Complex fluids are
mixture
In chemistry, a mixture is a material made up of two or more different chemical substances which can be separated by physical method. It is an impure substance made up of 2 or more elements or compounds mechanically mixed together in any proporti ...
s that have a coexistence between two
phases: solid–liquid (
suspensions or solutions of macromolecules such as polymers), solid–gas (
granular
Granularity (also called graininess) is the degree to which a material or system is composed of distinction (philosophy), distinguishable pieces, granular material, "granules" or grain, "grains" (metaphorically).
It can either refer to the exten ...
), liquid–gas (
foam
Foams are two-phase materials science, material systems where a gas is dispersed in a second, non-gaseous material, specifically, in which gas cells are enclosed by a distinct liquid or solid material. Note, this source focuses only on liquid ...
s) or liquid–liquid (
emulsion
An emulsion is a mixture of two or more liquids that are normally Miscibility, immiscible (unmixable or unblendable) owing to liquid-liquid phase separation. Emulsions are part of a more general class of two-phase systems of matter called colloi ...
s). They exhibit unusual mechanical responses to applied
stress or
strain due to the geometrical constraints that the phase coexistence imposes. The mechanical response includes transitions between solid-like and fluid-like behavior as well as fluctuations. Their mechanical properties can be attributed to characteristics such as high disorder, caging, and clustering on multiple length scales.
Example
Shaving cream is an example of a complex fluid. Without stress, the foam appears to be a solid: it does not flow and can support (very) light
loads. However, when adequate stress is applied, shaving cream flows easily like a fluid. On the level of individual bubbles, the flow is due to rearrangements of small collections of bubbles. On this scale, the flow is not smooth, but instead consists of fluctuations due to rearrangements of the bubbles and releases of stress. These fluctuations are similar to the fluctuations that are studied in
earthquake
An earthquakealso called a quake, tremor, or tembloris the shaking of the Earth's surface resulting from a sudden release of energy in the lithosphere that creates seismic waves. Earthquakes can range in intensity, from those so weak they ...
s.
Dynamics
The
dynamics of the particles in complex fluids are an area of current research. Energy lost due to friction may be a
nonlinear function of the velocity and normal forces. The topological inhibition to flow by the crowding of constituent particles is a key element in these systems. Under certain conditions, including high
densities and low
temperature
Temperature is a physical quantity that quantitatively expresses the attribute of hotness or coldness. Temperature is measurement, measured with a thermometer. It reflects the average kinetic energy of the vibrating and colliding atoms making ...
s, when externally driven to induce flow, complex fluids are characterized by irregular intervals of solid-like behavior followed by stress relaxations due to particle rearrangements. The dynamics of these systems are highly nonlinear in nature. The increase in stress by an
infinitesimal
In mathematics, an infinitesimal number is a non-zero quantity that is closer to 0 than any non-zero real number is. The word ''infinitesimal'' comes from a 17th-century Modern Latin coinage ''infinitesimus'', which originally referred to the " ...
amount or a small displacement of a single particle can result in the difference between an arrested state and fluid-like behavior.
Although many materials found in nature can fit into the class of complex fluids, very little is well understood about them; inconsistent and controversial conclusions concerning their material properties still persist. The careful study of these systems may lead to "new physics" and new states of matter. For example, it has been suggested that these systems can jam and a "jamming
phase diagram" can be used to consider how these systems can jam and unjam. It is not known whether further research will demonstrate these findings, or whether such a theoretical framework will prove useful. As yet this large body of theoretical work has been poorly supported with experiments.
References
External links
{{External links, date=March 2023
Stephan Herminghaus' Dynamics of Complex Fluids DepartmentDavid Weitz's Soft Condensed Matter Physics LaboratoryHoward Stone's Complex Fluids GroupBob Behringer's complex fluids pageHernán Alejandro Makse's complex fluids pageComplex Fluids/Nonlinear Dynamics LaboratoryFrancois Graner's complex fluids pageCarnegie Mellon University Center for Complex Fluids Engineering
Paulo Arratia's Complex Fluids Laboratory at PennComplex Fluids & Computational Polymer Physics at ETH ZurichUbaldo M. Córdova-Figueroa's Low Reynolds Fluid Mechanics Group at UPRMZhengdong Cheng's Soft Condensed Matter GroupNew England Complex Fluids (NECF) Workgroup
Fluid dynamics
Non-Newtonian fluids