The reason for this combination of properties is that ULC nanogels can be thought of as colloids in between flexible macromolecules and particles. In contrast, the stability against coalescence and the temperature-responsiveness closely resemble those of emulsions stabilised by regularly crosslinked pNIPAM nanogels. The resistance to flocculation of the ULC-stabilised emulsion droplets is similar to the one of emulsions stabilised by linear pNIPAM. These emulsions are resistant to droplet flocculation, stable against coalescence, and can be easily broken upon an increase in temperature. We show that ULC nanogels can efficiently stabilise emulsions already at low mass concentrations. Here, we study how the softest nanogels that can be synthesised with precipitation polymerisation of N-isopropylacrylamide (NIPAM), the ultra-low crosslinked (ULC) nanogels, stabilise oil-in-water emulsions. One of the factors that improves the emulsion stability is the nanogel softness. Micro- and nanogels are widely used to stabilise emulsions and simultaneously implement their responsiveness to the external stimuli. Furthermore, this review can give researchers dealing with soft colloids quantitative methods to define unambiguously which softness matters in their compound. The aim of this review is to look at the results on micro- and nanogels in a more quantitative way that allow us to explain the reported properties in terms of differences in colloidal softness. Concentrated solutions of nanogels are considered and we review the recent results in the literature concerning the phase behavior and flow properties of nanogels both in three and two dimensions, in the light of the different parameters we defined. The influence of the different synthetic routes on the softness of nanogels is discussed. New definitions of softness and new parameters, which depend on the particle-to-particle interactions, are introduced in terms of faceting and interpenetration. Then, concentrated solutions of soft colloids are considered.
This is done in terms of the energetic cost associated with the deformation and the capability of the colloid to isotropically deswell. Applying our criteria, we address the question what makes a nanomaterial soft? We discuss and introduce general criteria to quantify the different definitions of softness for an individual compressible colloid. cross-linked polymer networks swollen in water, a widely used model system for soft colloids. On the basis of these quantities, we review the recent literature on micro- and nanogels, i.e. Here, we propose different quantities that can be measured using scattering methods and microscopy experiments. Having quantitative parameters is fundamental to compare different systems and understand what the consequences of softness on the macroscopic properties are. However, we are missing a way to quantify what the term "softness" means in nanoscience. Softness plays a key role in determining the macroscopic properties of colloidal systems, from synthetic nanogels to biological macromolecules, from viruses to star polymers.
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