Christos Stamatopoulos


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Stamatopoulos

First Name

Christos

ORCID

0000-0002-8482-1150

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2020 - 20233
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Publications 1 - 3 of 3
  • Electric Field Mediated Contact Time Reduction of Impacting Drops on Cu(OH)2 Nanoneedle Clusters: Limitations and Implications for Anti-Icing and Pathogen-Containment Applications
    Item type: Journal Article
    Stamatopoulos, Christos; Suter, Reto; Franck, Christian (2022)
    ACS Applied Nano Materials
    In this study, the water drop impact on a copper-based nanotextured superhydrophobic surface inside a uniform electric field is investigated. Because of the wider attention that drop impact draws in the scientific community, this study gives emphasis on the effect of the electric field on the droplet’s residence time, a quantity that plays a key role in processes that involve heat and/or mass transport between the surface and impacting droplet. The reduction of the residence time is of vital importance especially for anti-icing and pathogen-transmission-containment applications. Shorter residence times enable droplets to rebound at supercooled surfaces before the occurrence of ice nucleation. Moreover, they restrict the likelihood of the deposition of viruses and bacteria for the case of pathogen-laden impacting droplets. Reduction of the residence time is achieved by a twofold strategy. The surface is textured in the nanoscale with the growth of a Cu(OH)2 nanoneedle cluster so that the nanoroughness topography in combination with the hydrophobic coating imparts to the surface an extreme water-repellent behavior and impalement resistance. Moreover, we introduce an additional external force exerted on the droplet, which originates from an electric field. We focus on the range of the electric Bond number 0 ≤ Boe ≤ 0.060. In this range, we observe two different interesting behaviors: (a) For 0 ≤ Boe ≤ 0.020, the contact time reduces with the applied electric field. We also conduct simulations to support our experimental findings concerning the effect of the electric field on the contact time. (b) For 0.025 ≤ Boe ≤ 0.060, the contact time increases. We demonstrate that this happens because of partial discharges that induce electrowetting, resulting in altering the wetting behavior of the droplet during retraction. Even though limitations exist, the application of electric fields can be considered to be a promising and flexible strategy for reducing the residence time because it can be applied on a wide range of superhydrophobic surfaces.
  • Droplet Self-Propulsion on Superhydrophobic Microtracks
    Item type: Journal Article
    Stamatopoulos, Christos; Milionis, Athanasios; Ackerl, Norbert; et al. (2020)
    ACS Nano
    Liquid transport (continuous or segmented) in microfluidic platforms typically requires pumping devices or external fields working collaboratively with special fluid properties to enable fluid motion. Natural liquid adhesion on surfaces deters motion and promotes the possibility of liquid or surface contamination. Despite progress, significant advancements are needed before devices for passive liquid propulsion, without the input of external energy and unwanted contamination, become a reality in applications. Here we present an unexplored and facile approach based on the Laplace pressure imbalance, manifesting itself through targeted track texturing, driving passively droplet motion, while maintaining the limited contact of the Cassie–Baxter state on superhydrophobic surfaces. The track topography resembles out-of-plane, backgammon-board, slowly converging microridges decorated with nanotexturing. This design naturally deforms asymmetrically the menisci formed at the bottom of a droplet contacting such tracks and causes a Laplace pressure imbalance that drives droplet motion. We investigate this effect over a range of opening track angles and develop a model to explain and quantify the underlying mechanism of droplet self-propulsion. We further implement the developed topography for applications relevant to microfluidic platform functionalities. We demonstrate control of the rebound angle of vertically impacting droplets, achieve horizontal self-transport to distances up to 65 times the droplet diameter, show significant uphill motion against gravity, and illustrate a self-driven droplet-merging process.
  • Out-of-Plane Biphilic Surface Structuring for Enhanced Capillary-Driven Dropwise Condensation
    Item type: Journal Article
    Stendardo, Luca; Milionis, Athanasios; Kokkoris, George; et al. (2023)
    Langmuir
    Rapid and sustained condensate droplet departure from a surface is key toward achieving high heat-transfer rates in condensation, a physical process critical to a broad range of industrial and societal applications. Despite the progress in enhancing condensation heat transfer through inducing its dropwise mode with hydrophobic materials, sophisticated surface engineering methods that can lead to further enhancement of heat transfer are still highly desirable. Here, by employing a three-dimensional, multiphase computational approach, we present an effective out-of-plane biphilic surface topography, which reveals an unexplored capillarity-driven departure mechanism of condensate droplets. This texture consists of biphilic diverging microcavities wherein a matrix of small hydrophilic spots is placed at their bottom, that is, among the pyramid-shaped, superhydrophobic microtextures forming the cavities. We show that an optimal combination of the hydrophilic spots and the angles of the pyramidal structures can achieve high deformational stretching of the droplets, eventually realizing an impressive “slingshot-like” droplet ejection process from the texture. Such a droplet departure mechanism has the potential to reduce the droplet ejection volume and thus enhance the overall condensation efficiency, compared to coalescence-initiated droplet jumping from other state-of-the-art surfaces. Simulations have shown that optimal pyramid-shaped biphilic microstructures can provoke droplet self-ejection at low volumes, up to 56% lower than superhydrophobic straight pillars, revealing a promising new surface microtexture design strategy toward enhancing the condensation heat-transfer efficiency and water harvesting capabilities.
Publications 1 - 3 of 3