posted on 2023-05-23, 21:33authored byMariana Rodríguez-Hakim, Luka Oblak, Jan Vermant
Stable foams that
can resist disproportionation for extended periods
of time have important applications in a wide range of technological
and consumer materials. Yet, legislative initiatives limit the range
of surface active materials that can be used for environmental impact
reasons. There is a need for technologies to efficiently produce multiphase
materials using more eco-friendly components, such as particles, and
for which traditional thermodynamics-based processing routes are not
necessarily efficient enough. This work describes an innovative foaming
technology that can produce ultrastable Pickering-Ramsden foams, with
bubbles of micrometer-sized dimensions, through pressure-induced particle
densification. Specifically, aqueous nanosilica-stabilized foams are
produced by foaming a suspension at subatmospheric pressures, allowing
for adsorption of the particles onto large bubbles. This is followed
by an increase back to atmospheric pressure, which induces bubble
shrinkage and compresses the adsorbed particle interface, forming
a strong elastoplastic network that provides mechanical resistance
against disproportionation. The foam’s interfacial mechanical
properties are quantified to predict the range of processing conditions
needed to produce permanently stable foams, and a general stability
criterion is derived by considering the interfacial rheological properties
under slow, unidirectional compression. Foams that are stable against
disproportionation are characterized by interfaces whose mechanical
resistance to compressive deformations can withstand their tendency
to minimize the interfacial stress by reducing their surface area.
Our ultrastable nanosilica foams are tested in real-life applications
by introducing them into concrete. In comparison to other commercial
air entrainers, our microfoam improves concrete’s freeze–thaw
resistance while supplying higher material strength, providing an
economically attractive, industrially scalable, and durable alternative
for use in real-life applications involving cementitious materials.
The applicability of our stability criterion to other rheologically
complex interfaces and the versatile nature of our foaming technology
enables usage for a broad class of materials, beyond the construction
industry.