Generation of Ultra-stable Microbubbles for Industrial Applications

Microbubbles (MBs) are fine bubbles with a radius of 1 – 100 μm. Microbubbles has been subject of interest for researchers and industries over the past few decades as they proved to be the most effective contrast agent for ultrasound radiography. More recently microbubbles potential for use in drug delivery, gene therapy and also for non-medical application such as food, thermal insulating material and low weight structure has been investigated. Development in applications of MBs increased the need for more advanced understanding of microbubble formation and stability. The thermodynamically unstable nature of microbubbles remains as an issue in all of their applications. Air bubbles in water stabilized by particles (‘Pickering’ bubbles) can be indefinitely stable, but the slower mass transport of particles to interfaces compared to molecules means that the final stable bubble size is a fine balance between the rates of bubble formation, particle coverage and shrinkage.
In this project the two types of Pickering nanoparticles (NPs) namely Class II hydrophobin (HFBII) and poly alkyl cyanoacrylate NPs were investigated in terms of their ability to form and stabilize microbubbles. HFBⅡ is an edible protein acts like a very small Janus nanoparticle since it is known not to unfold or denature at air-water interfaces and has a hydrophobic patch on one side. Microbubbles stabilized by HFBII alone are stable for a relatively long time (several days) but it is difficult to obtain a high volume fraction of bubbles due to HFBII aggregation. However, combining HFBII with other surfactants increases the overrun and a strong negative correlation is seen between the surface tension decrease resulting from the added surfactant and the volume fraction of microbubbles after 48 hours.
Different monomers from alkyl cyanoacrylate family (butyl (BCA), octyl (OCA) and Ethyl (ECA)) were polymerized under different conditions (pH, surfactant type and concentration) to form NPs, with the potential to stabilize MBs via Pickering mechanism. Poly butyl cyanoacrylates (PBCA) NPs, polymerized at pH 4 and in presence of 1 wt.% Tyloxapol were the best stabilizing NPs compared to other PACA NPs. MBs stabilized by this NPs were stable for the period of 10 weeks or more. The SEM micrographs indicated that MBs are stabilized via the self-assembly of PBCA NPs around the surface of microbubbles (contact angle 77 ± 10) during aeration via high shear mixer. Increasing the aeration steps led to increase the volume fraction of MBs up to 3 vol. %. On the other hand PBCA NPs polymerized in the absence of any surfactant were unable to stabilize any bubbles. Our results shows that roughly 25 wt.% of the Tyloxapol incorporate into the NPs during polymerization. This incorporation of Tyloxapol changes the physicochemical characteristic of the NPs in the way that NPs can adsorb to the air- water interface and protect MBs against shrinkage.
The PBCA + surfactant system was studied in more detail, by controlling the rate of formation and initial size of the bubbles more exactly, so a balance was achieved whereby very efficient microbubble production took place (e.g., > 10 vol.%).