to a fine understanding of shear thickening


Photo credit: LPENSL (CNRS / ENS de Lyon)

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

What Cook (Kitchen) has never experienced this paradoxical and embarrassing behavior of cornstarch “denying” itself. mixed (A mixture is a combination of two or more solid, liquid or gaseous substances…) ? If the strength of Corn (Corn (also called Indian corn in Canada) is a tropical plant…) is indeed highly concentratedwater (Water is a ubiquitous chemical compound on earth, essential to all…), the mixture will almost set if the movement of the spatula is a little too fast. However, this rheological behavior, called shear thickening, is not very common in complex fluids, where we observe it more frequentlyturning back (In mathematics, the inverse of an element x of a set endowed with a law of….)of materials (A material is a material of natural or artificial origin that man molds into…) that become liquid under shear (e.g. paints when brushed). This increase in cornstarch viscosity is due to the interactions between the cornstarch particles that make up the suspension (The process of suspending particles In chemistry, suspension denotes a dispersion of…).

However, the exact microscopic scenario that accompanies this sudden increase in viscosity is still poorly understood, particularly because the starch particles have the twin disadvantages of being individually poorly characterized and chemically aging during the process time (Time is a concept developed by humans to represent the…). Since this phenomenon is also found in other types of colloidal suspensions chemically very different from corn starch, various microscopic origins of shear thickening have been proposed in the literature, which could not be fully separated by experiments with sometimes conflicting results.


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

Through a new series of tests on suspensions of silica (Silicic acid consists of silicon dioxide, a chemical compound found in the…) (SiO2), of controlled size and roughness, a research team from the Laboratoire de physically (Physics (from 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 (According to the dictionary, science (lat. scientia, “knowledge”) is…) for energy and environment (2, CNRS/MIT) and MIT accurately quantified the relative contributions to friction shear thickening mechanically (Colloquially, 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 coarse particle suspensions and the chemical treatment of Pop up (A surface generally refers to the superficial layer of an object. The term a…) variable (In math 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 aggravation of this phenomenon by the presence of physicochemical interactions (typically hydrogen bonds) up to a response mechanics close to that of solid.

By varying the proportion of particles in the suspension that can form hydrogen bonds, the physicists have also managed to continuously control the intensity of the observed shear thickening. This opens the way to many applications, for example the formulation (Formulation is an industrial activity consisting in the manufacture of products…) Cements with more controlled rheological properties or the design of malleable and shock-absorbing materials such as bulletproof vests. These results were published in the journal Physical Review Research.

To obtain these results, researchers performed rheological measurements on four types of colloidal solutions of fumed silica (agglomerated SiO2 nanoparticles). polypropylene (Isotactic Polypropylene (or Polypropene), with the abbreviation PP (or PPi) and the formula…) Glycol, the particles differ in roughness and hydrophobicity, but all have one size mean (The average is a statistical measure that characterizes 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 for investigation, 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….) in the climax temperature (Temperature is a physical quantity that is measured with a thermometer and…) of SiO2 nanoparticles, whose size (10 and 25 nm in the described experiments) is precisely selected by the manufacturing process. Aggregates with more tortuous shapes are obtained the smaller the nanoparticles that make them up (see figure). This tortuosity allows them to develop considerable friction in the event of contact between them, the duration of which, and therefore the effectiveness in increasing 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 complicated-shape colloids are not well described by current theories and are therefore of some concern number (The concept of number in linguistics is dealt with in the article “Number…”.) fundamental questions whose answers are yet to be found.

Contacts:

– Thibaut Divoux – Responsible for research (Scientific research primarily refers to all measures taken to …)Physical laboratory of the ENS de Lyon – thibaut.divoux at ens-lyon.fr
communication (Communication affects both humans (intrapsychic, interpersonal communication, etc.) INP – inp.com and cnrs.fr

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