arXiv subjects

Dirk van den Ende

Publications and source records attributed to Dirk van den Ende.

5 recordsLinked to original sources

arXiv2020

Electric generation from drops impacting onto charged surfaces

The impact of liquid drops onto solid surfaces leads to conversion of kinetic energy of directed drop motion into various forms of energy including surface energy, vibrational energy, heat, and under suitable conditions, electrical energy. The latter has attracted substantial attention in recent years for its potential to directly convert energy from random environmental flows such as rainfall, spray, and wave motion on the sea to electrical energy. Despite the invention of numerous configurations of such energy harvesters, the underlying physical principles and optimum operation conditions have remained elusive. In this letter, we use a combination of high-speed electrical current and video imaging measurements to develop a parameter-free quantitative description of the energy harvesting process for an optimized electrode configuration. A novel electrowetting-assisted charge injection method, EWCI, enables highly stable surface charges and robust energy conversion for several months with record efficiencies exceeding 2.5 percent of the initial kinetic energy.

physics.app-ph↗

arXiv2019

Electrically controlled localized charge trapping at amorphous fluoropolymer-electrolyte interfaces

Charge trapping is a long-standing problem in electrowetting-on-dielectric (EWOD), causing reliability reduction and restricting its practical applications. Although this phenomenon has been investigated macroscopically, the microscopic investigations are still lacking. In this work, the trapped charges are proven to be localized at three-phase contact line region by using three detecting methods -- local contact angle measurements, electrowetting (EW) probe, and Kelvin Probe Force Microscopy (KPFM). Moreover, we demonstrate that this EW-induced charge trapping phenomenon can be utilized as a simple and low-cost method to deposit charges on fluoropolymer surfaces. Charge density near the three-phase contact line up to 0.46 mC/m2 and the line width with deposited charges ranging from 20 to 300 micrometer are achieved by the proposed method. Particularly, negative charge densities do not degrade even after harsh testing with a water droplet on top of the sample surfaces for 12 hours, as well as after being treated by water vapor for 3 hours. These findings provide an approach for applications which desire stable and controllable surface charges.

physics.app-ph↗

arXiv2019

Characterizing the fluid-matrix affinity in an organogel from the growth dynamics of oil stains on blotting paper

Grease, as used for lubrication of rolling bearings, is a two-phase organogel that slowly releases oil from its gelator matrix. Because the rate of release determines the operation time of the bearing, we study this release process by measuring the amount of extracted oil as a function of time, while we use absorbing paper, to speed up the process. The oil concentration in the resulting stain is determined by measuring the attenuation of light transmitted through the paper, using a modified Lambert-Beer law. For grease the timescale for paper imbibition is typically 2 orders of magnitude larger than for a bare drop of the same base oil. This difference results from the high affinity, \it i.e. wetting energy per unit volume, of the oil for the grease matrix. To quantify this affinity, we developed a Washburn-like model describing the oil flow from the porous grease into the paper pores. The stain radius versus time curves for greases at various levels of oil content collapse onto a single master curve, which allows us to extract a characteristic spreading time and the corresponding oil-matrix affinity. Lowering the oil content results in a small increase of the oil-matrix affinity yet in a significant change in the spreading timescale. Even an affinity increase by a few per mill doubles the timescale.

cond-mat.soft↗

arXiv2018

Controlling shedding characteristics of condensate drops using electrowetting

We show here that ac electrowetting (ac-EW) with structured electrodes can be used to control the gravity-driven shedding of drops condensing onto flat hydrophobic surfaces. Under ac-EW with straight interdigitated electrodes, the condensate drops shed with relatively smaller radii due to the ac-EW-induced reduction of contact angle hysteresis. The smaller shedding radius, coupled with the enhanced growth due to coalescence under EW, results in increased shedding rate. We also show that the condensate droplet pattern under EW can be controlled, and the coalescence can be further enhanced, using interdigitated electrodes with zigzag edges. Such enhanced coalescence in conjunction with the electrically-induced trapping effect due to the electrode geometry results in larger shedding radius, but lower shedding rate. However, the shedding characteristics can be further optimized by applying the electrical voltage intermittently. We finally provide an estimate of the condensate volume removed per unit time in order to highlight how it is enhanced using ac-EW-controlled dropwise condensation.

physics.flu-dyn↗

arXiv2018

Breath figures under electrowetting: electrically controlled evolution of drop condensation patterns

We show that electrowetting (EW) with structured electrodes significantly modifies the distribution of drops condensing onto flat hydrophobic surfaces by aligning the drops and by enhancing coalescence. Numerical calculations demonstrate that drop alignment and coalescence are governed by the drop size-dependent electrostatic energy landscape that is imposed by the electrode pattern and the applied voltage. Such EW-controlled migration and coalescence of condensate drops significantly alter the statistical characteristics of the ensemble of droplets. The evolution of the drop size distribution displays self-similar characteristics that significantly deviate from classical breath figures on homogeneous surfaces once the electrically-induced coalescence cascades set in beyond a certain critical drop size. The resulting reduced surface coverage, coupled with earlier drop shedding under EW, enhances the net heat transfer.

physics.flu-dyn↗