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Related Concept Videos

Diffusion01:21

Diffusion

7.2K
Diffusion is a type of passive transport. In passive transport, a substance tends to move from an area of high concentration to an area of low concentration until the concentration is equal across the space. For example, take the diffusion of substances through the air. When someone opens a perfume bottle in a room filled with people, the perfume is at its highest concentration in the bottle and is at its lowest at the edges of the room. The perfume vapor will diffuse, or spread away, from the...
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Diffusion01:12

Diffusion

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Diffusion is the passive movement of substances down their concentration gradients—requiring no expenditure of cellular energy. Substances, such as molecules or ions, diffuse from an area of high concentration to an area of low concentration in the cytosol or across membranes. Eventually, the concentration will even out, with the substance moving randomly but causing no net change in concentration. Such a state is called dynamic equilibrium, which is essential for maintaining overall...
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Behavior of Gas Molecules: Molecular Diffusion, Mean Free Path, and Effusion03:48

Behavior of Gas Molecules: Molecular Diffusion, Mean Free Path, and Effusion

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Although gaseous molecules travel at tremendous speeds (hundreds of meters per second), they collide with other gaseous molecules and travel in many different directions before reaching the desired target. At room temperature, a gaseous molecule will experience billions of collisions per second. The mean free path is the average distance a molecule travels between collisions. The mean free path increases with decreasing pressure; in general, the mean free path for a gaseous molecule will be...
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Passive Diffusion: Overview and Kinetics01:17

Passive Diffusion: Overview and Kinetics

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Passive diffusion is a critical process that allows small lipophilic drugs to cross the cell membrane along a concentration gradient. This mechanism's efficiency depends on four primary factors: the membrane's surface area, the drug's lipid-water partition coefficient, the concentration gradient, and the membrane's thickness.
When administered orally, drugs establish a substantial concentration gradient between the gastrointestinal (GI) lumen and the bloodstream, expediting...
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Protein Diffusion in the Membrane01:24

Protein Diffusion in the Membrane

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Proteins show rotational as well as lateral diffusion across the membrane. The lateral diffusion of proteins was confirmed through the cell fusion experiment where mouse and human cells were fused, resulting in hybrid cells. When the human and mouse cells fused, the specific membrane proteins on human and mouse cells were marked with the red and green-fluorescent markers, respectively. Initially, the red and green fluorescence was located on the respective hemisphere of the cell. As time...
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Capillarity in Fluid01:19

Capillarity in Fluid

1.5K
Capillarity describes the movement of liquid in small spaces without external forces acting on it. The capillarity is driven by surface tension and adhesive interactions between the liquid and surrounding solid surfaces. This effect is often seen in narrow tubes, porous materials, and fine particles.
Surface tension is crucial to capillarity. It results from cohesive forces between liquid molecules at the liquid-air boundary, forming a skin that resists external forces. When the capillary tube...
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Related Experiment Video

Updated: Mar 26, 2026

The Diffusion of Passive Tracers in Laminar Shear Flow
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The Diffusion of Passive Tracers in Laminar Shear Flow

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Particle diffusion in active fluids is non-monotonic in size.

Alison E Patteson1, Arvind Gopinath2, Prashant K Purohit1

  • 1Department of Mechanical Engineering & Applied Mechanics, University of Pennsylvania, Philadelphia, PA 19104, USA. parratia@seas.upenn.edu.

Soft Matter
|January 23, 2016
PubMed
Summary

Larger particles can diffuse faster than smaller ones in bacterial suspensions, a surprising finding that deviates from classical diffusion. This study reveals a peak in particle diffusivity related to size, impacting active fluid characterization.

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Related Experiment Videos

Last Updated: Mar 26, 2026

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Area of Science:

  • Soft Matter Physics
  • Active Matter Physics
  • Biophysics

Background:

  • Classical thermal diffusion describes particle motion driven by random thermal energy.
  • Active fluids, like bacterial suspensions, exhibit complex dynamics due to self-propelling microorganisms.
  • Understanding particle motion in active fluids is crucial for applications in microfluidics and biomaterials.

Purpose of the Study:

  • To experimentally investigate the influence of particle size on the dynamics of passive spheres in Escherichia coli suspensions.
  • To explore particle motion across various time scales, from short ballistic to long-term diffusive behavior.
  • To identify deviations from classical diffusion and understand the underlying mechanisms in active fluids.

Main Methods:

  • Experimental investigation of passive polystyrene spheres (0.6-39 microns) in Escherichia coli suspensions.
  • Analysis of particle dynamics at both short and long time scales.
  • Characterization of active contribution to diffusion using the Péclet number.

Main Results:

  • Particles exhibit super-diffusive ballistic behavior at short times, transitioning to diffusive behavior at longer times.
  • A peak in effective diffusivity was observed, where larger particles diffused faster than smaller ones, deviating from classical diffusion.
  • The Péclet number was identified as a key parameter controlling the active contribution to particle diffusion.

Conclusions:

  • The study reveals a non-classical diffusion regime in active fluids, with a size-dependent diffusivity peak.
  • A minimal model qualitatively explains the observed diffusivity peak and its dependence on bacterial concentration.
  • Findings have implications for characterizing active fluids and applying classical thermodynamics concepts to biological systems.