towards a fine understanding of shear thickening

towards a fine understanding of shear thickening


Credit: LPENSL (CNRS / ENS de Lyon)

Shear-thickening fluids are colloidal suspensions characterized by a viscosity which increases – sometimes very abruptly – with the intensity of the shear to which they are subjected. Researchers have shown experimentally that if this phenomenon emerges mainly from the intrinsic roughness of the colloids, it can be significantly amplified by the presence of bonds hydrogen (Hydrogen is a chemical element with symbol H and atomic number 1.).

What to cook (Kitchen) has never experienced this paradoxical and embarrassing behavior of cornstarch which “refuses” itself mixed (A mixture is an association of two or more solid, liquid or gaseous substances…) ? When the starch of corn (Corn (also called Indian corn in Canada) is a tropical plant…) is indeed highly concentrated inwater (Water is a ubiquitous chemical compound on Earth, essential for all…), the mixture becomes almost solid when the movement of the spatula is a little too fast. This rheological behavior, called shear thickening, is however not very common in complex fluids, where we more often observe thereverse (In mathematics, the inverse of an element x of a set endowed with a law of…)of the materials (A material is a material of natural or artificial origin that man shapes to…) which become fluid under shear (paints when they are spread, for example). This increase in cornstarch viscosity is due to the interactions between the cornstarch particles that make up the suspension (The act of suspending particles In chemistry, suspension designates a dispersion of…).

However, the precise microscopic scenario associated with this sudden increase in viscosity remains poorly understood, in particular because the starch particles have the double disadvantage of being insufficiently characterized individually and of aging chemically during the time (Time is a concept developed by human beings to apprehend the…). As this phenomenon is also found in other types of colloidal suspensions, chemically very different from cornstarch, different microscopic origins of shear thickening have been proposed in the literature, which experiments with sometimes contradictory results have not succeeded in separating entirely.


Scanning electron micrographs (the scale corresponds to 200 nm) showing the two types of colloids used, roughness different (In mathematics, the different is defined in algebraic theory of…) resulting from the aggregation of SiO2 nanoparticles with a radius of 25 nm (top) and 10 nm (bottom). The red circles highlight some of these primary nanoparticles. Credit: LPENSL (CNRS / ENS de Lyon)

By a new series of experiments on suspensions of silica (Silica is made up of silicon dioxide, a chemical compound that goes into the…) (SiO2), of controlled size and roughness, a team of researchers from the Laboratoire de physical (Physics (from the Greek φυσις, nature) is etymologically the…) the Ecole Normale Supérieure of Lyon (LPENSL, CNRS (The National Center for Scientific Research, better known by its acronym CNRS, is the largest…) / ENS de Lyon), from the joint international CNRS-MIT Multiscale materials laboratory science (Science (Latin scientia, “knowledge”) is, according to the dictionary…) for energy and environment (2, CNRS / MIT) and MIT precisely quantified the relative contributions to friction shear thickening mechanical (In everyday language, mechanics is the field of machines, engines, vehicles, organs, etc.) between the particles on the one hand and their physico-chemical interactions on the other.

By comparing the behavior of more or less rough particle suspensions and chemical treatment of surface (A surface generally refers to the superficial layer of an object. The term a…) variable (In mathematics and logic, a variable is represented by a symbol. It…)they demonstrate on the one hand the necessary role of the roughness of the particles to induce shear thickening and on the other hand the exacerbation of this phenomenon by the presence of physico-chemical interactions (typically hydrogen bonds) until a response mechanics close to the solid.

By varying the proportion of particles in suspension capable of forming hydrogen bonds, the physicists have also succeeded in continuously controlling the intensity of the shear thickening observed. This opens the way to many applications, for example the formulation (Formulation is an industrial activity consisting in manufacturing products…) cements with better controlled rheological properties or the design of malleable and shock absorbing materials such as bulletproof vests. These results are published in the journal Physical Review Research.

To obtain these results, the researchers carried out rheological measurements on four types of colloidal solutions of fumed silica (agglomerated SiO2 nanoparticles) in polypropylene (Isotactic polypropylene (or polypropene), with the acronym PP (or PPi) and the formula…) glycol, the particles differing in roughness and hydrophobicity, but all having a size mean (The average is a statistical measure characterizing the elements of a set of…) 300 nanometers (nm).

the control (The word control can have several meanings. It can be used as a synonym of examination, of…) of the roughness comes from the manufacturing process of the silica particles, resulting from a agglomeration (An agglomeration is an urban ensemble based on the continuity of the built environment….) at high temperature (Temperature is a physical quantity measured using a thermometer and…) of SiO2 nanoparticles whose size (10 and 25 nm in the experiments described) is precisely selected by the manufacturing process. Aggregates are obtained with shapes that are all the more tortuous as the nanoparticles composing them are small (see figure). This tortuosity enables them to develop significant friction in the event of contact between them, contact whose duration and therefore the effectiveness in increasing the viscosity can also be controlled by modifying the hydrophobicity of their surface by chemical treatment.

It is interesting to note that the results obtained with these colloids with complicated shapes are not well described by current theories and therefore raise a certain number (The notion of number in linguistics is treated in the article “Number…) fundamental questions whose answers have yet to be found.

contacts:

– Thibaut Divoux – Responsible for research (Scientific research designates in the first place all the actions undertaken with a view to…)Physics Laboratory of the ENS de Lyon – thibaut.divoux at ens-lyon.fr
Communication (Communication concerns both man (intra-psychic, interpersonal communication, etc.) INP – inp.com at cnrs.fr

Did you like this article? Do you wish to support us ? Share it on social networks with your friends and/or comment on it, this will encourage us to publish more similar topics!

#fine #understanding #shear #thickening

Leave a Comment

Your email address will not be published.