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Surface Tension, Capillary Action, and Viscosity02:57

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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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Coprecipitation is the contamination of a precipitate by otherwise soluble species and occurs via different processes. In colloidal precipitates, coprecipitation occurs via surface adsorption. For instance, barium sulfate has a primary layer of adsorbed barium ions and a secondary layer of nitrate counterions. This results in contamination of the precipitate by barium nitrate.
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Surface tension is defined as the force per unit length (γ) acting along the surface of a liquid. It arises due to strong intermolecular forces of attraction. A molecule located inside the bulk of the liquid is surrounded by other molecules and experiences equal forces in all directions. However, a molecule at the surface experiences unbalanced forces because there are more neighboring molecules below than above. This creates a net inward force that pulls surface molecules toward the interior,...

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Taking Advantage of Reduced Droplet-surface Interaction to Optimize Transport of Bioanalytes in Digital Microfluidics
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Contact line deposits in an evaporating drop

Deegan1, Bakajin, Dupont

  • 1James Franck Institute, 5640 South Ellis Avenue, Chicago, Illinois 60637, USA.

Physical Review. E, Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics
|November 23, 2000
PubMed
Summary

Solids in drying drops form rings at the edge due to outward flow. This study explains the physics behind this common phenomenon and predicts the resulting patterns.

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Area of Science:

  • Physics of fluid dynamics
  • Materials science
  • Surface chemistry

Background:

  • Solids dispersed in a drying drop often migrate to the edge, forming a characteristic ring-like stain.
  • This phenomenon, known as the "coffee ring effect," is observed across diverse solvents, solutes, and surfaces.
  • Understanding the underlying mechanisms is crucial for controlling deposition patterns in various applications.

Purpose of the Study:

  • To elucidate the physical mechanisms driving solute migration and ring formation in drying droplets.
  • To develop a theoretical model predicting the flow dynamics, ring growth rate, and solute distribution.
  • To validate theoretical predictions against experimental observations.

Main Methods:

  • Experimental observation of droplet drying on various surfaces.
  • Theoretical modeling of fluid flow driven by evaporation and droplet geometry.
  • Quantitative analysis of ring formation and solute distribution.

Main Results:

  • The outward flow within the drying drop, driven by solvent evaporation and maintaining droplet shape, causes solute migration.
  • A theoretical framework was established to predict flow velocity, ring growth rate, and solute distribution.
  • Experimental results demonstrated good agreement with the theoretical predictions.

Conclusions:

  • The coffee ring effect is primarily caused by an outward capillary flow within the drying droplet.
  • The developed theory accurately predicts the dynamics of ring formation and solute deposition.
  • This research provides fundamental insights into droplet evaporation and pattern formation.