Complex Fluids
Shampoos, paints, lubricants, and other commonly used complex fluids and soft materials have remarkable properties. Shampoo in a pump dispenser must be thick enough to stay in your hand, yet be thin enough to pump easily from the dispenser and spread into your hair. How the molecular structure of a complex fluid changes during shear thinning can be seen using neutrons. When the material is thick, the molecules are tangled, but when a one-directional force is applied (e.g., by a paint brush), the molecules become ordered under flow, making the material thin.
Using highly intense neutron beams to understand materials at the molecular level could speed the development of time-released, drug-delivery systems that target specific parts of the body.
Neutron scattering pattern shows how a soap solution thins by shear at a surface. Disentangled molecular structures (top) become aligned enough to flow freely (shown in peaks).
Another important property of complex fluids is viscoelasticity, a tendency for the material to recoil like a spring when displaced. Because of viscoelastic additives studied using neutrons, modern oils stick to moving metal parts, whereas older oils, when heated during start-up, would spread out and separate from the parts they were supposed to lubricate.
Complex fluids, such as blood and soft materials (such as the permeable walls of body cells and other membranes), are essential to the processes of life. Because these materials are composed of hydrogen and other light atoms, neutron scattering is useful for studying small samples of these materials. Using highly intense neutron beams to understand these materials at the molecular level could speed the development of time-released, drug-delivery systems that target specific parts of the body. Such drugs might be contained in vesicles (biological membrane sacs) and multilayered vesicles (sacs within sacs, like nested Russian dolls) whose walls are broken down by the body so that the drug is released in precise doses when and where it is needed. Neutron studies could expedite development of artificial blood-vesicles that mimic the action of human blood cells, thus avoiding the tough problems of screening blood for life-threatening viruses, finding the right type of blood in an emergency, and convincing people to donate blood.
Paint is another example of a complex fluid whose change in molecular structure during shear thinning can be studied with neutrons. To work well, paint must be both thick enough to stick to your brush and thin enough to spread smoothly over a wall.
Knowledge from neutron scattering studies is leading to advances in the development of microemulsions and vesicles for use in food processing and cosmetics, control of microscopic fluid flow to improve lubricants, development of additives to encourage or prevent mixing for controlling some industrial processes, and use of traces of self-assembling molecules to dramatically alter the viscosity of a mixture.
Neutron research on complex fluids is helping scientists:
- Understand how the structure of elastic materials relates to elasticity.
- Develop improved separation technologies through guided construction of membranes.
- Unraveling individual contributions of molecular structures to such complex behavior.
- Follow changes in the structure of a complex fluid when it is forced to flow or is placed under stresses.
- Better understand the relationship of fluid structure to fluid transport properties.
