title: Is a Knowledge of Surface Topology and Contact Angles Enough to Define the Drop Impact Outcome?
abstract: It is well known that a superhydrophobic surface may not be able to repel impacting droplets because of the so-called Cassie-to-Wenzel transition. It has been proven that a critical value of the receding contact angle (θR) exists for the complete rebound of water, recently experimentally measured to be 100° for a large range of impact velocities. On the contrary, in the present work, no rebound was observed when low-surface-tension liquids such as hexadecane (σ = 27.5 mN/m at 25 °C) are concerned, even for very low impact velocities and very high values of θR and low contact angle hysteresis. Therefore, the critical threshold of θR ≈ 100° does not sound acceptable for all liquids and for all hydrophobic surfaces. For the same Weber numbers, a Cassie-to-Wenzel state transition occurs after the impact as a result of the easier penetration of low-surface-tension fluids in the surface structure. Hence, a criterion for the drop rebound of low-surface-tension liquids must consider not only the contact angle values with surfaces but also their surface tension and viscosity. This suggests that, even if it is possible to produce surfaces with enhanced static repellence against oils and organics, generally the realization of synthetic materials with self-cleaning and antisticking abilities in dynamic phenomena, such as spray impact, remains an unsolved task. Moreover, it is demonstrated that the chemistry of the surface, the physicochemical interactions with the liquid drops, and the possible wettability gradient of the surface asperity also play important roles in determining the critical Weber number above which impalement occurs. Therefore, the classical numerical simulations of drop impact on dry surfaces are definitively not able to capture the final outcomes of the impact for all possible fluids if the surface topology and chemistry and/or the wettability gradient in the surface structure are not properly reflected.
authors.affiliation: Department of Engineering and Applied Sciences, University of Bergamo , Viale Marconi 5, 24044 Dalmine (BG), Italy.
authors.forename: Ileana
authors.initials: I
authors.lastname: Malavasi
authors.affiliation: ISTEC CNR - Institute of Science and Technology for Ceramics , Via Granarolo 64, 48018 Faenza (RA), Italy.
authors.forename: Federico
authors.initials: F
authors.lastname: Veronesi
authors.affiliation: ISTEC CNR - Institute of Science and Technology for Ceramics , Via Granarolo 64, 48018 Faenza (RA), Italy.
authors.forename: Aurora
authors.initials: A
authors.lastname: Caldarelli
authors.affiliation: Department of Physics, Politecnico di Milano , Piazza Leonardo da Vinci 32, 20133 Milano, Italy.|Center for Nano Science and Technology @Polimi, Istituto Italiano di Tecnologia (IIT) , Via G. Pascoli 70/3, 20133 Milano, Italy.
authors.forename: Maurizio
authors.initials: M
authors.lastname: Zani
authors.affiliation: ISTEC CNR - Institute of Science and Technology for Ceramics , Via Granarolo 64, 48018 Faenza (RA), Italy.
authors.forename: Mariarosa
authors.initials: M
authors.lastname: Raimondo
authors.affiliation: Department of Engineering and Applied Sciences, University of Bergamo , Viale Marconi 5, 24044 Dalmine (BG), Italy.|School of Computing, Engineering and Mathematics, University of Brighton , Lewes Road, BN2 4GJ Brighton, United Kingdom.
authors.forename: Marco
authors.initials: M
authors.lastname: Marengo