Carlo Antonini

On the Nanoengineering of Superhydrophobic and Impalement Resistant Surface Textures below the Freezing Temperature

The superhydrophobic behavior of nano- and microtextured surfaces leading to rebound of impacting droplets is of great relevance to nature and technology. It is not clear however, if and under what conditions this behavior is maintained when such surfaces are severely undercooled possibly leading to the formation of frost and icing. Here we elucidate key aspects of this phenomenon and show that the outcome of rebound or impalement on a textured surface is affected by air compression underneath the impacting drop and the time scale allowing this air to escape. Remarkably, drop impalement occurred at identical impact velocities, both at room and at very low temperatures (-30 °C) and featured a ringlike liquid meniscus penetration into the surface texture with an entrapped air bubble in the middle. At low temperatures, the drop contact time and receding dynamics of hierarchical surfaces were profoundly influenced by both an increase in the liquid viscosity due to cooling and a partial meniscus penetration into the texture. For hierarchical surfaces with the same solid fraction in their roughness, minimizing the gap between the asperities (both at micro- and nanoscales) yielded the largest resistance to millimetric drop impalement. The best performing surface impressively showed rebound at -30 °C for drop impact velocity of 2.6 m/s.

Physics of icing and rational design of surfaces with extraordinary icephobicity.

Icing of surfaces is commonplace in nature and technology, affecting everyday life and sometimes causing catastrophic events. Understanding (and counteracting) surface icing brings with it significant scientific challenges that requires interdisciplinary knowledge from diverse scientific fields such as nucleation thermodynamics and heat transfer, fluid dynamics, surface chemistry, and surface nanoengineering. Here we discuss key aspects and findings related to the physics of ice formation on surfaces and show how such knowledge could be employed to rationally develop surfaces with extreme resistance to icing (extraordinary icephobicity). Although superhydrophobic surfaces with micro-, nano-, or (often biomimetic) hierarchical roughnesses have shown in laboratory settings (under certain conditions) excellent repellency and low adhesion to water down to temperatures near or below the freezing point, extreme icephobicity necessitates additional important functionalities. Other approaches, such as lubricant-impregnated surfaces, exhibit both advantages and serious limitations with respect to icing. In all, a clear path toward passive surfaces with extreme resistance to ice formation remains a challenge, but it is one well worth undertaking. Equally important to potential applications is scalable surface manufacturing and the ability of icephobic surfaces to perform reliably and sustainably outside the laboratory under adverse conditions. Surfaces should possess mechanical and chemical stability, and they should be thermally resilient. Such issues and related research directions are also addressed in this article.

Drop impact and wettability: From hydrophilic to superhydrophobic surfaces

Experiments to understand the effect of surface wettability on impact characteristics of water drops onto solid dry surfaces were conducted. Various surfaces were used to cover a wide range of contact angles (advancing contact angle from 48° to 166°, and contact angle hysteresis from 5° to 56°). Several different impact conditions were analyzed (12 impact velocities on 9 different surfaces, among which 2 were superhydrophobic). Results from impact tests with millimetric drops show that two different regimes can be identified: a moderate Weber number regime (30 200), in which wettability effect is secondary, because capillary forces are overcome by inertial effects. In particular, results show the role of advancing contact angle and contact angle hysteresis as fundamental wetting parameters to allow understanding of different phases of drop spreading and beginning of recoiling. It is also shown that drop spreading on hydrophilic and superhydrophobic surfaces occurs with different time scales. Finally, if the surface is superhydrophobic, eventual impalement, i.e., transition from Cassie to Wenzel wetting state, which might occur in the vicinity of the drop impact area, does not influence drop maximum spreading.

Supercooled Water Drops Impacting Superhydrophobic Textures

Understanding the interaction of supercooled metastable water with superhydrophobic surface textures is of fundamental significance for unraveling the mechanisms of icing as well as of practical importance for the rational development of surface treatment strategies to prevent icing. We investigate the problem of supercooled water drops impacting superhydrophobic textures for drop supercooling down to -17 °C and find that increased viscous effects significantly influence all stages of impact dynamics, in particular, the impact and meniscus impalement behavior, with severe implications to water retention by the textures (sticky versus rebounding drop) and possible icing. Viscous effects in water supercooling conditions cause a reduction of drop maximum spreading (∼25% at an impact speed of 3 m/s for a millimetric drop) and can significantly decrease the drop recoil speed when the meniscus partially penetrates into the texture, leading to an increase of the contact time up to a factor of 2 in supercooling conditions compared to room temperature. We also show that meniscus penetration upon drop impact occurs with full penetration at the center, instead of ring shape, common to room temperature drop impact. To this end, we describe an unobserved mechanism for superhydrophobicity breakdown: unlike for room temperature drops, where transition from bouncing to sticky (impaled) behavior occurs sharply at the condition of full texture penetration, with a bubble captured at the point of impact, under supercooled conditions, the full penetration velocity threshold is increased markedly (increasing by ∼25%, from 2.8 to 3.5 m/s) and no bubble is entrapped. However, even though only partial texture penetration takes place, failure to completely dewet because of viscous effects can still prohibit complete supercooled drop rebound.

General Methodology for Evaluating the Adhesion Force of Drops and Bubbles on Solid Surfaces

The shortcomings of the current formulation for calculating the adhesion force for drops and bubbles with noncircular contact lines are discussed. A general formulation to evaluate the adhesion force due to surface forces is presented. Also, a novel methodology, that is, IBAFA, image based adhesion force analysis, was developed to allow implementation of the general formulation. IBAFA is based on the use of multiple profile images of a drop. The images are analyzed (1) to accurately reconstruct the contact line shape, which is analytically represented by a Fourier cosine series, and (2) to measure contact angles at multiple locations along the contact line and determine the contact angle distribution based on a linear piecewise interpolation routine. The contact line shape reconstruction procedure was validated with both actual experiments and simulated experiments. The procedure for the evaluation of the adhesion force was tested using simulated experiments with synthetic drops of known shapes. A comparison with current methods showed that simplifying assumptions (e.g., elliptical contact line or linear contact angle distribution) used in these methods result in errors up to 76% in the estimated adhesion force. However, the drop adhesion force evaluated using IBAFA results in small errors on the order of 1%.

Unraveling wetting transition through surface textures with X-rays: Liquid meniscus penetration phenomena

In this report we show that synchrotron X-ray radiography is a powerful method to study liquid-air interface penetration through opaque microtextured surface roughness, leading to wetting transition. We investigate this wetting phenomenon in the context of sessile drop evaporation, and establish that liquid interface sinking into the surface texture is indeed dictated by the balance of capillary and Laplace pressures, where the intrinsically three-dimensional nature of the meniscus must be accounted for. Air bubble entrapment in the texture underneath impacting water drops is also visualized and the mechanisms of post-impact drop evaporation are discussed.

Understanding the drop impact phenomenon on soft PDMS substrates

The dynamics of water droplets impacting on a soft dry substrate was studied experimentally. Both the drop evolution (i.e. spreading, recoiling, as well as oscillations) upon impact on a soft gel substrate (i.e. polydimethylsiloxane, PDMS) and substrate deformation caused by the impact force (for Weber numbers 110 < We < 520) were analyzed. Results were compared with similar tests carried out using hard, non-deformable PDMS substrates (two orders of magnitude more rigid than soft substrates). A sensor was designed based on the principle of a two-degree of freedom lever and successfully implemented to measure the maximum deformation of soft substrates during drop impact. Analysis of drop deformation highlighted that the recoil phase on a soft surface is slower, the final diameter of the resting drop is larger, and consequently the final resting contact angle is lower, when compared to impacts on hard substrates. Also, the oscillation time is significantly reduced, i.e. by half on soft substrates, compared to hard ones. Differences are likely due to the absorbance of impact energy due to substrate deformation. These findings are significant as for the first time it is shown that drop impact dynamics can be changed by manipulating the substrate stiffness. In addition, the following were found: drop impact can cause significant deformation of soft substrates, on the order of tens of micrometer. Different analytical models borrowed from solid mechanics of deformable bodies and drop impact on non-deformable surfaces were used and compared to estimate the force upon impact at the liquid–solid interface; in addition, a numerical model to study soft substrate deformation dynamics was developed. Analytical models significantly disagree on the magnitude of the impact force at the liquid–solid interface: model assumptions are critically discussed to explain shortcomings of such models. Numerical simulations showed that in strongly dynamic conditions, the PDMS substrate may respond to loads more rigidly than in static conditions.

Prediction of the dynamic response of complex transmission line systems for unsteady pressure measurements

Among the measurement and control systems of gas turbine engines, a recent new issue is the possibility of performing unsteady pressure measurements to detect flow anomalies in an engine or to evaluate loads on aerodynamic surfaces. A possible answer to this demand could be extending the use of well known and widely used transmission line systems, which have been applied so far to steady monitoring, to unsteady measurements thanks to proper dynamic modeling and compensation. Despite the huge number of models existing in the literature, a novel method has been developed, which is at the same time easy-to-handle, flexible and capable of reproducing the actual physics of the problem. Furthermore, the new model is able to deal with arbitrary complex networks of lines and cavities, and thus its applicability is not limited to series-connected systems. The main objectives of this paper are to show the derivation of the model, its validation against experimental tests and example of its applicability.

Water Touch-and-Bounce from a Soft Viscoelastic Substrate: Wetting, Dewetting, and Rebound on Bitumen.

Understanding the interaction between liquids and deformable solid surfaces is a fascinating fundamental problem, in which interaction and coupling of capillary and viscoelastic effects, due to solid substrate deformation, give rise to complex wetting mechanisms. Here we investigated as a model case the behavior of water drops on two smooth bitumen substrates with different rheological properties, defined as hard and soft (with complex shear moduli in the order of 10(7) and 10(5) Pa, respectively, at 1 Hz), focusing both on wetting and on dewetting behavior. By means of classical quasi-static contact angle measurements and drop impact tests, we show that the water drop behavior can significantly change from the quasi-static to the dynamic regime on soft viscoelastic surfaces, with the transition being defined by the substrate rheological properties. As a result, we also show that on the hard substrate, where the elastic response is dominant under all investigated conditions, classical quasi-static contact angle measurements provide consistent results that can be used to predict the drop dynamic wetting behavior, such as drop deposition or rebound after impact, as typically observed for nondeformable substrates. Differently, on soft surfaces, the formation of wetting ridges did not allow to define uniquely the substrate intrinsic advancing and receding contact angles. In addition, despite showing a high adhesion to the soft surface in quasi-static measurements, the drop was surprisingly able to rebound and escape from the surface after impact, as it is typically observed for hydrophobic surfaces. These results highlight that measurements of wetting properties for viscoelastic substrates need to be critically used and that wetting behavior of a liquid on viscoelastic surfaces is a function of the characteristic time scales.

Detergency and Its Implications for Oil Emulsion Sieving and Separation

Separating petroleum hydrocarbons from water is an important problem to address in order to mitigate the disastrous effects of hydrocarbons on aquatic ecosystems. A rational approach to address the problem of marine oil-water separation is to disperse the oil with the aid of surfactants in order to minimize the formation of large slicks at the water surface and to maximize the oil-water interfacial area. Here we investigate the fundamental wetting and transport behavior of such surfactant-stabilized droplets and the flow conditions necessary to perform sieving and separation of these stabilized emulsions. We show that, for water-soluble surfactants, such droplets are completely repelled by a range of materials (intrinsically underwater superoleophobic) due to the detergency effect; therefore, there is no need for surface micro-/nanotexturing or chemical treatment to repel the oil and prevent fouling of the filter. We then simulate and experimentally investigate the effect of emulsion flow rate on the transport and impact behavior of such droplets on rigid meshes to identify the minimum pore opening (w) necessary to filter a droplet with a given diameter (d) in order to minimize the pressure drop across the mesh-and therefore maximize the filtering efficiency, which is strongly dependent on w. We define a range of flow conditions and droplet sizes where minimum droplet deformation is to be expected and therefore find that the condition of w ≈ d is sufficient for efficient separation. With this new understanding, we demonstrate the use of a commercially available filter-without any additional surface engineering or functionalization-to separate oil droplets (d < 100 μm) from a surfactant-stabilized emulsion with a flux of ∼11,000 L m-2 h-1 bar-1. We believe these findings can inform the design of future oil separation materials.