Bioinspired surfaces for robust submerged superhydrophobicity: insights from molecular dynamics

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1 Bioinspired surfaces for robust submerged superhydrophobicity: insights from molecular dynamics PRACE project SLIP - Salvinia-like surfaces in action: slip, cavitation, drag reduction Alberto Giacomello alberto.giacomello@uniroma1.it Dipartimento di Ingegneria Meccanica e Aerospaziale PRACEdays17 Barcelona, 16 th May 2017 A. Giacomello (DIMA, Sapienza) Submerged superhydrophobicity PRACEdays17, Barcelona 1 / 22

2 Superhydrophobicity en plein air The Lotus effect copyright W. Barthlott MICROSCOPIC ORIGIN surface microstructure hydrophobic chemistry superhydrophobic Cassie state scales involved: from nm to mm [Barthlott & Neinhuis, 1997] A. Giacomello (DIMA, Sapienza) Submerged superhydrophobicity PRACEdays17, Barcelona 2 / 22

3 Submerged superhydrophobicity Cassie state air cushion Slip in laminar and turbulent flows Robustness & durability A. Giacomello (DIMA, Sapienza) Submerged superhydrophobicity PRACEdays17, Barcelona 3 / 22

4 A new vegetable muse: Salvinia molesta [Barthlott et al., Adv. Mater. 2010] Understanding the Salvinia paradox Design criteria for durable submerged superhydrophobicity Stabilize superhydrophobicity against cavitation and intrusion A. Giacomello (DIMA, Sapienza) Submerged superhydrophobicity PRACEdays17, Barcelona 4 / 22

5 Superhydrophobicity breaks down Two kinds of stability KINETIC stability against thermally activated barrier crossing: τ exp [ F (P)/(k B T )] MECHANICAL stability against spinodal transition F (P) 0 F Wenzel Cassie z A. Giacomello (DIMA, Sapienza) Submerged superhydrophobicity PRACEdays17, Barcelona 5 / 22

6 Unraveling the Salvinia Paradox: design principles for submerged superhydrophobicity Vol. 2 No. 14 September [Amabili et al., Adv. Mater. Interfaces 2015] & [Amabili et al., Soft Matter 2016] A. Giacomello (DIMA, Sapienza) Submerged superhydrophobicity PRACEdays17, Barcelona 6 / 22

7 Molecular Dynamics (MD) setup T-shaped geometry: MD model: Modified Lennard Jones potential Cavity of 5 nm 100k atoms Free energy via restrained MD 10M CPU hours (Fermi, CINECA) A. Giacomello (DIMA, Sapienza) Submerged superhydrophobicity PRACEdays17, Barcelona 7 / 22

8 Systems under investigation 3 different chemistries in submerged conditions: Hydrophobic Salvinia like Hydrophilic constant N,P,T simulation: negative pressures: P = P P vap < 0 positive pressures: P = P P vap > 0 A. Giacomello (DIMA, Sapienza) Submerged superhydrophobicity PRACEdays17, Barcelona 8 / 22

9 Intrusion and extrusion experiment a) 8 intrusion b) 4 extrusion P [MP a] 0 4 P [MP a] texp t[ns] φv P [MP a] intrusion extrusion [Amabili et al., J. Phys.: Condens. Matter 2017] Intrusion: Two Transitions! Extrusion: Wenzel irreversible even at P 0 A. Giacomello (DIMA, Sapienza) Submerged superhydrophobicity PRACEdays17, Barcelona 9 / 22

10 Intrusion and extrusion experiment Quasi static interpretation: Ω/kBT P = 4.2MP a P = 3.0MP a P = 0MP a P = 2.1MP a P = 4.2MP a φ v φv P [MP a] intrusion extrusion Cassie Wenzel P range [MPa] Cassie inner-cassie Wenzel P > < P < < P < < P < 2.0 P < 3.5 For hydrophobic chem. : P sp Cv > Psp Wv = NO RECOVERY A. Giacomello (DIMA, Sapienza) Submerged superhydrophobicity PRACEdays17, Barcelona 10 / 22

11 Restrained molecular dynamics Adapted from [Maragliano & Vanden-Eijnden, J. Chem. Phys. 2006] Bias potential (physical+umbrella) keeps the system at ε(x) = z U k (r, z) = V (r) + 1 κ(z ε(r))2 2 Mean force equals negative free energy gradient: lim κ κ(ε(r) z) bias = ( Ω/ z) z Biased NPT simulations provide statistics for N replicas A. Giacomello (DIMA, Sapienza) Submerged superhydrophobicity PRACEdays17, Barcelona 11 / 22

12 Free-energy at coexistence ( P 0) 500 Ω/(kBT ) hydrophilic hydrophobic Salvinia like Φ Salvinia-like free-energy is a superimposition of hydrophobic and hydrophilic ones A. Giacomello (DIMA, Sapienza) Submerged superhydrophobicity PRACEdays17, Barcelona 12 / 22

13 Effect of pressure 1000 Ω/(kBT ) P = MPa P = 0 MPa P = 1.68 MPa Φ Pressure tilts FE profiles by a term Φ P A. Giacomello (DIMA, Sapienza) Submerged superhydrophobicity PRACEdays17, Barcelona 13 / 22

14 Kinetics of transition: the barriers life-time of a metastable state τ exp ( Ω /k B T ) Ω /(kbt ) Pmin hydrophilic hydrophobic Salvinia like Pmax P [MPa] Salvinia has the highest nucleation and intrusion barriers Salvinia resists in the Cassie state over the broadest pressure range Optimal kinetic and mechanical stability of the superhydrophobic Cassie state A. Giacomello (DIMA, Sapienza) Submerged superhydrophobicity PRACEdays17, Barcelona 14 / 22

15 Macroscopic scaling via sharp-interface calculations Spinodals approach P = 0 with increasing sizes: P sp = ±2γ/w Barriers decrease to k B T only for 1 nm cavities A. Giacomello (DIMA, Sapienza) Submerged superhydrophobicity PRACEdays17, Barcelona 15 / 22

16 Conclusions: design criteria A. Giacomello (DIMA, Sapienza) Submerged superhydrophobicity PRACEdays17, Barcelona 16 / 22

17 Can Salvinia-like surfaces be fabricated? [Tricinci et al., ACS Appl. Mater. Interf. 2015] A. Giacomello (DIMA, Sapienza) Submerged superhydrophobicity PRACEdays17, Barcelona 17 / 22

18 Can Salvinia-like surfaces be fabricated? Nanoscale + Re-entrant geometry: Checco et al. Tuteja et al. Heterogeneous chemistry: Yao et al. Varanasi et al. A. Giacomello (DIMA, Sapienza) Submerged superhydrophobicity PRACEdays17, Barcelona 17 / 22

19 Recovering superhydrophobicity A. Giacomello (DIMA, Sapienza) Submerged superhydrophobicity PRACEdays17, Barcelona 18 / 22

20 Leveraging thermal fluctuations in extreme confinement (a) w σ/2 (b) 1 P 4 P 5 extrusion Wenzel h Φ 0.5 P sp P c P c intrusion Cassie-Baxter 0 0 P P 2 P (c) P 1 P 2 P σ 3 /(k B T) P 0 P P 0 P 3 P P 1 P 2 P0 P w = 6 σ w = 21σ microscopic classical DFT for grooves with 1.5 nm < w < 6 nm Wenzel eliminated over a broad pressure range: passive strategy for SH recovery! [Giacomello, Schimmele, Dietrich, Tasinkevych, Perpetual Superhydrophobicity, Soft Matter 2016] A. Giacomello (DIMA, Sapienza) Submerged superhydrophobicity PRACEdays17, Barcelona 19 / 22

21 How to scale up perpetual SH perpetually-sh nanoscale roughness allows for θ app > 135 µm scale for drag reduction (wedge drying perpetual) see also [Li, Quéré, Lv, Zheng, Monostable superantiwettability, PNAS 2017] A. Giacomello (DIMA, Sapienza) Submerged superhydrophobicity PRACEdays17, Barcelona 20 / 22

22 Conclusions Computational challenges 1 large MD systems 2 long timescales 3 free-energy methods Rare-event MD simulations show that: 1 metastable SH is kinetically robust: τ exp [ F (P) /(k B T )] 2 the smaller the better for robustness drag-reduction maximum! 3 tuning SHS robustness via surface geometry and chemistry: Salvinia paradigm 4 µm-scale perpetual SH via hierarchical surfaces A. Giacomello (DIMA, Sapienza) Submerged superhydrophobicity PRACEdays17, Barcelona 21 / 22

23 Acknowledgements C.M. Casciola M. Amabili A. Giacomello E. Lisi A. Giacomello (DIMA, Sapienza) S. Meloni A. Tinti S. Marchio Submerged superhydrophobicity (CINECA) PRACEdays17, Barcelona 22 / 22