Interaction of bubbles and droplets with solids surfaces is part of our everyday life and has been a source of intrigue for millennia. The formation of vapor bubbles (boiling) is considered as the most effective heat transfer mechanism and is fundamental in steam production (power plants) and thermal separation processes as well as indispensable for cooling of high-powered electronics. Meanwhile, behavior of droplets on solids is important in condensation heat transfer, microfluidics, biomedical engineering, and environmental sciences, to name a few. Bubble/droplet interactions with solids can be controlled through surface functionalization, which can enhance many important processes. However, the future progress is constrained by the surface robustness and the lack of fundamental understanding of bubble/droplet interaction with functionalized surfaces, which is related to the lack of appropriate diagnostic techniques and physical or semi-empirical models.

DroBFuSE project addresses some of the most important knowledge gaps today, simultaneously for bubbles and droplets, through development of robust surface functionalization approaches, advanced diagnostic tools, data processing algorithms, and mechanistic modelling through the force balance approach and heat flux partitioning schemes. Among others, project deliverables include development of:

• Novel advanced experimental approaches for in-liquid temperature measurements during boiling (through temperature dependent absorption of near infrared light) and droplet wettability transition investigations on thick/thin substrates (including utilization of thin-film piezoelectric devices and laser interferometry).
• Robust hydrophobic and superhydrophobic surfaces for long-term enhanced boiling heat transfer and condensation applications. Surfaces will be created through combination of scalable surface functionalization techniques, including direct laser texturing, oxidation, hydrothermal treatment, and pos-processing via hydrophobic monolayers.
• Novel environmentally friendly and robust superhydrophobic surfaces based on organic hydrophobic coatings with optimized micro/nanostructure for environmental applications (i.e., water harvesting).
• Proof-of-concept water harvesting device aimed at capturing water from fog and process streams that include droplets in micrometer size range.
• Mechanistic models and sub-models able to predict the relevant bubble parameters during boiling, such as bubble departure frequency and departure diameter. These advanced models will include contributions from the thermal boundary layer and heat transfer through the liquid/vapor interface in combination with substrate properties (thermophysical properties and wettability).
• Model of droplet behavior on superhydrophobic surfaces under the influence of elevated pressure induced by external forces.
• Project results will provide completely new insights about the effects of surface structure/wettability and substrates’ thermophysical properties on bubble dynamics, allow a breakthrough progress on boiling surface optimization to reach heat fluxes beyond our current capabilities, and inclusion of developed modelling framework to future numerical codes. Investigations of droplets on functionalized surfaces will allow future development of environmental technologies like water harvesters, enhanced condensers, self-cleaning and anti/bacterial interfaces, unlimitedly applicable in future scientific research and our everyday life.

Highly ambitious objectives will be achieved through the (i) well-defined work programme, (ii) systematic collaboration between excellent Slovenian researchers and top-tier foreign institutions (including MIT, KU Leuven, Silicon Austria Labs, Univ. of Pisa and Univ. of Toulouse), and (iii) by integrating knowledge from previous and current projects of the DroBFuSE research group members, including the ones funded by ESA (RUBI experiment) and ARRS (J2-2486, N2-0251, J2-3052, J2-1741, L2-1833).

WP2: Development of Advanced Diagnostic Tools (M1–M18)

T2.1 Experimental system for temperature measurement in liquid around a boiling bubble (M1–M16)
The design, procurement, and installation of the equipment were successfully completed. Preliminary results, especially using a micro-thermocouple, have been successfully obtained.

T2.2 Development of image processing algorithms for boiling process analysis (M6–M18)
Algorithms related to data synchronization and detection of the vapor-liquid interface layer have been developed. The remaining task involves determining the temperature field from light absorption measurements in the NIR spectrum.

T2.3 Experimental system for evaluation of wettability transition (M1–M14)
The experimental system has been successfully established and is operational. It enables monitoring of the wettability transition process on both thin and thick samples.

T2.4 Development of image processing algorithms for water droplet experiments (M6–M12)
The algorithms, successfully implemented in MATLAB and Python, allow detection of the liquid-vapor interface during compression of water droplets between two superhydrophobic surfaces. Synchronization with a force sensor was achieved, with data acquisition carried out in the Dewesoft X environment.

WP3: Interfacial Phenomena Investigations of Bubbles on Functionalised Surfaces

T3.1 Fabrication of samples with functional surfaces for boiling tests (M6–M10)
Samples made from thin metal foils and various materials were successfully processed using a combination of laser texturing and low-surface-energy coatings. These have been integrated into boiling experiments, where their effectiveness in generating vapor bubbles at predefined superhydrophobic sites has been demonstrated.

T3.2 Execution of boiling experiments on thin and thick samples using advanced diagnostics (M8–M16)
A comprehensive matrix of experiments was conducted. The most promising results were obtained using a micro-thermocouple, which enabled successful diagnostics of the thermal boundary layer—an essential and often missing element in the development of models for predicting vapor bubble growth.

T3.3 Analysis of wettability effects on bubble dynamics and heat transfer mechanisms (M17–M30)
This task is currently beginning, in accordance with the planned timeline.

T3.4 Modeling of combined effects of surface wettability and heating surface properties (M22–M34)
This task is scheduled for implementation at the end of the second year and during the third year of the project.

WP4: Droplets on Functional Surfaces and Proof of Concept for Water Harvesting Applications

T4.1 Development of functional surfaces (M9–M15)
This task has been successfully completed. Hierarchical (stochastic) structures with superhydrophobic coatings were fabricated on various substrates, along with deterministic structures produced on silicon using lithographic techniques.

T4.2 Experiments on wettability transition using advanced diagnostics (M10–M16)
This task has been successfully completed. Wettability transition experiments were successfully performed. The most promising results were achieved on silicon superhydrophobic surfaces. A comparison of force measurements from a high-resolution system with analytical results shows a high degree of correlation.

T4.3 Data processing and modeling (M17–M27)
This task is currently starting, in accordance with the planned timeline.

T4.4 Conceptual device for atmospheric water harvesting (M28–M36)
This task is scheduled for execution in the final year of the project.

WP5: Dissemination and Exploitation of Project Results

T5.1 Presentation of project results to the general public (M3–M36)
The general public has been informed through five conference contributions, where the audience included not only the scientific community but also broader professional and general public. The results are published in open access, with three of the five contributions written in Slovene.

T5.2 Presentation and dissemination of results to the scientific community (M6–M36)
So far, 7 scientific journal papers have been published, along with 5 scientific conference papers and 3 conference presentations without proceedings. In addition, three lectures have been delivered: two at foreign universities and one at a boiling heat transfer winter school for PhD students in France.

T5.3 Presentation and dissemination of results to industrial partners (M30–M36)
This task is scheduled for implementation during the final year of the project.

• BUCCI, Mattia, BERCE, Jure, ZUPANČIČ, Matevž, MOŽE, Matic, GOLOBIČ, Iztok. Dry spot propagation during boiling crisis on thin metal heaters. Applied thermal engineering. Dec. 2024, vol. 257, part a, [article no.] 124207, str. 1-12, ilustr. ISSN 1359-4311. https://www.sciencedirect.com/science/article/pii/S1359431124018751, DOI: 10.1016/j.applthermaleng.2024.124207.
• BUCCI, Mattia, ZUPANČIČ, Matevž, MOŽE, Matic, GOLOBIČ, Iztok. Influence of thin heater properties on wall heat transfer dynamics during quasi-steady bubble life cycle : presentation at the conference ‘Micro and Nanoscale Phase Change Phenomena’, Gordon Research Conference, January 12-17, 2025, Pomona, California, United States.
• JEREB, Samo, MOŽE, Matic, ZUPANČIČ, Matevž, GOLOBIČ, Iztok. Eksperimentalno ovrednotenje Laplaceovega tlaka kapljice pri prehodu iz heterogenega v homogeno omočenje površine. V: Akademija strojništva 2024 : inženirstvo – inovativen trajnostni razvoj z visoko dodano vrednostjo : Ljubljana, Cankarjev dom ter prek spleta, 10. december 2024.
• SHANG, Yuheng, MOŽE, Matic, TANSU AKSOY, Yunus, CASTAGNE, Sylvie, SEVENO, David, GOLOBIČ, Iztok, VETRANO, Maria Rosaria. Effect of surface wettability on dynamic behaviors and freezing mechanisms of water droplets impacting cold surfaces. Physics of fluids. Sep. 2024, vol. 36, iss. 9, [article no.] 092004, str. 1-22, ilustr. ISSN 1070-6631. DOI: 10.1063/5.0222155.
• RODIČ, Peter, KOVAČ, Nina, KRALJ, Slavko, JEREB, Samo, GOLOBIČ, Iztok, MOŽE, Matic, MILOŠEV, Ingrid. Anti-corrosion and anti-icing properties of superhydrophobic laser-textured aluminum surfaces. Surface & coatings technology. [Print ed.]. 2024, vol. 494, art. 131325, str. 1-15, ilustr. ISSN 0257-8972. https://www.sciencedirect.com/science/article/pii/S0257897224009563, DOI: 10.1016/j.surfcoat.2024.131325.
• LOVŠIN, Robert, JEREB, Samo, ZUPANČIČ, Matevž, MOŽE, Matic. Influence of laser-fabricated surface features on droplet spreading, rebound behaviour, and ice adhesion on superhydrophobic surfaces. V: Akademija strojništva 2024 : inženirstvo – inovativen trajnostni razvoj z visoko dodano vrednostjo : Ljubljana, Cankarjev dom ter prek spleta, 10. december 2024. Ljubljana: Zveza strojnih inženirjev Slovenije – ZSIS, 2024. Letn. 13, št. 1/6, str. 76-77, ilustr. Svet strojništva, letn. 13, št. 1/6. ISSN 1855-6493.  DOI: 10.62020/svet.str.as2024017.
• ARHAR, Klara, MOŽE, Matic, ZUPANČIČ, Matevž, GOLOBIČ, Iztok. Rast vodikovih mehurčkov pri elektrolizi na platinasti mikroelektrodi. V: Akademija strojništva 2024 : inženirstvo – inovativen trajnostni razvoj z visoko dodano vrednostjo : Ljubljana, Cankarjev dom ter prek spleta, 10. december 2024. Ljubljana: Zveza strojnih inženirjev Slovenije – ZSIS, 2024. Letn. 13, št. 1/6, str. 118-119, ilustr. Svet strojništva, letn. 13, št. 1/6. ISSN 1855-6493. DOI: 10.62020/svet.str.as2024038.
• MOŽE, Matic, JEREB, Samo, LOVŠIN, Robert, SHANG, Yuheng, ZUPANČIČ, Matevž, VETRANO, Maria Rosaria, GOLOBIČ, Iztok. Reducing contact time of impacting droplets and enhancing freezing resistance with rationally microengineered superhydrophobic surfaces with micropillar arrays : presentation at the conference ‘Micro and Nanoscale Phase Change Phenomena’, Gordon Research Conference, January 12-17, 2025, Pomona, California, United States.
• ŽALEC, Domen, MOŽE, Matic, ZUPANČIČ, Matevž, GOLOBIČ, Iztok. Elucidating the effects of surface wettability on boiling heat transfer using hydrophilic and hydrophobic surfaces with laser-etched microchannels. Case studies in thermal engineering. May 2024, vol. 57, [article no.] 104357, str. 1-17, ilustr. ISSN 2214-157X. DOI: 10.1016/j.csite.2024.104357.
• ŽALEC, Domen, HADŽIĆ, Armin, MOŽE, Matic, GOLOBIČ, Iztok. Hierarchical surfaces with open microchannels and laser-induced microcavities for enhancement of pool boiling critical heat flux. International journal of heat and mass transfer. Dec. 2024, vol. 235, [article no.] 126192, str. 1-12, ilustr. ISSN 1879-2189. DOI: 10.1016/j.ijheatmasstransfer.2024.126192.
• HADŽIĆ, Armin, MOŽE, Matic, ZUPANČIČ, Matevž, GOLOBIČ, Iztok. Aluminum micropillar surfaces with hierarchical micro- and nanoscale features for enhancement of boiling heat transfer coefficient and critical heat flux. Nanomaterials. [Online ed.]. Apr. 2024, vol. 14, iss. 8, [article no.] 667, str. 1-19, ilustr. ISSN 2079-4991. DOI: 10.3390/nano14080667.
• BERCE, Jure, HADŽIĆ, Armin, ZUPANČIČ, Matevž, MOŽE, Matic, GOLOBIČ, Iztok. Obstojne superhidrofobne bakrene površine za izboljšan prenos toplote pri mehurčkastem vrenju. V: Akademija strojništva 2024 : inženirstvo – inovativen trajnostni razvoj z visoko dodano vrednostjo : Ljubljana, Cankarjev dom ter prek spleta, 10. december 2024. Ljubljana: Zveza strojnih inženirjev Slovenije – ZSIS, 2024. Letn. 13, št. 1/6, str. 52-53, ilustr. Svet strojništva, letn. 13, št. 1/6. DOI: 10.62020/svet.str.as2024005.
• HADŽIĆ, Armin, ŽALEC, Domen, ZUPANČIČ, Matevž, MOŽE, Matic, GOLOBIČ, Iztok. Pool boiling heat transfer performance enhancement on copper surfaces with laser-textured open microchannel. V: Akademija strojništva 2024 : inženirstvo – inovativen trajnostni razvoj z visoko dodano vrednostjo : Ljubljana, Cankarjev dom ter prek spleta, 10. december 2024. Ljubljana: Zveza strojnih inženirjev Slovenije – ZSIS, 2024. Letn. 13, št. 1/6, str. 80-81, ilustr. Svet strojništva, letn. 13, št. 1/6. DOI: 10.62020/svet.str.as2024019.
• BUCCI, Mattia, ZUPANČIČ, Matevž, MOŽE, Matic, GOLOBIČ, Iztok. Influence of thin heater properties on wall heat transfer dynamics during quasi-steady bubble life cycle : presentation at the conference ‘Micro and Nanoscale Phase Change Phenomena’, Gordon Research Conference, January 12-17, 2025, Pomona, California, United States.
• BERCE, Jure, HADŽIĆ, Armin, ZUPANČIČ, Matevž, GOLOBIČ, Iztok. Towards long-term durability of superhydrophobic laser-microengineered surfaces for enhanced nucleate pool boiling : presentation at the conference ‘Micro and Nanoscale Phase Change Phenomena’, Gordon Research Conference, January 12-17, 2025, Pomona, California, United States.
• JEREB, Samo, MOŽE, Matic, ZUPANČIČ, Matevž, GOLOBIČ, Iztok. Eksperimentalno ovrednotenje Laplaceovega tlaka kapljice pri prehodu iz heterogenega v homogeno omočenje površine. V: Akademija strojništva 2024 : inženirstvo – inovativen trajnostni razvoj z visoko dodano vrednostjo : Ljubljana, Cankarjev dom ter prek spleta, 10. december 2024.