Influence of Surface Wettability on Microbubble Formation

The production and utilization of microbubbles are rapidly becoming of major importance in a number of global applications, from biofuel production to medical imaging contrast agents. Many aspects of bubble formation have been studied, with diffuser characteristics (such as pore size, pore orientation) and gas flow rate all being shown to influence the bubble formation process. However, very little attention has been paid to the influence of surface wettability of the diffuser and the detailed role it plays at the triple interface of gas–liquid–diffuser. Here, we investigate how the wettability of the diffuser surface impacts upon the dynamics of the bubble formation process and examine the effect both at the orifice and upon the bubble cloud produced as a result of the engineered wetting variations. Experimental data shown here indicate the presence of a switching point at a contact angle of θ = 90°, where bubble size vastly changes. When a surface exhibits a contact angle below 90°, bubbles emitted from it are considerably smaller than those emitted from a surface with an angle in excess of 90°. This effect is observable over flow rates ranging from 2.5 to 60 mL min^–1 from a single pore, an array of controlled pores, and the industrially relevant and commercially available sintered metals and sintered ceramic diffusers. It is also observed for both thiol and silane modified surfaces, encompassing a range of contact angles from 10° to 110°. In addition, the importance of the diffuser plate’s surface topography is discussed, with elevated roughness acting to reduce the effect of surface chemistry in some instances

Determination of Solvent–Polymer and Polymer–Polymer Flory–Huggins Interaction Parameters for Poly(3-hexylthiophene) via Solvent Vapor Swelling

We report the use of solvent vapor swelling of ultrathin polymer films to determine Flory–Huggins solvent–polymer and polymer–polymer interaction parameters (χ_<i>i–j</i>) for poly­(3-hexylthiophene) (P3HT) and polystyrene (PS) over a wide solvent composition range. From the calculated interaction parameters, we constructed a polymer/polymer/solvent phase diagram that was validated experimentally. χ_{tetrahydrofuran–P3HT} (1.04 ± 0.04) and χ_{CHCl₃–P3HT} (0.99 ± 0.01) were determined through swelling of ultrathin P3HT films. Similar experiments using PS films gave χ_{tetrahydrofuran–PS} = 0.41 ± 0.02 and χ_CHCl₃–PS = 0.39 ± 0.01, consistent with literature values. As expected, these χ_<i>i–j</i> parameters indicated that P3HT is less compatible than PS with either solvent. From δ_PS (17.9 ± 0.2 MPa^1/2) and δ_P3HT (14.8 ± 0.2 MPa^1/2), determined through regular solution theory, we calculated χ_PS–P3HT = 0.48 ± 0.06 at 23 °C. The resulting phase diagram was validated by solution-based transmission measurements of PS/P3HT blends in <i>o</i>-xylene. Although we focused on PS/P3HT blends in this work, this approach is easily adaptable to other polymer/polymer combinations of interest

The Relationship between Charge Density and Polyelectrolyte Brush Profile Using Simultaneous Neutron Reflectivity and In Situ Attenuated Total Internal Reflection FTIR

We report on a novel experimental study of a pH-responsive polyelectrolyte brush at the silicon/D₂O interface. A poly­[2-(diethylamino)­ethyl methacrylate] brush was grown on a large silicon crystal which acted as both a substrate for a neutron reflectivity solid/liquid experiment but also as an FTIR-ATR spectroscopy crystal. This arrangement has allowed for both neutron reflectivities and FTIR spectroscopic information to be measured in parallel. The chosen polybase brush shows strong IR bands which can be assigned to the N–D⁺ stretch, D₂O, and a carbonyl group. From such FTIR data, we are able to closely monitor the degree of protonation along the polymer chain as well as revealing information concerning the D₂O concentration at the interface. The neutron reflectivity data allows us to determine the physical brush profile normal to the solid/liquid interface along with the corresponding degree of hydration. This combined approach makes it possible to quantify the charge on a polymer brush alongside the morphology adopted by the polymer chains