattern had approximately 30 times larger volume than the largest droplets formed on the surface when electrowetting was not applied. Finer patterns exhibited a larger overall condensation rate, where the electrowetting method showed up to a 56% increase in overall water condensation.

Increased droplet coalescence using electrowetting on dielectric (EWOD)

Quantum well infrared photodetector (QWIP) technology is an excellent solution for tactical applications due to the unmatched image uniformity, stability, and pixel operability (typically &lt;99.9%). IRnova have a high-volume production of QWIP detectors in both QVGA and VGA formats since 1999. In 2017 a VGA format QWIP with 15 μm pitch was released, which has demonstrated excellent sensitivity (expressed as noise equivalent temperature difference (NETD)) of 30 mK at F/2 and 100 Hz frame rate. QWIPs also enable compact designs for polarimetric imaging by implementing polarization-sensitive lamellar gratings directly onto the detector pixels. Such polarimetric LWIR detectors (peak absorption at 8.5 μm) have been fabricated at IRnova as both QVGA and VGA format FPAs on 30 and 15 μm pitch, respectively. Polarimetric imaging with these detectors is demonstrated in this work, showing polarimetric contrast as high as 57±5 % for the QVGA format detectors, while the VGA format arrays provide enhanced spatial resolution at the expense of a lower polarization contrast (≈21±2 %). Finally, to meet the demand of highly sensitive and reliable HD LWIR detectors, a 1280×1024 QWIP detector on 10 μm pitch is the next targeted format of IRnova’s QWIP sensors. In this work, the anticipated performance of such detectors will be presented based on the simulation result using the in-house developed tools. The modelling takes into account the relevant ROIC parameters, as well as scaling of the detector performance with pixel size. 

Sara Högnadóttir

Papers

Increased droplet coalescence using electrowetting on dielectric (EWOD)

QWIP: still the best solution for high-end applications in LWIR