Mechanical Durability of Low Ice Adhesion Polydimethylsiloxane Surfaces.

Elastomeric surfaces and oil-infused elastic surfaces reveal low ice adhesion, in part because of their deformability. However, these soft surfaces might jeopardize their mechanical durability. In this work, we analyzed the mechanical durability of elastic polydimethylsiloxane (PDMS) surfaces with different balances between elasticity and deicing performances. The durability was studied in terms of shear/tensile ice adhesion strength before and after different wear tests. These tests consisted of abrasion/erosion cycles using standard procedures aimed to reproduce different environmental wearing agents. The main objective is to evaluate if our PDMS surfaces can become long-lasting solutions for ice removal in real conditions. We found that our elastic surfaces show excellent durability. After the wear tests, the ice adhesion strength values remained low or even unaltered. Although the oil-infused PDMS surface was the softest one, it presented considerable durability and excellent low ice adhesion, being a promising solution.

Ice adhesion of PDMS surfaces with balanced elastic and water-repellent properties.

HYPOTHESIS: Ice adhesion to rigid materials is reduced with low energy surfaces of high receding contact angles. However, their adhesion strength values are above the threshold value to be considered as icephobic materials. Surface deformability is a promising route to further reduce ice adhesion. EXPERIMENTS: In this work, we prepared elastomer surfaces with a wide range of elastic moduli and hydrophobicity degree and we measured their ice adhesion strength. Moreover, we also explored the deicing performance of oil-infused elastomeric surfaces. The ice adhesion was characterized by two detachment modes: tensile and shear. FINDINGS: The variety of elastomeric surfaces allowed us to simultaneously analyze the ice adhesion dependence with deformability and contact angle hysteresis. We found that the impact of these properties depends on the detachment mode, being deformability more important in shear mode and hydrophobicity more relevant in tensile mode. In addition, oil infusion further reduces ice adhesion due to the interfacial slippage. From an optimal balance between deformability and hydrophobicity, we were able to identify surfaces with super-low ice adhesion.

Contact line relaxation of sessile drops on PDMS surfaces: A methodological perspective.

HYPOTHESIS: Characterization of contact angle hysteresis on soft surfaces is sensitive to the measurement protocol and might present adventitious time-dependencies. Contact line dynamics on solid surfaces is altered by the surface chemistry, surface roughness and/or surface elasticity. We observed a "slow" spontaneous relaxation of static water sessile drops placed on elastic surfaces. This unexpected drop motion reveals unresolved equilibrium configurations that may affect the observed values of contact angle hysteresis. Drop relaxation on deformable surfaces is partially governed by a viscoelastic dissipation located at the contact line. EXPERIMENTS: In this work, we studied the natural relaxation of water drops formed on several smooth PDMS surfaces with different elastic moduli. We monitored in time the contact angle and contact radius of each drop. For varying the initial contact angle, we used the growing-shrinking drop method. FINDINGS: We postulate that the so-called "braking effect", produced by the surface deformability, affects the contact line velocity and in consequence, the contact angle measurements. We conclude that the wetting properties of elastic surfaces should be properly examined with reliable values of contact angle measured after drop relaxation.

Testing the performance of superhydrophobic aluminum surfaces.

The analysis of wetting properties of superhydrophobic surfaces may be a difficult task due to the restless behavior of drops on this type of surfaces and the limitations of goniometry for high contact angles. A method to validate the performance of superhydrophobic surfaces, rather than standard goniometry, is required. In this work, we used bouncing drop dynamics as a useful tool to predict the water repellency of different superhydrophobic surfaces. From bouncing drop experiments conducted over a wide range of superhydrophobic surfaces, we found that those surfaces with a proper roughness degree and homogeneous chemical composition showed higher water-repellency. We also conducted a drop condensation study at saturating conditions aimed to determine whether there is direct correlation between water repellency and condensation delay. We found that the drop condensation process is strongly related to the surface topography, as well as the intrinsic wettability. The condensation is promoted on rough surfaces but it is delayed on intrinsically hydrophobic surfaces. However, the differences found in condensation delay between the superhydrophobic surfaces explored in this study cannot be justified by their chemical homogeneity nor their efficiency as water repellent surfaces, separately.

Interfacial Activity of Gold Nanoparticles Coated with a Polymeric Patchy Shell and the Role of Spreading Agents.

Gold patchy nanoparticles (PPs) were prepared under surfactant-free conditions by functionalization with a binary ligand mixture of polystyrene and poly(ethylene glycol) (PEG) as hydrophobic and hydrophilic ligands, respectively. The interfacial activity of PPs was compared to that of homogeneous hydrophilic nanoparticles (HPs), fully functionalized with PEG, by means of pendant drop tensiometry at water/air and water/decane interfaces. We compared interfacial activities in three different spreading agents: water, water/chloroform, and pure chloroform. We found that the interfacial activity of PPs was close to zero (∼2 mN/m) when the spreading agent was water and increased to ∼14 mN/m when the spreading agent was water/chloroform. When the nanoparticles were deposited with pure chloroform, the interfacial activity reached up to 60 mN/m by compression. In all cases, PPs exhibited higher interfacial activity than HPs, which were not interfacially active, regardless of the spreading agent. The interfacial activity at the water/decane interface was found to be significantly lower than that at the water/air interface because PPs aggregate in decane. Interfacial dilatational rheology showed that PPs form a stronger elastic shell at the pendant drop interface, compared to HPs. The significantly high interfacial activity obtained with PPs in this study highlights the importance of the polymeric patchy shell and the spreading agent.

Particle Segregation at Contact Lines of Evaporating Colloidal Drops: Influence of the Substrate Wettability and Particle Charge-Mass Ratio.

Segregation of particles during capillary/convective self-assembly is interesting for self-stratification in colloidal deposits. In evaporating drops containing colloidal particles, the wettability properties of substrate and the sedimentation of particles can affect their accumulation at contact lines. In this work we studied the size segregation and discrimination of charged particles with different densities. We performed in-plane particle counting at evaporating triple lines by using fluorescence confocal microscopy. We studied separately substrates with very different wettability properties and particles with different charge-mass ratios at low ionic strength. We used binary colloidal suspensions to compare simultaneously the deposition of two different particles. The particle deposition rate strongly depends on the receding contact angle of the substrate. We further observed a singular behavior of charged polystyrene particles in binary mixtures under "salt-free" conditions explained by the "colloidal Brazil nut" effect.

In-plane particle counting at contact lines of evaporating colloidal drops: effect of the particle electric charge.

Complete understanding of colloidal assembly is still a goal to be reached. In convective assembly deposition, the concentration gradients developed in evaporating drops or reservoirs are usually significant. However, collective diffusion of charge-stabilized particles has been barely explored. The balance between convective and diffusive flows may dictate the particle dynamics inside evaporating colloidal drops. In this work we performed in situ counting of fluorescent particles in the vicinity of the triple line of evaporating sessile drops by using confocal laser scanning microscopy. We used particles of different sizes, with different charge response over the pH scale and we focused on charged and nearly uncharged particles. Two substrates with different receding contact angles were used. Binary colloidal mixtures were used to illustrate simultaneously the accumulation of particles with two different charge states at the triple line. The deposition rate close to the triple line was different depending on the electric state of the particle, regardless of the substrate used.

Transition from stripe-like patterns to a particulate film using driven evaporating menisci.

Better control of colloidal assembly by convective deposition is particularly helpful in particle templating. However, knowledge of the different factors that can alter colloidal patterning mechanisms is still insufficient. Deposit morphology is strongly ruled by contact line dynamics, but the wettability properties of the substrate can alter it drastically. In this work, we experimentally examined the roles of substrate contact angle hysteresis and receding contact angle using driven evaporating menisci similar to the dip-coating technique but at a low capillary number. We used smooth substrates with very different wettability properties and nanoparticles of different sizes. For fixed withdrawal velocity, evaporation conditions, and nanoparticle concentration, we analyzed the morphology of the deposits formed on each substrate. A gradual transition from stripe-like patterns to a film was observed as the contact angle hysteresis and receding contact angle were lowered.

Using AFM to probe the complexation of DNA with anionic lipids mediated by Ca(2+): the role of surface pressure.

Complexation of DNA with lipids is currently being developed as an alternative to classical vectors based on viruses. Most of the research to date focuses on cationic lipids owing to their spontaneous complexation with DNA. Nonetheless, recent investigations have revealed that cationic lipids induce a large number of adverse effects on DNA delivery. Precisely, the lower cytotoxicity of anionic lipids accounts for their use as a promising alternative. However, the complexation of DNA with anionic lipids (mediated by cations) is still in early stages and is not yet well understood. In order to explore the molecular mechanisms underlying the complexation of anionic lipids and DNA we proposed a combined methodology based on the surface pressure-area isotherms, Gibbs elasticity and Atomic Force Microscopy (AFM). These techniques allow elucidation of the role of the surface pressure in the complexation and visualization of the interfacial aggregates for the first time. We demonstrate that the DNA complexes with negatively charged model monolayers (DPPC/DPPS 4 : 1) only in the presence of Ca(2+), but is expelled at very high surface pressures. Also, according to the Gibbs elasticity plot, the complexation of lipids and DNA implies a whole fluidisation of the monolayer and a completely different phase transition map in the presence of DNA and Ca(2+). AFM imaging allows identification for the first time of specific morphologies associated with different packing densities. At low surface coverage, a branched net like structure is observed whereas at high surface pressure fibers formed of interfacial aggregates appear. In summary, Ca(2+) mediates the interaction between DNA and negatively charged lipids and also the conformation of the ternary system depends on the surface pressure. Such observations are important new generic features of the interaction between DNA and anionic lipids.

Modeling the corrugation of the three-phase contact line perpendicular to a chemically striped substrate.

We model an infinitely long liquid bridge confined between two plates chemically patterned by stripes of the same width and different contact angle, where the three-phase contact line runs, on average, perpendicular to the stripes. This allows us to study the corrugation of a contact line in the absence of pinning. We find that, if the spacing between the plates is large compared to the length scale of the surface patterning, the cosine of the macroscopic contact angle corresponds to an average of cosines of the intrinsic angles of the stripes, as predicted by the Cassie equation. If, however, the spacing becomes on the order of the length scale of the pattern, there is a sharp crossover to a regime where the macroscopic contact angle varies between the intrinsic contact angle of each stripe, as predicted by the local Young equation. The results are obtained using two numerical methods, lattice Boltzmann (a diffuse interface approach) and Surface Evolver (a sharp interface approach), thus giving a direct comparison of two popular numerical approaches to calculating drop shapes when applied to a nontrivial contact line problem. We find that the two methods give consistent results if we take into account a line tension in the free energy. In the lattice Boltzmann approach, the line tension arises from discretization effects at the diffuse three phase contact line.

Mechanical derivation of the Wenzel and Cassie equations using a statistical interpretation of drop dispensation.

The global mechanical equilibrium condition of a liquid on a rough and chemically heterogeneous surface was derived for three-dimensional situations from a statistical outlook of dispensation of many drops and the assumption of local mechanical equilibrium. Unlike the conventional thermodynamic derivations from variational methods, the current proof is based on vector algebra rather than differential geometry. The mechanics-based derivation becomes less intricate although the minimum energy condition is not established. An effective contact angle is computed from the directional sampling of three-phase lines after local drop dispensations. The final expression is a combined mechanical version of the Wenzel and Cassie equations.

Wettability and bonding of self-etching dental adhesives. Influence of the smear layer.

OBJECTIVES: To evaluate dentin wettability and bonding of self-etching and total-etch adhesives on smear layer-covered and smear layer-free dentin. METHODS: Three self-etching adhesives (Clearfil SE Bond, AdheSE and Xeno III) and one total-etch adhesive (SingleBond) were evaluated. The substrates were mid coronal smear layer-covered and smear layer-free dentin. Dentin wettability by resins was studied from contact angle measurement using sessile drop method and Axisymmetric Drop Shape Analysis (ADSA). Shear-bond strength was evaluated using a push-out technique (ad hoc design). Data were analysed with two-way ANOVA and Tukey's test. RESULTS: Similar values of dentin wettability were obtained for all adhesives tested regardless the presence of smear layer. Even though, Xeno III and AdheSE exhibited slightly lower wettability (higher contact angles values) on smear layer-free dentin. Likewise, the presence of smear layer did not affect the shear-bond strength. Total-etch adhesive obtained higher shear-bond strength than self-etching adhesives, which obtained similar values. SIGNIFICANCE: Wettability is similar between self-etching and total-etch adhesives. The smear layer affects slightly the wettability of self-etching adhesives. Shear-bond strength is not sensitive to the smear layer presence. Total-etch adhesion is stronger than self-etching adhesion. There is no clear relationship between wettability and bond strength.

Effect of acid etching on dentin wettability and roughness: self-etching primers versus phosphoric acid.

OBJECTIVE: To compare the effect of self-etching primers and phosphoric acid on the wettability and roughness of smear layer-covered and smear layer-free dentin. MATERIALS AND METHOD: Three self-etching primers (Clearfil SE Bond, AdheSE, and Xeno III) and 10% (w/w) phosphoric acid (H(3)PO(4)) solution were evaluated. The substrates were midcoronal dentin with and without smear layer. For each liquid, pH, density, and surface tension were determined. Water wettability of dentin and roughness were measured before and after each etching. Wettability of self-etching primers and phosphoric acid was measured on untreated dentin. RESULTS: Water wettability increased after acid conditioning similarly for all the liquids used. On smear layer-covered surfaces, self-etching primers achieved a comparable wetting but with greater contact angles than phosphoric acid. However, on smear layer-free surfaces, the increasing sequence of contact angle was Clearfil SE < AdheSE < Xeno III < Phosphoric acid. The treatment with phosphoric acid (lowest pH) produced the highest roughness increase on both dentin substrates. The roughening effect of the self-etching adhesives was more evident for AdheSE and Xeno III. CONCLUSIONS: Self-etching primers and phosphoric acid promote similar water wettability increase. However, self-etching primers provided lower dentin roughness increase than pretreatment with phosphoric acid. The presence of smear layer did not affect the results of self-etching and phosphoric acid treatments.