Experiments established that chemically patterning a microchannel wall with hydrophobic dots on a hydrophilic background enhances subcooled flow-boiling heat transfer by about 22% over a homogeneous hydrophilic channel, and a force-balance model explained the bubble-mobility mechanism. What remains unresolved is the predictive design problem: no validated quantitative framework determines how pattern geometry, hydrophobic (non-wetting) fraction, and pitch control heat transfer, flow-regime transitions, long-term stability, and dryout/critical-heat-flux limits as channel size, working fluid, and operating conditions vary. Later work has produced single-setup optima, departure-force models, and CHF-enhancement demonstrations, but no validated general predictive theory for patterned-surface flow boiling has yet been established. The problem is to establish and validate such a framework, turning wettability patterning from empirical trial-and-error into quantitative design.

Background

Two-phase microchannel heat sinks are a leading approach to cooling high-power-density electronics because boiling removes large heat fluxes at nearly constant wall temperature. In subcooled flow boiling the incoming liquid is kept below its saturation temperature, so vapor bubbles nucleate at the heated wall while the bulk liquid remains cooler than saturation; heat removal combines latent-heat exchange near the wall with forced convection. Performance is commonly quantified by the heat-transfer coefficient (HTC, heat removed per unit area per degree of wall superheat) and bounded by the critical heat flux (CHF), the heat flux at which vapor blankets or dries out the wall and the wall temperature rises abruptly.

Wall wettability — the contact angle a liquid drop makes on the surface — controls both limits. Hydrophobic (poorly wetting) surfaces trap vapor in surface cavities, so bubble nucleation starts at low wall superheat, but they rewet poorly and fail by dryout at low CHF. Hydrophilic (well-wetting) surfaces rewet vigorously and sustain high CHF but nucleate poorly. A wettability-patterned (biphilic or chemically patterned) surface combines the two: hydrophobic patches act as engineered, positioned nucleation sites on a hydrophilic background. The pitch is the center-to-center spacing of the patches and the non-wetting (hydrophobic) fraction is the fraction of the heated area that is hydrophobic.

Wang, Yang, He and Qiu (2019) studied subcooled flow boiling of water in a single microchannel of 311.76 µm hydraulic diameter and 20 mm heated length, with mass fluxes of 103–203 kg/m2^2s and heat fluxes of 2.56–78.61 W/cm2^2, comparing a wall patterned with hydrophobic dots against a homogeneous hydrophilic wall. The patterned channel raised the HTC by about 22%. The mechanism they identified couples nucleation and bubble transport: bubbles nucleate preferentially on the hydrophobic dots, and on the patterned wall they exhibit higher mobility (including jump-type motion across wettability contrasts) and a longer three-phase contact line (the line along which liquid, vapor, and solid meet), both of which promote thin-film evaporation and liquid supply. They also developed an analytical model of the local forces governing bubble departure, showing that patterning reduces the detaching force needed compared with a uniformly hydrophobic wall.

The study left open what a designer needs. The optimal pattern geometry, hydrophobic fraction, and pitch were not determined, and no map exists of how these design variables shift with channel size, working fluid, or heated length. The experiments covered one channel and a narrow operating window, so the generality of the 22% enhancement and of the force-model prediction is untested. The nucleation, bubble-mobility, and contact-line mechanisms were established only qualitatively, and beyond them no predictive theory exists for flow-regime transition boundaries (the progression from bubbly to slug to annular flow — discrete bubbles, then elongated vapor slugs, then a vapor core lined by a liquid film — that governs microchannel boiling), long-term stability (constancy of the heat-transfer coefficient and absence of pressure and flow oscillations over the operating period, not the chemical durability of the surface treatment), or dryout and CHF limits on patterned surfaces in flow boiling. Whether a wettability-patterned surface can deliver both a higher heat-transfer coefficient and a high critical heat flux under flow-boiling conditions was already posed as the open motivating question of the same group's earlier conference study (Wang, He and Qiu, 2018), and later reviews identify stability and the synergy between flow pattern and heat transfer as open directions.

Problem Statement

Establish a validated predictive framework for subcooled flow boiling in microchannels whose walls are chemically patterned with hydrophobic features on a hydrophilic background, replacing the current empirical, single-setup findings with quantitative design capability. The framework addresses the design-optimization gap left open by Wang, Yang, He and Qiu (2019) together with the flow-regime, stability, and dryout/CHF questions posed by the same group's research program and by later reviews. Each numbered item below is a separately resolvable sub-question: a quantitative treatment of any one item settles that sub-question, and the framework is complete only when all four are settled.

  1. Determine how the pattern design variables — patch geometry (shape and size), non-wetting (hydrophobic) fraction, pitch, and pattern orientation — control the heat-transfer coefficient and the underlying bubble dynamics (nucleation, mobility/jump motion, contact-line length, coalescence), and identify the corresponding optima (the pattern designs that maximize the heat-transfer coefficient subject to the critical heat flux not falling below that of the homogeneous hydrophilic baseline), together with how these optima shift with the operating conditions (mass flux, heat flux, inlet subcooling).

  2. Predict the flow-regime transition boundaries (bubbly/slug/annular and the transitions among them) on patterned surfaces, extending the experimentally established qualitative nucleation, bubble-mobility, and contact-line mechanisms into a quantitative criterion.

  3. Predict the long-term stability of patterned-surface flow boiling and its dryout and critical-heat-flux limits, including how these limits depend on the pattern design variables.

  4. Establish the domain of validity of these predictions — how the optima, transition boundaries, and dryout/CHF limits of items 1–3 shift with channel size and working fluid, and the ranges of channel size, working fluid, and heated length within which they hold — the generality that the single-channel, restricted mass- and heat-flux window of the original experiments could not test.

References

  1. Hongzhao Wang, Yinchuang Yang, Minghao He, Huihe Qiu (2019). Subcooled flow boiling heat transfer in a microchannel with chemically patterned surfaces. International Journal of Heat and Mass Transfer 140:587-597. DOI: 10.1016/j.ijheatmasstransfer.2019.06.027. https://doi.org/10.1016/j.ijheatmasstransfer.2019.06.027
  2. Hongzhao Wang, Yinchuang Yang, Ying Wang, Christopher Y.H. Chao, Huihe Qiu (2022). Effects of non-wetting fraction and pitch distance in flow boiling heat transfer in a wettability-patterned microchannel. International Journal of Heat and Mass Transfer 190:122753. DOI: 10.1016/j.ijheatmasstransfer.2022.122753. https://doi.org/10.1016/j.ijheatmasstransfer.2022.122753
  3. Jonghyun Kim, Jae Yong Cho, Joon Sang Lee (2020). Flow boiling enhancement by bubble mobility on heterogeneous wetting surface in microchannel. International Journal of Heat and Mass Transfer 153:119631. DOI: 10.1016/j.ijheatmasstransfer.2020.119631. https://doi.org/10.1016/j.ijheatmasstransfer.2020.119631
  4. Akam Aboubakri, Vahid Ebrahimpour Ahmadi, Suleyman Celik, Abdolali K. Sadaghiani, Khellil Sefiane, Ali Kosar (2021). Effect of Surface Biphilicity on FC-72 Flow Boiling in a Rectangular Minichannel. Frontiers in Mechanical Engineering 7:755580. DOI: 10.3389/fmech.2021.755580. https://doi.org/10.3389/fmech.2021.755580
  5. Christopher Salmean, Huihe Qiu (2022). Flow Boiling Enhancement Using Three-Dimensional Contact-Line Pinning on Hierarchical Superbiphilic Micro/Nanostructures. Nano Letters 22:8487-8494. DOI: 10.1021/acs.nanolett.2c02915. https://doi.org/10.1021/acs.nanolett.2c02915
  6. Christopher Salmean, Huihe Qiu (2023). Flow Boiling Heat Transfer Enhancement Using Tuned Geometrical Contact-Line Pinning. ACS Applied Materials & Interfaces 15:23844-23859. DOI: 10.1021/acsami.2c21813. https://doi.org/10.1021/acsami.2c21813
  7. Shuang Cao, Hui Yang, Luxing Zhao, Tao Wang, Jian Xie (2021). Recent Advances of Surface Wettability Effect on Flow Boiling Heat Transfer Performance. Frontiers in Heat and Mass Transfer 17:17. DOI: 10.5098/hmt.17.17. https://doi.org/10.5098/hmt.17.17
  8. Huiqing Shang, Guodong Xia, Lixin Cheng, Shanshan Miao (2025). Comprehensive review of enhancement techniques and mechanisms for flow boiling in micro/mini-channels. Applied Thermal Engineering 258:124783. DOI: 10.1016/j.applthermaleng.2024.124783. https://doi.org/10.1016/j.applthermaleng.2024.124783
  9. Cong Deng, Xiaoping Luo, Zhiwei Sun, Jinxin Zhang, Yijie Fan, Donglin Liu (2025). Investigation on Critical Heat Flux of Flow Boiling in Rectangular Microchannels: A Parametric Study and Assessment of New Prediction Method. Energies 18:4866. DOI: 10.3390/en18184866. https://doi.org/10.3390/en18184866
  10. Yuting Jia, Jiakang Wang, Zihao Lin, Jingtao Wang (2026). Effects of alternating hydrophilic/hydrophobic structure on flow boiling heat transfer and flow stability in microchannels. Applied Thermal Engineering 297:130913. DOI: 10.1016/j.applthermaleng.2026.130913. https://doi.org/10.1016/j.applthermaleng.2026.130913
  11. Yuting Jia, Zihao Lin, Jiakang Wang, Jingtao Wang, Zhiming Xu (2026). Experimental investigation on flow boiling heat transfer characteristics in microchannels with heterogeneous modified surfaces. International Journal of Heat and Mass Transfer 255:127752. DOI: 10.1016/j.ijheatmasstransfer.2025.127752. https://doi.org/10.1016/j.ijheatmasstransfer.2025.127752
  12. Shikun Liu, Hitoshi Asano, Katsumi Sugimoto (2026). Investigation on hysteresis effect of enhanced subcooled flow boiling by surface modification in horizontal narrow channel. Experimental Heat Transfer (online first, pp. 1-19). DOI: 10.1080/08916152.2026.2633425. https://doi.org/10.1080/08916152.2026.2633425
  13. Thalles Coimbra Borba Roldão, Cristiano Bigonha Tibiriçá (2025). Experimental local heat transfer coefficient for subcooled flow boiling in a microchannel and new predictive method. Applied Thermal Engineering 268:125826. DOI: 10.1016/j.applthermaleng.2025.125826. https://doi.org/10.1016/j.applthermaleng.2025.125826
  14. Hongzhao Wang, Minghao He, Huihe Qiu (2018). Bubble Dynamics and Flow Boiling Characteristics in a Chemically Patterned Microchannel. Proceedings of the 4th World Congress on Mechanical, Chemical, and Material Engineering (MCM'18), Madrid, Spain, Paper HTFF 130. DOI: 10.11159/htff18.130. https://doi.org/10.11159/htff18.130