Study of the aggregation behavior of Janus particles by coupling experiments and Brownian dynamics simulations.

HYPOTHESIS: New colloids such as inverse patchy particles or Janus particles are considered as smart building blocks in the development of innovative and performant materials. For example, the control of the self-assembly of oxide-based charged Janus particles is interesting for ceramic shaping. Thus, the synthesis of silica based Janus particles as well as a detailed study of their behavior in suspension are presented in this paper. EXPERIMENTS: Fluorescent silica particles are partially modified in surface by grafting amine groups using a Pickering emulsion route. Zeta potential measurements, sedimentation tests and confocal microscopy observations are carried out to analyze the aggregation of the obtained particles in aqueous suspension as a function of the patch size and of the pH. Brownian dynamics simulations are also performed to better understand the aggregate structures. FINDINGS: The aggregation of the synthesized silica-based Janus particles can be tuned by modifying the experimental parameters, and elongated or on the contrary more compact structures could be observed. This control of aggregation makes such particles promising to build new ceramic materials.

Brownian dynamics simulations of one-patch inverse patchy particles.

Inverse patchy particles are promising colloids to develop new architectures in ceramic materials based on their self-assembly. Nonetheless, a good understanding of their aggregation is required. Several previous studies have shown that the behavior of ceramic colloids can be well described by the DLVO interaction potential. In the present paper, we develop new coarse-grained Brownian dynamics simulations, where particles are represented by an assembly of beads interacting via DLVO interactions, whose parameters can be directly linked to experimental characterization. First, the validity of the simulations is proved by studying the heteroaggregation of homogeneously charged particles. Then, simulations are applied to one-patch inverse patchy particles to study the effect of the patch size. They show that the smaller the patch, the more elongated the aggregates. Simulations are also performed to understand the role of the Debye screening length in the particular case of large patches and they show that aggregation leads always to compact aggregates.

Influence of different surfactants on Pickering emulsions stabilized by submicronic silica particles.

HYPOTHESIS: Pickering emulsions were prepared using wax and silica submicronic particles (650 nm), as a first step towards the synthesis of Janus particles. Surfactants added to silica particles control the emulsion stability and particles arrangement, i.e. their penetration depth into the wax and their ability to form a monolayer. Thus, a systematic study of surfactants is proposed. EXPERIMENTS: Zeta potential measurements and sedimentation tests are conducted to evaluate interactions of two cationic (CTAB: hexadecyltrimethylammonium bromide and DDAB: didodecyldimethylammonium bromide) and two polymeric surfactants with silica surface. Surfactants affinity for the wax is estimated by contact angle measurements. Emulsions stability is compared to evaluate the ability of particles to stabilize wax droplets. Through microscopic analyses, the penetration depth into the wax is measured as well as the ability to form a monolayer or multilayers/aggregates. FINDINGS: All surfactants modify silica surface properties, but only CTAB and DDAB give stable Pickering emulsions. Because of a better affinity for the wax, DDAB presents the best characteristics for Janus particle synthesis, allowing a larger variation of particles penetration depth into the wax.