Context:

Researchers at the Raman Research Institute (RRI), an autonomous institute under the Department of Science and Technology (DST), have developed a novel experimental method to study how complex fluids—particularly wormlike micellar fluids (WMFs) formed by surfactant molecules—behave at microscopic levels.

About Wormlike micellar fluids (WMFs):

Wormlike micellar fluids (WMFs) are a class of complex fluids formed by the self-assembly of surfactant molecules into long, flexible, cylindrical aggregates.

• • These micelles entangle to form a transient network, granting the fluid unique viscoelastic (having both viscous and elastic properties) and shear-thinning characteristics that resemble polymer solutions.
  • These fluids are widely used in oil recovery, cosmetics, shampoos, gels, and polymer-based products, making their detailed study valuable for industry.
  • Unlike simple Newtonian fluids such as water, where falling objects eventually reach a stable terminal velocity, non-Newtonian fluids often do not allow terminal velocity.
  • Instead, objects show chaotic, fluctuating motion because local fluid structures continuously form and break around them.

Key Observations by scientists:

RRI scientists built a special setup inside a rheometer where a needle-like probe moved through the fluid between two cylinders. This allowed them to measure the force on the probe and see fluid structures in real time.

·        At low speeds, the force stayed steady like in normal fluids. But at higher speeds, the force rose slowly and then dropped suddenly, creating a “sawtooth” pattern.

·        Images showed this happened because a tail-like structure formed behind the probe and then snapped off, like an elastic band.

About Terminal velocity:

Terminal velocity is the maximum constant speed an object reaches when falling through a fluid (like air or water). It is achieved when the downward force of gravity is exactly balanced by the upward forces of fluid resistance (drag) and buoyancy, resulting in a net force of zero and no further acceleration. 

Conclusion:

This research provides a powerful tool to study the mechanics of complex materials across different length scales. By understanding how small probes move through structured fluids, scientists can better design and optimize oilfield surfactants, cosmetic formulations, gels, and polymer solutions. The flexibility of the experimental setup also opens avenues for studying a wide range of materials with probes of varying sizes.