![]() ![]() Usually, in colloidal systems, thermal gelation is achieved in grafting long molecules to the surface of the particles, depending on the temperature, these molecules will interact differently with the suspending media and either have steric repulsion (liquid macroscopic behavior) or will induce the particles to aggregate into clusters (gel-like macroscopic behavior). An external trigger that can be controlled in most measurement systems is the temperature, which is why we proceeded in looking for a gelation strategy, where we could use the above-mentioned rough particles in combination with thermal gelation. Therefore, even using a syringe or pipette to load the gel into the measurement cell, would have biased the comparison of the rough and smooth primary particle gels. The latter was most important to the study because colloidal gels are highly thixotropic, meaning that their mechanical properties depend on their processing and shear history. This can be done in many ways, however we had a few design requirements for the gel system: (i) as the measurements can be long, evaporation needed to be limited, therefore water was not a good suspending medium (ii) the interaction forces had to be the same for the smooth and rough particle system in order to best compare the results of both (iii) most importantly, the gelation needed to be reversible through an independent trigger in order to form a nascent gel inside the measurement cell. In a next step, the particles had to be assembled into a gel, which means that attractive interactions must be induced between the particles. This is how the rough particles were synthesized in this work. If negatively charged smaller particles are then added to this suspension, they will in turn adsorb to the positively charged cores. The synthesis of the rough particles was built on a previously developed heterocoagulation method, which uses electrostatic effects to add small particles to the surface of large particles: as the particles are negatively charged in water, adding a positively charged molecule to the suspension will make the molecule adsorb to the particle surface and make it positive. By understanding the particle properties, the features can be utilized to engineer more complex geometries, a kind of “nano-lego”. Often, particles can be made of smaller molecules that naturally assemble to smooth spherical structures, which is dictated by surface energy. With this mental image in mind, the next step was to realize these particles in a reproducible manner, make the gel and find the right experiments to prove the hypothesis, that smooth and rough particles will behave different when exposed to shear stresses.Ĭhemistry is a very powerful tool when designing nanoparticles. The thinking was that if the particles would behave similar to a jammed assembly of gears, they would interlock and resist to rolling when exposed to stresses. In this work, the initial idea was to render a gel tougher by introducing surface topography to the building blocks of the gel. ![]() These are only a few examples of the “knobs to turn” so we can engineer a gel with targeted properties. By tuning the particle material, shape, topography as well as the suspending media properties, the network properties will change, which in turn also change the macroscopic properties. This material is visco-elastic, meaning that it has liquid-like and solid-like characteristics, where the particle network contributes to the elasticity and the suspending medium contributes to the viscous behavior. The particles then form a spanning network which macroscopically forms a gel. Colloidal gels are formed, when attractive interactions are induced in a suspension of colloidal particles (which typically are in the size range 10 nm - 10 µm in diameter). Material science is at the juncture of chemistry, physics and engineering, and all three disciplines were important when understanding the behavior of rough particle colloidal gels. Of course, all these methods will have their advantages and limitations, which is why they are all needed in order for a complete mental picture to be formed. We need to stitch together the “truth” from the different snapshots that we get using various measurement methods. Understanding mechanisms arising from phenomena six orders of magnitudes in length scale below what we can see, is challenging. ![]()
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