Intermittent dynamics of bubble dissolution due to interfacial growth of fat crystals.

Foams are inherently unstable objects, that age and disappear over time. The main cause of foam aging is Ostwald ripening: smaller air bubbles within the foam empty their gas content into larger ones. One strategy to counter Ostwald ripening consists in creating armored bubbles, where solid particles adsorbed at the air/liquid interface prevent bubbles from shrinking below a given size. Here, we study the efficiency of coating air bubbles with fat crystals to prevent bubble dissolution. A monoglyceride, monostearin, is directly crystallized at the air/oil interface. Experiments on single bubbles in a microfluidic device show that the presence of monostearin fat crystals slows down dissolution, with an efficiency that depends on the crystal size. Bubble ripening in the presence of crystals exhibits intermittent dissolution dynamics, with phases of arrest, when crystals jam at the interface, followed by phases of dissolution, when monostearin crystals are ejected from the interface. In the end, crystals do not confer enough mechanical strength to the bubbles to prevent them from fully dissolving.

Fatty Acid Monolayers on Randomly Nanostructured Inorganic Surfaces: Interplay of Wettability, Chemistry, and Topography.

Understanding the wetting properties of chemically modified inorganic surfaces with random nanoscale topographies is of fundamental importance for diverse applications. This issue has hitherto continuously been the subject of considerable controversies. Herein, we report a thorough investigation of the wettability-topography-chemistry balance for a nanostructured surface with random topography, the main challenge being decoupling topography from surface chemistry. For this purpose, we use a superficially nanostructured aluminum substrate chemically modified by fatty acid monolayers. From atomic force microscopic data, we extract a variety of parameters describing the surface topography by means of variogram calculations, a method originally developed by geostatisticians to explore large surfaces. Moreover, by using log and power transforms, we establish a consistent relationship relating wettability, topography, and surface chemistry. Interestingly, we demonstrate that the water contact angle comprises a contribution due to the surface composition, originating from hydrophobization through alkyl chains, and a contribution due to the surface topography, particularly its stochastic feature. This model is valid in the Wenzel region; it provides guidelines for tuning the wetting properties of inorganic surfaces with random nanoscale topographies.

Capture of colloidal particles by a moving microfluidic bubble.

Foams can be stabilized for long periods by the adsorption of solid particles on the liquid-gas interfaces. Although such long-term observations are common, mechanistic descriptions of the particle adsorption process are scarce, especially in confined flows, in part due to the difficulty of observing the particles in the complex gas-liquid dispersion of a foam. Here, we characterise the adsorption of micron-scale particles onto the interface of a bubble flowing in a colloidal aqueous suspension within a microfluidic channel. Three parameters are systematically varied: the particle size, their concentration, and the mean velocity of the colloidal suspension. The bubble coverage is found to increase linearly with position in the channel for all conditions but with a slope that depends on all three parameters. The optimal coverage is found for 1 µm particles at low flow rates and high concentrations. In this regime the particles pass the bubbles through the gutters between the interface and the channel corners, where the complex 3D flow leads them onto the interface. The largest particles cannot enter into the gutters and therefore provide very poor coverage. In contrast, particle aggregates can sediment onto the microchannel floor ahead of the bubble and get swept up by the advancing interface, thus improving the coverage for both large and medium particle sizes. These observations provide new insight on the influence of boundaries for particle adsorption at an air-liquid interface.

Arresting dissolution by interfacial rheology design.

A strategy to halt dissolution of particle-coated air bubbles in water based on interfacial rheology design is presented. Whereas previously a dense monolayer was believed to be required for such an "armored bubble" to resist dissolution, in fact engineering a 2D yield stress interface suffices to achieve such performance at submonolayer particle coverages. We use a suite of interfacial rheology techniques to characterize spherical and ellipsoidal particles at an air-water interface as a function of surface coverage. Bubbles with varying particle coverages are made and their resistance to dissolution evaluated using a microfluidic technique. Whereas a bare bubble only has a single pressure at which a given radius is stable, we find a range of pressures over which bubble dissolution is arrested for armored bubbles. The link between interfacial rheology and macroscopic dissolution of [Formula: see text] 100 [Formula: see text]m bubbles coated with [Formula: see text] 1 [Formula: see text]m particles is presented and discussed. The generic design rationale is confirmed by using nonspherical particles, which develop significant yield stress at even lower surface coverages. Hence, it can be applied to successfully inhibit Ostwald ripening in a multitude of foam and emulsion applications.

Lipid Layers on Nanoscale Surface Topography: Stability and Effect on Protein Adsorption.

Herein, we report the coating of a surface with a random nanoscale topography with a lipid film formed by an anchoring stearic acid (SA) monolayer and phospholipid (DPPC) layers. For this purpose, different procedures were used for phospholipid coating, including adsorption from solution, drop deposition, and spin-coating. Our results reveal that the morphology of the obtained lipid films is strongly influenced by the topography of the underlying substrate but also impacted by other factors, including the coating procedure and surface wettability (modulated by the presence of SA). These coated surfaces showed a remarkable antifouling behavior toward proteins, with different yields of repellency (Yrp) depending on the amount/organization of DPPC on the nanostructured substrate. The interaction between the proteins and phospholipids involves a partial detachement of the film. The use of characterization techniques with different charcateristics (accuracy, selectivity, analysis depth) did not reveal any obvious vertical heterogenity of the probed interface, indicating that the lipid film acts as a nonfouling coating on the whole surface, including the outermost part (nanoprotrusions) and deeper regions (valleys).

Self-assembly of fatty acids on hydroxylated Al surface and effects of their stability on wettability and nanoscale organization.

The self-assembly of fatty acids (FA) on the surfaces of inorganic materials is a relevant way to control their wetting properties. While the mechanism of adsorption on model flat substrate is well described in the literature, interfacial processes remain poorly documented on nanostructured surfaces. In this study, we report the self-assembly of a variety of FA on a hydroxylated Al surface which exhibits a random nanoscale organization. Our results revealed a peculiar fingerprint due to the FA self-assembly which consists in the formation of aligned nanopatterns in a state of hierarchical nanostructuration, regardless of the molecular structure of the FA (chain length, level of unsaturation). After a significant removal of adsorbed FA using UV/O3 treatment, a complete wetting was reached, and a noticeable disturbance of the surface morphology was observed, evidencing the pivotal role of FA molecules to maintain these nanostructures. The origin of wetting properties was investigated prior to and after conditioning of FA-modified samples taking into account key parameters, namely the surface roughness and its composition. For this purpose, the Wenzel roughness, defined as the third moment of power spectral density, was used, as it is sensitive to high spatial frequency and thus to the obtained hierarchical level of nanostructuration. Our results revealed that no correlation can be made between water contact angles (θ(w)) and the Wenzel roughness. By contrast, θ(w) strongly increased with the amount of -CHx- groups exhibited by adsorbed FA. These findings suggest that the main origin of hydrophobization is the presence of self-assembled molecules and that the surface roughness has only a small contribution to the wettability.

Ordered nanostructures on a hydroxylated aluminum surface through the self-assembly of fatty acids.

We investigate the mechanism of self-assembly of fatty acids (FA) and methyl oleate on an Al oxy-hydroxide surface with a view to deciphering the role and nature of interfacial processes (adsorption, chemical binding, molecular organization, etc.). For this purpose, we focus on parameters related to intrinsic properties of molecules, namely the level of unsaturation and the nature of the head group (carboxylic acid or ester). After the FA adsorption, the presence of coordinative bonded carboxylate species on the Al oxy-hydroxide surface is demonstrated by means of PM-IRRAS analysis. We observe that contact of methyl oleate with the surface leads to its chemical transformation through a saponification reaction. As a consequence, it binds to the surface in a manner similar to that for fatty acids. Through an innovative mode of atomic force microscopy (AFM), the organization of the adsorbed molecules is demonstrated. Our results reveal the existence of highly ordered nanostructures guided by the FA self-assembly. The size of these nanostructures was determined with accuracy, suggesting that it exceeds one FA monolayer. By contrast, no organization was observed with methyl oleate.