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Related Concept Videos

Viscosity01:17

Viscosity

5.7K
When water is poured into a glass, it falls freely and quickly, whereas if honey or maple syrup is poured over a pancake, it flows slowly and sticks to the surface of the container. This difference in the flow of different kinds of liquids arises due to the fluid friction between the liquid layers and the liquid and the surrounding material. This property of fluids is called fluid viscosity. In this example, water has a lower viscosity than honey and maple syrup.
The SI unit of viscosity is...
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Types of Fluids01:27

Types of Fluids

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Fluids can be classified into Newtonian and non-Newtonian fluids based on their response to shear stress. Newtonian fluids have a linear relationship between shear stress and the shear strain rate, following Newton's law of viscosity. Their viscosity remains constant regardless of the shear rate, making their behavior predictable and easier to analyze. Common examples include water, air, oil, and gasoline.
In contrast, non-Newtonian fluids do not follow Newton's law of viscosity, and...
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Surface Tension, Capillary Action, and Viscosity02:57

Surface Tension, Capillary Action, and Viscosity

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Surface Tension
The various IMFs between identical molecules of a substance are examples of cohesive forces. The molecules within a liquid are surrounded by other molecules and are attracted equally in all directions by the cohesive forces within the liquid. However, the molecules on the surface of a liquid are attracted only by about one-half as many molecules. Because of the unbalanced molecular attractions on the surface molecules, liquids contract to form a shape that minimizes the number...
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Thin-Walled Hollow Shafts01:15

Thin-Walled Hollow Shafts

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In analyzing a thin-walled hollow shaft subjected to torsional loading, a segment with width dx is isolated for examination. Despite its equilibrium state, this segment faces torsional shearing forces at its ends. These forces are quantitatively described by the product of the longitudinal shearing stress on the segment's minor surface and the area of this surface, leading to the concept of shear flow. This shear flow is consistent throughout the structure, indicating a uniform distribution...
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Frictional Force01:07

Frictional Force

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When a body is in motion, it encounters resistance because the body interacts with its surroundings. This resistance is known as friction, a common yet complex force whose behavior is still not completely understood. Friction opposes relative motion between systems in contact, but also allows us to move. Friction arises in part due to the roughness of surfaces in contact. For one object to move along a surface, it must rise to where the peaks of the surface can skip along the bottom of the...
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Viscosity of Fluid01:19

Viscosity of Fluid

225
Viscosity measures the resistance a fluid offers to flow and deformation. It results from internal friction between layers of fluid moving relative to one another. Dynamic viscosity, denoted by the Greek letter mu (μ), quantifies the force needed to move one fluid layer over another. For Newtonian fluids like water and air, the relationship between the shearing stress and the rate of shearing strain is linear, meaning their viscosity remains constant regardless of the applied stress.
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Related Experiment Video

Updated: May 14, 2025

Experimental Measurement of Settling Velocity of Spherical Particles in Unconfined and Confined Surfactant-based Shear Thinning Viscoelastic Fluids
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Experimental Measurement of Settling Velocity of Spherical Particles in Unconfined and Confined Surfactant-based Shear Thinning Viscoelastic Fluids

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Active Saffman-Taylor Viscous Fingering.

Akash Ganesh1, Carine Douarche1, Harold Auradou1

  • 1FAST, Université Paris-Saclay, CNRS, 91405 Orsay, France.

Physical Review Letters
|April 11, 2025
PubMed
Summary

Swimming bacteria in a liquid can reduce its viscosity to zero, creating complex, fingerlike patterns during fluid displacement. This instability occurs under specific conditions of bacterial concentration and shear rate.

Area of Science:

  • Fluid dynamics
  • Microbiology
  • Rheology

Background:

  • Swimming microorganisms can alter the bulk properties of fluids.
  • The reduction of effective shear viscosity to zero in bacterial suspensions is a known phenomenon.
  • Classical Saffman-Taylor instability describes viscous fingering in simple fluid systems.

Purpose of the Study:

  • To investigate if the zero-viscosity property of bacterial suspensions leads to viscous fingering.
  • To characterize the dynamic behavior of fluid displacement fronts in such systems.
  • To determine the conditions under which instability occurs.

Main Methods:

  • Experimental observation of fluid displacement fronts.
  • Systematically varying bacterial volume fraction and imposed shear rate.

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Fabricating High-viscosity Droplets using Microfluidic Capillary Device with Phase-inversion Co-flow Structure
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  • Analysis of the resulting dynamic characteristics and instability formation.

    • Main Results:

    • The system exhibits complex dynamic characteristics beyond classical Saffman-Taylor instability.
    • Viscous fingerlike displacement fronts are observed.
    • Instability occurs when bacterial volume fraction exceeds a critical value and shear rate is below a critical value, coinciding with zero viscosity.

    Conclusions:

    • Bacterial suspensions exhibiting zero effective viscosity can lead to complex fluid displacement patterns.
    • The observed instability is distinct from the classical Saffman-Taylor instability.
    • Critical bacterial concentration and shear rate are key parameters for instability onset.