Conrad Aiken, though a new contributor, will not need an introduction to poetry lovers. A native of Savannah, he lives now in England. He has published volumes of poetry since 1914, as well as short stories and novels. Frederic Prokosch, whose poems have often appeared in the Quarterly, is a Pennsylvanian. Geoffrey Johnson is an English poet, whose work is also familiar to our readers.

Oil foams have been stabilized by using particles of oligomer of tetrafluoroethylene (OTFE). OTFE particles were dispersed in oil mixtures prior to aeration, to exclude the oil-repellency nature of the particles due to the formation of the metastable Cassie-Baxter state and properly evaluate the effects of contact angle on the foaming behavior. The particle contact angle (Y) against air/oil surfaces were controlled by changing a composition of two oils with different surface tension (n-heptane and methyl salicylate). The Y value increases with increasing a mole fraction of methyl salicylate, from 42 (for pure n-heptane) to 89 (for pure methyl salicylate). The air volume incorporated in the oils after aerating OTFE dispersions in the oil mixtures shows a maximum when Y = 55. The flocculation of OTFE particles in bulk oils is responsible for the unexpected behavior of foaming observed when Y is relatively high. The increase in the degree of the flocculation reduces the effective concentration of OTFE particles in bulk oil, leading to the inefficient bubble stabilization. These findings suggest the efficient oil foaming using particles as a stabilizer is achieved by optimizing both the particle contact angle and the degree of flocculation in oils.

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Compared to particle-stabilized aqueous foams, particle-stabilized oil foams have been sparsely studied (Friberg, 2010; Fameau, 2017). The stabilization of oil foams have been reported using particles with low surface energy (particles with fluorinated surfaces and fluoro-particles such as PTFE) and various kinds of pure oils (Thareja et al., 2008; Binks et al., 2011, 2015; Binks and Tyowua, 2013). The types of materials formed by mixing air, the oils and the particles have been considered in terms of air-oil surface tension, particle surface energy and mixing methods. However, the mechanisms regarding oil foam stabilization, for example, the dependence of foam volume on the contact angle, has not been studied. In our previous study, we have prepared oil foams using oligomer of tetrafluoroethylene (OTFE) particle (Murakami and Bismarck, 2010). The particle contact angle was controlled by mixing two oils with different surface tension. The foam volume shows a maximum when the particle intrinsic (Young's) contact angle (Y), which is the contact angle determined by the particle surface chemistry, is around 46. When the contact angle is relatively high, say, between 50 and 80, oil foams are barely formed, instead, agglomerated particles containing small amount of trapped air bubbles are obtained. From the viewpoint of G, the foaming efficiency is expected to increase with increasing Y when Y < 90, as is found for particle-stabilized aqueous foams (Binks and Horozov, 2005; Binks and Murakami, 2006). The reason of the poor oil foaming at a relatively high Y is that OTFE particles are oil-repellent due to the metastable Cassie-Baxter state. The particles are highly agglomerated in air due to the surface force between the particles and air/oil surfaces can be suspended between the primary particles in the agglomerates, leading to the formation of the metastable Cassie-Baxter state, when Y is higher than a critical angle (46). Bubbles formed during aeration are barely adsorbed by OTFE particles, as they are not readily wetted by oils when Y is higher than 46. Such oil-repellent agglomerated particles preferably stabilize oil marbles and dry oils, instead of oil foams.

We have hypothesized that if OTFE particles are properly dispersed in oil prior to aeration, oil foam volume would increase with increasing Y and hence with increasing G. In this study, we prepare dispersions of OTFE particles in two oils with a low surface tension oil (n-heptane) and a high surface tension one (methyl salicylate). To eliminate the effects of the metastable Cassie-Baxter state on oil foaming behavior, a dispersion of OTFE particles in methyl salicylate is prepared by using the solvent replacement method; initially the particles are dispersed in an oil with a low surface tension (hence low Y) and the oil is gradually replaced with an oil with a high surface tension (hence high Y). The two dispersions of OTFE particles are mixed at a fixed content of OTFE particles and Y is increased with increasing a mole fraction of methyl salicylate in the oil mixture. By aerating the dispersions of OTFE particles with showing different Y, we investigate the dependence of oil foaming on the contact angle and discuss the oil foaming mechanism. The aim of this study is to find out important factors to control properties of oil foams stabilized by particulate materials.

By mixing the OTFE dispersions in n-heptane and methyl salicylate, we have prepared OTFE dispersions with desired mole fractions of methyl salicylate (xMS) and particle concentrations (1.0, 2.0 and 3.0 vol.% relative to the total volume). The liquid volume was fixed at 5.0 mL. Aeration of OTFE dispersions in glass vials with a volumetric capacity of about 20 mL was carried out by shaking the dispersions by hand at 4 Hz for 15 s, at room temperature (22  2C).

The air volume incorporated in oil mixtures was measured manually at a certain time. The error for the volume measurement is typically  0.1 mL. Photographs of samples were taken with a CX2 digital camera (Ricoh). Optical micrographs of samples placed on a glass slide with a depression were taken with an Olympus BX51 transmission/reflection microscope fitted with a CMOS camera (Moticam 2000, Shimazu). SEM observation without prior conductive coating was conducted using a Hitachi S-3400N Variable Pressure SEM at an accelerating voltage of 15 kV and at 60-80 Pa with an environmental secondary electron detector.

Digital photographs of vials containing 3.0 vol.% dispersions of OTFE particles in oil mixtures at different xMS (and Y) before and after aeration. Dotted line is guide for apparent volume of sedimented OTFE particles.

Air volume incorporated into the dispersion with different OTFE concentration after aeration plotted as a function of: (A) Y and (B) OTFE concentration. Air volume measured after 1 month is plotted.

Figure 4B shows that the incorporated air volume monotonically increases with increasing OTFE concentration at a fixed Y. Binks and Tyiwua observed an increase in volume of particle-stabilized oil foams with increasing a particle concentration and pointed out the resemblance to surfactant foaming agents below the critical micelle concentration (Binks and Tyowua, 2013). It is assumed that during aeration bare bubbles are initially formed and if the bubbles are adsorbed by enough amount of particles during aeration, they remain as particle-stabilized bubbles after the cease of aeration. The size of bubbles stabilized by particles might be larger than the original bare bubble due to the limited coalescence (Arditty et al., 2003; Tcholakova et al., 2008). The increase in the particle concentration facilitate the bubble stabilization, which is driven by an increase in the potential total area to be covered by particles, leading to an increase in the incorporated air volume. However the drawback of the increase in the particle concentration is a decrease in the rate of energy dissipation, which is consumed by the formation bubble surfaces and the agitation of the total mass (Tsabet and Fradette, 2015). The air volume incorporated in oils is determined by a balance between these two factors; the air volume could initially increase and practically reach a plateau with increasing the particle concentration.

Here we propose that the flocculation of the particles is closely related to the foaming behavior. One of the possible reasons for less foaming with highly flocculated particles could be a decrease in the effective particle concentration with increasing the degree of flocculation. In the field of mineral flotation, it is well-known that the rate of flotation is a first order to a bulk particle concentration;(3)dNdt=-kN,where N is the number of floatable particles and k is the rate constant (Mao and Yoon, 1997; Arai et al., 2009). Upon aeration, as stated in the previous section (the dependence of OTFE concentration on the incorporated air volume), bare bubbles could be initially formed and the bubbles become stable against coalescence if the bubbles are enoughly covered by the particles during aeration. The finding that the incorporated air volume increases with increasing the bulk OTFE concentration has suggested that the efficiency of the coverage could increase with increasing the particle concentration around bubbles. Not well-covered bubbles could be formed at a low effective particle concentration for flocculated particles and they are prone to coalesce during aeration, eventually less amount of foam is produced compared to the more dispersed particles. On one hand, the low degree of flocculation might be advantageous to the formation of oil foams. The degree of the flocculation could monotonically increase with increasing the contact angle in this study (see Figure 6). The maximum foaming at the intermediate contact angle (55) might be associated with the particle adsorption energy but also the moderately flocculated particles that can retard drainage and improve the stability of oil foams. Another reason might be that flocculated particles are so large that they are more slowly transferred to an air/oil surface than the deflocculated ones. 589ccfa754

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