Interaction of oil drops with surfaces of different interfacial energy and topography

Probing the interfacial behaviors of interfacially active and non-active asphaltenes and their impact on emulsion stability

HYPOTHESIS

Asphaltenes subfractions with distinct interfacial behaviors may play different roles in stabilizing oil-water emulsions.

EXPERIMENTS

In this work, whole asphaltenes were separated into interfacially active asphaltenes (IAA) and interfacially non-active asphaltenes (INAA). Employing advanced nanomechanical techniques, we have explored the compositions, morphologies, sizes, adsorption, and interfacial behaviors of IAA and INAA.

FINDINGS

IAA exhibits a high and unevenly distributed oxygen content, distinguishing it from INAA. In toluene, the diameters of IAA and INAA are about 60 nm and 6 nm, respectively. When adsorbed irreversibly on mica surfaces, the thickness of the IAA and INAA film was measured at ∼5.5 nm or 1 nm, respectively; while in a toluene solution, the film thickness reached ∼46 nm and 3.1 nm for IAA and INAA, respectively. IAA demonstrates superior interfacial activity, and elastic/viscous moduli compared to INAA at the water-toluene interface. Quantified surface force measurements reveal that IAA stabilizes water droplets in toluene at a concentration of only 10 mg/L, while INAA requires a higher concentration of 100 mg/L. This work provides the first comprehensive investigation into the adsorption and interfacial behaviors of asphaltene subfractions and provides useful insights into the asphaltenes-stabilization mechanism of emulsions.

Shapes of Nonsymmetric Capillary Bridges

Here we study the shapes of droplets captured between chemically distinct parallel plates. This work is a preliminary step toward characterizing the influence of second-phase bridging between biomolecular surfaces on their solution contacts, i.e., capillary attraction or repulsion. We obtain a simple, variable-separated quadrature formula for the bridge shape. The technical complication of double-ended boundary conditions on the shapes of nonsymmetric bridges is addressed by studying waists in the bridge shape, i.e., points where the bridge silhouette has zero derivative. Waists are generally expected with symmetric bridges, but waist points can serve to characterize shape segments in general cases. We study how waist possibilities depend on the physical input to these problems, noting that these formulas change with the sign of the inside-outside pressure difference of the bridge. These results permit a variety of different interesting shapes, and the development below is accompanied by several examples.

Understanding the Wetting and Water-Induced Dewetting Behaviors of Bitumen on Rough Aggregate Surfaces

The interaction of bitumen colloidal (a form of heavy oil) with inorganic solids, for example, mineral aggregates, in both air and water environments is ubiquitous in nature and engineering. However, our knowledge of the underlying physical mechanism of bitumen-/solid-wetting phenomena is still very limited. The current study aims to reveal how the mineralogy and topography of aggregate surfaces affect the wetting and water-induced dewetting of bitumen on aggregate surfaces. For this, contact angle tests were performed to measure the surface energies of bitumen and aggregate surfaces varying in both mineralogy and roughness. Based on the measurements, both qualitative and quantitative analyses were conducted for the interaction of bitumen/aggregate interface in air and water environments. Complete wetting and complete dewetting hold for the air/bitumen/aggregate and water/bitumen/aggregate interfaces, respectively. The negative interfacial adhesive energy for the air/bitumen/aggregate interface and the interfacial debonding energy for the water/bitumen/aggregate interface imply that both bitumen wetting and water-induced bitumen dewetting on flat surfaces are thermodynamically favorable. The Wenzel model approximation holds up for the rough aggregate surface interface systems. The interfacial adhesive energy and interfacial debonding energy are enhanced geometrically by the roughness factor r, which indicates that the textured aggregate surface is in favor of force-induced interfacial cracking resistance but shows an adverse effect to moisture damage resistance. The findings from the current study provide guidelines for materials design in pavement engineering.

The mechanisms of filter feeding on oil droplets: Theoretical considerations

Filter feeding animals capture food particles and oil droplets from the fluid environment using cilia or appendages composed of arrays of fibers. Here we review the theoretical models that have provided a foundation for observations on the efficiency of particle capture. We then provide the mathematical theoretical framework to characterize the efficient filtration of oil droplets. In the aquatic and marine environments oil droplets are released from the decay of organisms or as hydrocarbons. Droplet size and flow velocity, oil-to-water viscosity ratio, oil-water interfacial tension, oil and water density difference, and the surface wettability, or surface texture, of the filter fiber are the key parameters for oil droplet capture. Following capture, capillary force maintains the droplet at its location due to the oil-water interfacial tension. If the oil-coated fiber is subject to any external force such as viscous or gravitational forces, it may deform and separate from the fiber and re-enter the fluid stream. We show oil droplet capture in Daphnia and the barnacle Balanus glandula, and outline some of the ecological unknowns regarding oil capture in the oceans. Awareness of these mechanisms and their interrelationships will provide a foundation for investigations into the efficiency of various modes of filter feeding on oil droplets.

Hierarchical patterning of hydrogels by replica molding of impregnated breath figures leads to superoleophobicity

The surface chemistry and topography govern the spreading of liquids on a solid. When an oil drop makes a contact angle, θ > 90° on a solid surface, the solid is termed as oleophobic. Adding roughness to an inherently oleophobic surface enhances its oil dewetting and can lead to superoleophobicity when θ > 150°. In this study, we introduce the concept of a two-tier hierarchical roughness on the surface of soft materials such as hydrogels by forming the patterned inverse replica of breath figure polymer films impregnated with nanoparticles. The directed deposition of nanoparticles in the breath figure pores is accomplished by an aerosol assisted technique that exclusively leads to deposition within the pores and filling of the pores. The inverse replica of such impregnated films exhibits a close packed hexagonally structured second tier of surface roughness which directly leads to a superoleophobic surface. Since these structures have well defined geometries, it is possible to estimate the contact angle by assuming a partial wetting of the oil drop in a 'fakir' state on the rough surface. The estimation is in good agreement with the experimental contact angle value. While the work demonstrates a facile method to impart superoleophobicity to a hydrogel surface, it also demonstrates new methods to imbue breath figure pores with functional materials that can be easily transferred to the pores of the inverse replica.

Hydrogel Inverse Replicas of Breath Figures Exhibit Superoleophobicity Due to Patterned Surface Roughness

The wetting behavior of a surface depends on both its surface chemistry and the characteristics of surface morphology and topography. Adding structure to a flat hydrophobic or oleophobic surface increases the effective contact angle and thus the hydrophobicity or oleophobicity of the surface, as exemplified by the lotus leaf analogy. We describe a simple strategy to introduce micropatterned roughness on surfaces of soft materials, utilizing the template of hexagonally packed pores of breath figures as molds. The generated inverse replicas represent micron scale patterned beadlike protrusions on hydrogel surfaces. This added roughness imparts superoleophobic properties (contact angle of the order of 150° and greater) to an inherently oleophobic flat hydrogel surface, when submerged. The introduced pattern on the hydrogel surface changes morphology as it swells in water to resemble morphologies remarkably analogous to the compound eye. Analysis of the wetting behavior using the Cassie-Baxter approximation leads to estimation of the contact angle in the superoleophobic regime and in agreement with the experimental value.

A Surface Texture Design to Obstruct the Liquid Migration Induced by Omnidirectional Thermal Gradients

Thermo-capillary migration is a phenomenon in which surface thermal gradients drive a liquid to flow from warm to cold regions without external forces. It is important to prevent the migration of liquid lubricants on rubbing surfaces. In this paper, a pattern of microdimples was proposed to obstruct the liquid migration induced by an omnidirectional thermal gradient. Microdimple patterns were fabricated on the surfaces of SUS316 stainless steel. Experiments were performed to investigate the influence of microdimple patterns with different geometric parameters on the migration behavior of paraffin oil. In particular, this study focused on the interfacial flowing near the microdimples. The results demonstrated that microdimples have a significant obstructive effect on migration, whereas dimples have a retaining effect, and the adjacent dimples have an interacting effect.