Simple method for the direct measurement of cohesive forces between microscopic particles

Area of Science:

• Colloid and Interface Science
• Soft Matter Physics
• Physical Chemistry

Background:

• Understanding cohesive interactions between particles is crucial in various scientific fields.
• Measuring weak forces, particularly at the pico-Newton scale, presents experimental challenges.
• Depletion interactions are a key mechanism driving self-assembly and structuring in colloidal systems.

Purpose of the Study:

• To present a simple, inexpensive, and accessible method for quantifying weak cohesive interactions.
• To apply this method to measure the depletion interaction between oil droplets in an aqueous solution.
• To demonstrate the method's versatility by varying droplet size and depletant concentration.

Main Methods:

• A custom chamber was designed to hold a string of oil droplets dispersed in an aqueous solution.
• A single droplet was immobilized, while nearest neighbors were held by depletion forces.
• The chamber's angle was gradually increased to apply a controlled gravitational/buoyant force until the droplet string detached.

Main Results:

• The method successfully measured cohesive forces on the tens of pico-Newton scale.
• Variations in droplet radii and depletant concentration directly impacted the measured cohesive forces.
• The detachment force was found to be directly proportional to the cohesive bond strength.

Conclusions:

• The developed technique provides a simple, direct, and reproducible way to measure weak cohesive forces.
• This method is applicable to various systems beyond oil-water droplets and depletion interactions.
• The study validates the technique's utility in exploring fundamental colloidal interactions.