Shear modulus and yield stress of foams: contribution of interfacial elasticity

Tuning stability, rheology, and fire-extinguishing performance of advanced firefighting foam material by inorganic nanoparticle flame retardants

HYPOTHESIS

Nanoparticle-stabilized foams are extremely stable, and flame retardant inorganic nanoparticles should be able to add sealing capacity of firefighting foams on flammable liquid fuels, and hence enhance fire extinguishment performance on liquid fuel fire. In practice, how do flame retardant nanoparticles resist the destructive effect of oil molecules on foam and tune foam properties?

EXPERIMENTS

We have prepared a nanoparticle-enhanced foam comprising of hydrocarbon surfactant, short-chain fluorocarbon surfactant, and nanoparticles. The interactions among nanoparticles and surfactant molecules were characterized by using dynamic surface tension and conductivity. Stability, rheology, and oil resistivity on liquid fuel of the nanoparticle-enhanced foam were evaluated systematically. Fire suppression effectiveness of the foams was verified based on a standard experiment.

FINDINGS

Foam stability and oil resistivity were enhanced due to self-assembled network structures formed by jammed aggregates composed by nanoparticles and surfactants in Plateau borders and bubble films, providing structural recoverability and enhanced viscoelasticity within foam. Foams containing nano-SiO2, nano-CaCO3, nano-Al(OH)3, and nano-Mg(OH)2 show difference in fire extinguishment due to different ability to enhance foam properties. Foam containing nano-Al(OH)3 shows the strongest adaptation and could shorten fire extinguishing time by 2 times and prolong burn-back time by 2.3 times compared with commercial product.

Scientific gastronomy: On the mechanism by which garlic juice and allicin (thio-2-propene-1-sulfinic acid S-allyl ester) stabilize meringues

Garlic juice (GJ) contains allicin, a bioactive that stabilizes egg white (EW) foams without potassium bitartrate (cream-of-tartar), meeting the aim to identify clean-label ingredients that alter foam characteristics. 0.001 wt% allicin in EWs forms stiff foams with the highest overrun (∼800 %) and delayed drainage (60 min). Whipping EW with GJ or allicin changes the disulfide-bridge conformation of the EW foam from 56, 44, and 0% gauche-gauche-gauche (SSg-g-g), trans-gauche-trans (SSt-g-t), and trans-gauche-gauche (SSt-g-g) to 11, 52, and 37%. Raman microspectrometry (RM) found higher relative percentages of SSg-g-g disulfide bonds coincide with rapid foam collapse, while the tyrosine I850/I830 band ratios, a measure of tyrosine surface exposure, remained constant. These changes in spectra illustrate that polypeptide chain displacement and protein unfolding are essential in stabilizing bubble interfaces. Few clean-label functional compounds alter foam stability and functionality, and compounds such as allicin could lead to entirely new culinary techniques and dishes.

Scraping of foam on a substrate

HYPOTHESIS

Foam is not only an industrially important form of matter, but also one of soft jammed system such as colloidal suspensions and emulsion suspensions. Since foams are composed of gas bubbles and liquid, it is expected that the coupling of bubbles and liquid strongly affects rheology of foams, which is different from simple liquids. To reveal this coupling effect and considering the importance of foam applications, we studied the scraping of foam by a rigid plate on a substrate.

EXPERIMENT

We mainly used 5.0 wt% solution of ionic surfactant TTAB (tetradecyltrimethylammonium bromide). We systematically investigate the scraping behaviors by the rigid plate as a function of scraping velocity, gap height, confinement length, amount of foam and wettability of the substrate.

FINDINGS

The results show that there are three distinguishable scraping patterns: homogeneous scraping, no scraping, and slendered scraping. These behaviors are clearly different from those of simple liquid systems. Of these, the upper limit of homogeneous scraping could be explained theoretically by the competition between dewetting and shear, which is not previously discussed. Furthermore, the theory is applicable to the scraping of other soft jammed systems such as colloidal and emulsion suspensions.

Foaming and rheological properties of aqueous solutions: an interfacial study

Although aqueous foam is composed of simple fluids, air and water, it shows a complex rheological behavior. It exhibits solid-like behavior at low shear and fluid-like behavior at high shear rate. Therefore, understanding such behavior is important for many industrial applications in foods, pharmaceuticals, and cosmetics. Additionally, air–water interface of bubble surface plays an important role in the stabilizing mechanism of foams. Therefore, the rheological properties associated with the aqueous foam highly depend on its interfacial properties. In this review, a systematic study of aqueous foam are presented primarily from rheology point of view. Firstly, foaming agents, surfactants and particles are described; then foam structure was explained, followed by change in structure under applied shear. Finally, foam rheology was linked to interfacial rheology for the interface containing particles whose surface properties were altered by surfactants.