Slippery Liquid-infused Porous Surfaces Research Articles

Slippery Passive Radiative Cooling Supramolecular Siloxane Coatings.

Polymer coatings with comprehensive properties including passive radiative cooling, anti-fouling, and self-healing constitute a promising energy-saving strategy but have not been well documented yet. Herein, we reported a class of novel multifunctional supramolecular polysiloxane composite coatings showing the combination of these features. The coatings have a hybrid structure with a slippery liquid-infused porous surface and a gradient polymer-Al2O3 composite matrix constructed by reversible hydrogen bonding. The gradient matrix consists of a polymer-rich top and a particle-rich bottom favoring coating attachment on rigid substrates. Such a complex structure can be obtained by simply casting the suspending solutions of the polydimethylsiloxane (PDMS)-urea copolymer and Al2O3 on substrates followed by swelling silicone oil. Obtained coatings display good passive daytime radiative cooling (a temperature drop of ∼2 °C), self-healing ability, and anti-fouling properties. Since the comprehensive performances and the facile fabrication, the coatings should have application potential for various thermal management purposes.

Construction of ultrasmooth PTFE membrane for preventing bacterial adhesion and cholestasis

Bacterial adhesion and bile sludge accumulation can increase the risk of complications such as stent restenosis after biliary stent implantation. Compared with active and passive antimicrobial surfaces, a significant advantage of slippery liquid-infused porous surfaces (SLIPSs) is their recoverable anti-adhesive properties. According to the mechanism of SLIPSs and the application environments of the biliary system, a polytetrafluoroethylene (PTFE) electrospun fibrous membrane-impregnated silicone-oil system was developed to construct an ultrasmooth surface. Experimental results indicated that a PTFE SLIPS with 350 cSt of silicone oil had an extremely small roll angle (< 5°) and a high slip rate (4.8 ± 0.1 mm/s) and maintained excellent sliding stability after 7 d of immersion in model bile system. Thus, it can inhibit the adhesion of proteins, Pseudomonas aeruginosa, Staphylococcus aureus, Escherichia coli, and bile sludge. Moreover, when human fibroblasts were cultured on the PTFE SLIPS, it exhibited good cytocompatibility. Therefore, the proposed ultrasmooth PTFE membranes provide a promising alternative for biliary system to prevent bacterial adhesion and bile sludge accumulation.

Applicability of anti-corrosion for slippery liquid-infused porous surface using a double-layer ZnO nanostructure on Al foil

Applicability of anti-corrosion for slippery liquid-infused porous surface using a double-layer ZnO nanostructure on Al foil

Effective corrosion and wear protection of slippery liquid-infused porous surface on AZ31 Mg alloy

Magnesium alloys possess many exceptional advantages and attract much interest from researchers. However, the poor corrosion resistance significantly restricts their extensive applications in industry. Herein, a slippery liquid-infused porous surface (SLIPS), with good surface hydrophobicity, anti-corrosion, anti-wear, self-cleaning and self-healing performances, was developed on AZ31 Mg alloy. MgAl-layered double hydroxides (LDHs) with nanoporous structure were first in-situ synthesized on AZ31. Especially, an environmentally friendly corrosion inhibitor of sodium benzoate was intercalated into MgAl-LDHs to improve the corrosion resistance. After low-surface-energy modification and infusion of lubricant, the desired SLIPS with a homogeneous liquid layer was finally formed. The SLIPS showed good repellency to water to provide superior corrosion protection for the underlying AZ31, with the lowest corrosion current density of 1.93 × 10−9 A cm−2 and the largest impedance modulus of 1.18 × 107 Ω cm2. In comparison with the bare AZ31, the SLIPS exhibited enhanced friction-reducing and anti-wear performance because of the lubricating effect of the infused silicone oil and nanosheets of MgAl-LDHs. In addition, the SLIPS performed well in the self-cleaning and self-healing tests, showing potential to be widely used to enlarge the industrial applications of Mg alloys.

Nature-Inspired Superwettability Achieved by Femtosecond Lasers

Wettability is one of a solid surface’s fundamental physical and chemical properties, which involves a wide range of applications. Femtosecond laser microfabrication has many advantages compared to traditional laser processing. This technology has been successfully applied to control the wettability of material surfaces. This review systematically summarizes the recent progress of femtosecond laser microfabrication in the preparation of various superwetting surfaces. Inspired by nature, the superwettabilities such as superhydrophilicity, superhydrophobicity, superamphiphobicity, underwater superoleophobicity, underwater superaerophobicity, underwater superaerophilicity, slippery liquid-infused porous surface, underwater superpolymphobicity, and supermetalphobicity are obtained on different substrates by the combination of the femtosecond laser-induced micro/nanostructures and appropriate chemical composition. From the perspective of biomimetic preparation, we mainly focus the methods for constructing various kinds of superwetting surfaces by femtosecond laser and the relationship between different laser-induced superwettabilities. The special wettability of solid materials makes the femtosecond laser-functionalized surfaces have many practical applications. Finally, the significant challenges and prospects of this field (femtosecond laser-induced superwettability) are discussed.

Preparation of slippery liquid-infused porous surface based on MgAlLa-layered double hydroxide for effective corrosion protection on AZ31 Mg alloy

BackgroundThe relatively poor corrosion resistance of magnesium (Mg) alloys significantly restricts their extensive applications in industry despite their superior properties such as good electromagnetic shielding, low density, and good biocompatibility. The formation of a protective coating, especially a hydrophobic coating, can provide corrosion protection for the underlying Mg alloys. MethodsHerein, a slippery liquid-infused porous surface (SLIPS) with good hydrophobicity was developed by infusing MgAlLa-layered double hydroxide (LDH) with silicone oil. In particular, the MgAlLa-LDH was intercalated with sodium benzoate to further improve the corrosion resistance. Significant FindingsA homogeneous liquid layer was generated on the SLIPS, which can effectively prevent corrosion from external corrosive species. Compared with a superhydrophobic coating, the SLIPS showed better corrosion protection in both electrochemical and immersion tests, attributed to the better stability of liquid layer than air pockets. After immersion in 3.5 wt.% NaCl solution for 7 days, no obvious corrosion was observed on the SLIPS. Moreover, the SLIPS presents good self-cleaning and self-healing properties. These good performances made the SLIPS possible to be extensively applied in practice.

Condensation heat transfer on phase change slippery liquid-infused porous surfaces

Phase change slippery liquid-infused porous surfaces (PC-SLIPSs) are presented with emphasis on surface wetting characteristics and droplet dynamics influencing the water vapor condensation and the corresponding heat transfer. The functionalized nano-porous copper plate is infused with paraffin wax-xylene solution via dip coating method to prepare PC-SLIPSs., where the droplet dynamics are found to be affected by low adhesion (sliding angle α of 45±5°), high adhesion (α of 60±5°) and slippery (α of 3 ± 1°) states enabled by solid (at 48 °C), mush (at 58 °C) and liquid (at 66 °C) phases, respectively. Besides the wettability and droplet adhesion characterization, the condensation heat transfer is particularly explored on PC-SLIPSs in custom-built vacuum-assisted condensation rig. Two major dropwise condensation mechanisms are unveiled depending on the phase of the infused phase change material, such as coalescence-induced droplet shedding coupled with droplet sweeping in the solid and mush phases, while discrete-droplet shedding and sweeping in the absence of additional coalescence are reported in the liquid phase. Approximately, 136.8% higher heat transfer coefficients for PC-SLIPS in the liquid phase are reported when compared with the pristine copper surface at low sub-cooling temperatures. In the liquid phase of PC-SLIPSs, theoretical heat transfer modeling on dropwise condensation has been further carried out to elucidate the heat transfer mechanism via a single droplet coupled with the droplet number density. The operational durability of PC-SLIPSs has been found to last for 8 ± 1 h as confirmed during rigorous condensation experiments. In summary, different wetting features, various droplet shedding mechanisms, promising dropwise condensation modes, high heat transfer coefficient in the liquid phase, and effective time-dependent durability are the salient findings associated with PC-SLIPSs, rendering them competitive alternatives. The most important implication is that the dropwise condensation mode can also underperform if the droplet dynamics is inefficient.

Slippery Surface with Petal-like Structure for Protecting Al Alloy: Anti-corrosion, Anti-fouling and Anti-icing

The harsh working environment affects the performance and usage life of Al and its alloys, thus limiting their application. In recent years, Slippery Liquid-infused Porous Surface (SLIPS) has attracted much attention due to excellent anti-corrosion, anti-fouling and anti-icing properties. This may be an effective way to improve the properties of Al and its alloys. Here, the SLIPS with petal-like structure was constructed on the Al alloy via simple hydrothermal reaction, Stearic Acid (STA) modification and lubricant injection. A variety of droplets (including oil-in-water emulsions) can slide on the SLIPS at a low angle, even the Sliding Angle (SA) of the water droplet is only 3°. Furthermore, the SLIPS exhibits outstanding mechanical and chemical properties. It can maintain fine oil-locking ability under high shearing force and keep slippery stability after immersion in acid/alkaline solutions. In addition, the SLIPS possesses excellent anti-corrosion, anti-fouling and anti-icing properties, which provides a new way to promote the application of Al and its alloys. Therefore, the SLIPS is expected to be an effective way to improve the properties of Al and its alloys, as well as play a role in anti-fouling and self-cleaning in construction, shipbuilding and automotive manufacturing industries, thereby expanding the practical application of Al and its alloys.

Efficient Bubble Transport on Bioinspired Topological Ultraslippery Surfaces.

Slippery liquid-infused porous surfaces (SLIPS) with micro-/nanostructures inspired by the Nepenthes pitcher plant exhibit excellent characteristics in terms of liquid repellency, self-healing, pressure tolerance, and so forth. In particular, stable bubble transport on SLIPS can be achieved when the surface is submerged in water. However, more precise and sophisticated bubble manipulations on SLIPS still remain challenging. In this research, a three-dimensional topological SLIPS combined with a submillimeter rice leaf-like groove array is fabricated to guide the underwater bubble motion precisely. The dynamic behavior and wetting state of bubbles on SLIPS were investigated experimentally. Furthermore, topological SLIPS with different geometric textures were designed and created for sophisticated bubble manipulations, such as fast bubble directional transport and collection. The results indicated that a lubricant with low surface tension and low viscosity could improve the adhesion force to bubbles and the transport velocity of bubbles, simultaneously. The current findings are helpful to deepen the cognition of interaction between bubbles and SLIPS and to promote their wide applications in the field of smart bubble manipulation and catalytic chemistry.

The mechanisms of anti-icing properties degradation for slippery liquid-infused porous surfaces under shear stresses

HypothesisLoss of anti-icing properties of slippery liquid-infused porous surfaces (SLIPS) in conditions of repetitive shear stresses is the intrinsic process related to peculiarities of SLIPS structure. ExperimentsThe study of the evolution of the ice adhesion strength to superhydrophobic surfaces (SHS) and SLIPS during repetitive icing/de-icing cycles measured by a centrifugal method was supplemented with the estimation of change in capillary pressure inside the pores, and SEM analysis of the effect of multiple ice detachments on surface morphology. FindingsObtained data indicated that although for freshly prepared SLIPS, the ice shear adhesion strength at −25 °C was several times lower than for SHS, repetitive icing-deicing cycles resulted in dramatic SLIPS degradation. In contrast, SHS showed weak degradation at least during 50 cycles. Additional to the depletion of an impregnating oil layer, other mechanisms of SLIPS degradation were hypothesized and tested. It was shown that lower capillary pressure required to displace air by water from the surface texture for SLIPSs compared to SHSs resulted in deeper water/ice penetration inside the grooves. The accelerated destruction of the mechanical texture caused by the Rehbinder effect constitutes another mechanism of SLIPSs degradation.

Slippery liquid-infused microphase separation surface enables highly robust anti-fouling, anti-corrosion, anti-icing and anti-scaling coating on diverse substrates

Slippery liquid-infused porous surface (SLIPS) strategy has demonstrated great potential in the field of both biomedical and industrial applications. However, the fabrication of large-area SLIPS with a combined competence of strong bonding to diverse substrates as well as robust oil-locking ability especially those with special shape and structure remains highly challenging. Herein, SLIPS was facilely constructed on various substrates and shapes in bulk via a step-by-step deposition strategy. Collective results demonstrated that the facile and feasible formation of SLIPS coating implemented the combination of multiple excellent properties, including anti-fouling, anti-corrosion, anti-icing, anti-scaling, and readily self-healing. In particular, the spontaneous microphase separation on the surface induced by the prominent polar differences between the superhydrophobic fluorinated segments and hydrophilic hydroxyl and amino groups enabled ultra-stable oil-locking ability, ensuring high robustness under conditions of blood flow, in vivo transplantation, hydrodynamic shear stress even accompanied with boiling and ultrasound, and mechanical wear. Therefore, such desirable properties will endow this SLIPS coating strategy promising practical applications in surgical equipment, medical implants, marine and industrial facilities.

Scalable wear resistant 3D printed slippery liquid infused porous surfaces (SLIPS)

Surface wettability is a measure of adhesion and repulsion of liquids on a material’s surface, where surface wettability may be altered by creating a new interfacial surface between the base material and the target liquid on top. Where the interface comprises of a liquid that is locked in the pores or ridges of the base material, it is then termed as liquid infused surface (LIS). LIS alters the wettability due to the distinctive properties of the interfacial liquid which now forms the new surface. If the interfacial liquid is repellent towards polar (hydrophobic) and non-polar (oleophobic) liquids, the overall surface becomes slippery (amphiphobic) and prevents any new target liquids from adhering. This phenomenon leads to the definition of slippery liquid infused porous surfaces (SLIPS), where this term not only describes its role but also its fabrication route. Here a two-step facile method is presented to quickly transform any 3D printable polymeric material into robust SLIPS, irrespective of the wettability properties of the original polymer. The complex geometrical porosity required for locking the interfacial liquid is achieved using Fused Deposition Modelling (FDM) setup. Surface wettability characteristics of the 3D printed porous structure are then enhanced by increasing the liquid-adsorption sites where the locking of the infused interfacial (repellent) liquid takes place. The SLIPS demonstrate low rolling-off (sliding) angles with both polar and non-polar solvents of up to 2 degrees with high resistance to mechanical abrasion undergoing sliding frictional wear. The robust SLIPS as produced can be quickly scaled up using existing processes used in the laboratory.

Self-healing dual biomimetic liquid-infused slippery surface in a partition matrix: Fabrication and anti-corrosion capability for magnesium alloy

Biomimetics affords versatile ways to develop new routes for protecting metals from corrosion. In this report, focusing on the inhibition of corrosion for magnesium alloy, different biomimetic strategies were studied. Using a facile immersion method followed by surface modification, a lotus leaf-inspired superhydrophobic surface (SHS) is achieved. However, in the water phase, the SHS is found to be vulnerable, and a hydrophilic defect spot can irreversibly deteriorate the corrosion inhibition capability. Based on the infusion of dimethyl silicone oil into a SHS matrix, a dual biomimetic slippery liquid-infused porous surface (SLIPS) in a partition matrix inspired by both Nepenthes pitcher and pomegranate fruit was prepared. In a 3.5 wt% NaCl solution, the corrosion resistance of the SLIPS is found to be four orders of magnitude larger than that of the untreated and superhydrophobic magnesium alloys. In comparison with the vulnerable SHS, the SLIPS can tolerate repeat mechanical damage, exhibiting prominent self-healing properties. A facile route leads to dual biomimetic slippery liquid-infused surface in partition matrix onto magnesium alloy. Slippery liquid-infused porous surface has long-term corrosion inhibition and good self-healing capability. • Immersion and surface modification leads to superhydrophobic AZ31B. • Durability of superhydrophobicity is short for corrosion inhibition. • Slippery liquid-infused porous surface protects AZ31B from corrosion.

Icephobic and Anticorrosion Coatings Deposited by Electrospinning on Aluminum Alloys for Aerospace Applications.

Anti-icing or passive strategies have undergone a remarkable growth in importance as a complement for the de-icing approaches or active methods. As a result, many efforts for developing icephobic surfaces have been mostly dedicated to apply superhydrophobic coatings. Recently, a different type of ice-repellent structure based on slippery liquid-infused porous surfaces (SLIPS) has attracted increasing attention for being a simple and effective passive ice protection in a wide range of application areas, especially for the prevention of ice formation on aircrafts. In this work, the electrospinning technique has been used for the deposition of PVDF-HFP coatings on samples of the aeronautical alloy AA7075 by using a thickness control system based on the identification of the proper combination of process parameters such as the flow rate and applied voltage. In addition, the influence of the experimental conditions on the nanofiber properties is evaluated in terms of surface morphology, wettability, corrosion resistance, and optical transmittance. The experimental results showed an improvement in the micro/nanoscale structure, which optimizes the superhydrophobic and anticorrosive behavior due to the air trapped inside the nanotextured surface. In addition, once the best coating was selected, centrifugal ice adhesion tests (CAT) were carried out for two types of icing conditions (glaze and rime) simulated in an ice wind tunnel (IWT) on both as-deposited and liquid-infused coatings (SLIPs). The liquid-infused coatings showed a low water adhesion (low contact angle hysteresis) and low ice adhesion strength, reducing the ice adhesion four times with respect to PTFE (a well-known low-ice-adhesion material used as a reference).

Fabrication of Slippery Liquid-Infused Coatings in Flexible Narrow-Bore Tubing.

We report a layer-by-layer suction-and-flow approach that enables the fabrication of polymer-based "slippery" liquid-infused porous surfaces (SLIPS) in the confined luminal spaces of flexible, narrow-bore tubing. These SLIPS-coated tubes can prevent or strongly reduce surface fouling after prolonged contact, storage, or flow of a broad range of complex fluids and viscoelastic materials, including many that are relevant in the contexts of medical devices (e.g., blood and urine), food processing (beverages and fluids), and other commercial and industrial applications. The robust and mechanically compliant nature of the nanoporous coating used to host the lubricating oil phase allows these coated tubes to be bent, flexed, and coiled repeatedly without affecting their inherent slippery and antifouling behaviors. Our results also show that SLIPS-coated tubes can prevent the formation of bacterial biofilms after prolonged and repeated flow-based exposure to the human pathogen Staphylococcus aureus and that the anti-biofouling properties of these coated tubes can be further improved or prolonged by coupling this approach with strategies that permit the sustained release of broad-spectrum antimicrobial agents. The suction-and-flow approach used here enables the application of slippery coatings in the confined luminal spaces of narrow-bore tubing that are difficult to access using several other methods for the fabrication of liquid-infused coatings and can be applied to tubing of arbitrary length and diameter. We anticipate that the materials and approaches reported here will prove useful for reducing or preventing biofouling, process fouling, and the clogging or occlusion of tubing in a wide range of consumer, industrial, and healthcare-oriented applications.

Coral-like silicone nanofilament coatings with extremely low ice adhesion

Passive icephobic surfaces can provide a cost and energy efficient solution to many icing problems that are currently handled with expensive active strategies. Water-repellent surface treatments are promising candidates for this goal, but commonly studied systems, such as superhydrophobic surfaces and Slippery Liquid Infused Porous Surfaces (SLIPS), still face challenges in the stability and durability of their properties in icing environments. In this work, environmental icing conditions are simulated using an Icing Wind Tunnel, and ice adhesion is evaluated with a Centrifugal Adhesion Test. We show that superhydrophobic coral-like Silicone Nanofilament (SNF) coatings exhibit extremely low ice adhesion, to the point of spontaneous ice detachment, and good durability against successive icing cycles. Moreover, SNFs-based SLIPS show stably low ice adhesion for the whole duration of the icing test. Stability of surface properties in a cold environment is further investigated with water wettability at sub-zero surface temperature, highlighting the effect of surface chemistry on superhydrophobicity under icing conditions.

Fabrication of a robust slippery liquid infused porous surface on Q235 carbon steel for inhibiting microbiologically influenced corrosion

Improving the mechanical stability and oil stability of slippery liquid infused porous surface (SLIPS) is very urgent to its long-term application in preventing bacteria settlement and mitigating the following microbiologically influenced corrosion (MIC) for metallic material in marine environment. Herein, a robust superhydrophobic surface was first fabricated through spraying SiO2 nanoparticles on the natural resin shellac adhesive. The superhydrophobic surface could retain superhydrophobicity after 160 sandpaper abrasion cycles (abrasive length of 64 m), which was attributed to the strong binding force between SiO2 nanoparticles and shellac via hydrogen bonding. Thus, a robust SLIPS was fabricated through infusing lubricant oil into abrasion-resistant superhydrophobic surface. And the fabricated slippery surface exhibited stable slippery performance after spin treatment and damaged by a scratch. Furthermore, scanning kelvin probe (SKP) test verified the scratch was able to heal through the self-healing performance of lubricant oil. Finally, the bacterial settlement and electrochemical experiment results indicated the SLIPS could effectively mitigate sulfate reducing bacteria (SRB) settlement and following MIC after immersing in SRB solution for 14 days. This facile method provides a guidance to fabricate robust slippery surface for its long term application.

Architecture-Driven Fast Droplet Transport without Mass Loss.

Spontaneous droplet transport without mass loss has great potential applications in the fields of energy and biotechnology, but it remains challenging due to the difficulty in obtaining a sufficient driving force for the transport while suppressing droplet mass loss. Learning from the slippery peristome of Nepenthes alata and wedge topology of a shorebird beak that can spontaneously feed water against gravity, a combined system consisting of two face-to-face hydrophilic slippery liquid-infused porous surfaces (SLIPS) with variable beak-like opening and spacing was proposed to constrain the droplet in-between and initiate fast droplet transport over a long distance of 75 mm with a maximum speed of 12.2 mm·s-1 without mass loss by taking advantage of the Laplace pressure gradient induced by the asymmetric shape of the constrained droplet. The theoretical model based on the Navier-Stokes equation was developed to interpret the corresponding mechanism of the droplet transport process. In addition, in situ sophisticated droplet manipulations such as droplet mixing are readily feasible when applying flexible 304 stainless foil as the substrate of SLIPS. It is believed that extended research would contribute to new references for the precise and fast droplet motion control intended for energy harvest and water collection devices.

Evaluation of the Durability of Slippery, Liquid-Infused Porous Surfaces in Different Aggressive Environments: Influence of the Chemical-Physical Properties of Lubricants

Liquid-repellent surfaces have been extensively investigated due to their potential application in several fields. Superhydrophobic surfaces achieve outstanding water repellence, however their limited durability in severe operational conditions hinders their large-scale application. The Slippery, Liquid-Infused Porous Surface (SLIPS) approach solves many of the durability problems shown by superhydrophobic surfaces due to the presence of an infused liquid layer. Moreover, SLIPS show enhanced repellence towards low surface tension liquids that superhydrophobic surfaces cannot repel. In this perspective, SLIPS assume significant potential for application in harsh environments; however, a systematic evaluation of their durability in different conditions is still lacking in the literature. In this work, we report the fabrication of SLIPS based on a ceramic porous layer infused with different lubricants, namely perfluoropolyethers with variable viscosity and n-hexadecane; we investigate the durability of these surfaces by monitoring the evolution of their wetting behavior after exposure to severe environmental conditions like UV irradiation, chemically aggressive solutions (acidic, alkaline, and saline), and abrasion. Chemical composition and viscosity of the infused liquids prove decisive in determining SLIPS durability; especially highly viscous infused liquids deliver enhanced resistance to abrasion stress and chemical attack, making them candidates for applicable, long-lasting liquid-repellent surfaces.

Surface Acoustic Waves to Control Droplet Impact onto Superhydrophobic and Slippery Liquid-Infused Porous Surfaces.

Superhydrophobic coatings and slippery liquid-infused porous surfaces (SLIPS) have shown their potentials in self-cleaning, anti-icing, anti-erosion, and antibiofouling applications. Various studies have been done on controlling the droplet impact on such surfaces using passive methods such as modifying the lubricant layer thickness in SLIPS. Despite their effectiveness, passive methods lack on-demand control over the impact dynamics of droplets. This paper introduces a new method to actively control the droplet impact onto superhydrophobic and SLIPS surfaces using surface acoustic waves (SAWs). In this study, we designed and fabricated SLIPS on ZnO/aluminum thin-film SAW devices and investigated different scenarios of droplet impact on the surfaces compared to those on similar superhydrophobic-coated surfaces. Our results showed that SAWs have insignificant influences on the impact dynamics of a porous and superhydrophobic surface without an infused oil layer. However, after infusion with oil, SAW energy could be effectively transferred to the droplet, thus modifying its impact dynamics onto the superhydrophobic surface. Results showed that by applying SAWs, the spreading and retraction behaviors of the droplets are altered on the SLIPS surface, leading to a change in a droplet impact regime from deposition to complete rebound with altered rebounding angles. Moreover, the contact time was reduced up to 30% when applying SAWs on surfaces with an optimum oil lubricant thickness of ∼8 μm. Our work offers an effective way of applying SAW technology along with SLIPS to effectively reduce the contact time and alter the droplet rebound angles.