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Related Concept Videos

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Cohesion

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Cohesion is the attraction between molecules of the same type, such as water molecules. Water molecules have an overall neutral charge but are polar molecule. An oxygen atom in one water molecule has a partial negative charge that can bind to a hydrogen atom with a partial positive charge in a second water molecule, forming a hydrogen bond. Each water molecule can form up to four hydrogen bonds with other water molecules. Hydrogen bonds are responsible for water's cohesive nature.
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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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Surface Tension of Fluid01:22

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Surface tension is a fundamental property of fluids, occurring at the boundary between a liquid and a gas or between two immiscible liquids. This phenomenon arises from the cohesive forces between molecules at the fluid's surface, creating an effect similar to a stretched elastic membrane. Inside each fluid, molecules are equally attracted in all directions by neighboring molecules, but surface molecules experience a net inward force, resulting in surface tension.
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Buoyancy00:59

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When an object is placed in a fluid, it either floats or sinks. All objects in a fluid experience a buoyant force. For example, a metal ball sinks, while a rubber ball floats. Similarly, a submarine can sink and float by adjusting its buoyancy.  The concept of buoyancy raises several interesting questions. For instance, where does this buoyant force come from? How much buoyant force is required to make an object sink or float? Do objects that sink get any support at all from the...
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The shape of a small drop of liquid can be considered spherical, neglecting the effect of gravity. This drop can further be considered as two equal hemispherical drops put together due to surface tension. The forces acting on the spherical drop are due to the pressure of the liquid inside the drop, the pressure due to air outside the drop, and the force due to the surface tension acting on the two hemispherical drops.
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Buoyancy-driven attraction of active droplets.

Yibo Chen1, Kai Leong Chong2, Haoran Liu1

  • 1Physics of Fluids Group, Max Planck Center for Complex Fluid Dynamics and J.M.Burgers Center for Fluid Dynamics, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands.

Journal of Fluid Mechanics
|February 16, 2024
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This study reveals that buoyancy effects, often overlooked, drive active oil droplet attraction and clustering, contrasting with the typical Marangoni repulsion. Increased solutal Rayleigh number (Ra) promotes attraction and collisions, while Galileo number (Ga) delays them.

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Area of Science:

  • Soft Matter Physics
  • Fluid Dynamics
  • Chemical Engineering

Background:

  • Active oil droplets in ambient liquids typically exhibit repulsive interactions due to the Marangoni effect.
  • Buoyancy effects arising from density differences are often neglected in studies of active droplet interactions.
  • Recent experiments suggest buoyancy-driven convection can lead to active droplet clustering.

Purpose of the Study:

  • To numerically investigate the role of buoyancy effects on the interaction and behavior of active oil droplets.
  • To analyze how buoyancy, in addition to Marangoni flow, influences droplet attraction and repulsion.
  • To determine the dependence of droplet interactions on key control parameters: Péclet number (Pe), Galileo number (Ga), and solutal Rayleigh number (Ra).

Main Methods:

  • Numerical simulations were employed to model active oil droplet dynamics.
  • The study incorporated both Marangoni-driven propulsion (Pe) and buoyancy effects (Ga, Ra).
  • Analysis focused on the attractive and repulsive behaviors of neighboring droplets under varying parameter conditions.

Main Results:

  • Marangoni effect causes droplet repulsion, while buoyancy of the reaction product induces attraction.
  • Sufficiently high solutal Rayleigh number (Ra) can lead to droplet collisions.
  • Increasing Galileo number (Ga) delays droplet collisions.
  • Attractive velocity (Red) is proportional to Ra^(1/4)/(ℓ/R), and repulsive velocity (Rerep) is proportional to Pe*Ra^(-0.38).

Conclusions:

  • Buoyancy effects, particularly from the diffusing product, are crucial in governing active droplet interactions and can overcome Marangoni repulsion.
  • The interplay between attractive (buoyancy) and repulsive (Marangoni) forces dictates droplet aggregation and collision dynamics.
  • A balance condition (Pe ~ Ra^0.63) accurately predicts the transition between non-colliding and colliding regimes.