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Efecto "cheerios" y la tensi贸n superficial de la leche

Escrito por agsens en 16-08-08

Why the Cheerios floating on the top of the milk have a tendency to clump together?

This behavior is caused by the surface tension of milk. The molecules at the liquid surface exhibit stronger cohesive bonding than the molecules below the surface. As a result, the surface behaves like a thin film. Since the Cheerios are floating, an upward meniscus forms at the liquid/Cheerio interface. Since the Cheerios are buoyant, they want to move to the highest point - the center of the bowl, to the edge, and to each other.

A contact angle forms where the milk touches the liquid as a result of strong surface energy and poor wetting. The resulting meniscus is upward due to the buoyancy of the Cheerio while the meniscus on of paper clip curves downward due to gravity and its greater density.

The Cheerios effect is useful to understand how self-assembly can work with nanoscopically small parts of uniform shape.

On a related note, while we were studying Cheerios, we became curious about the surface tension of milk. So we took some measurements using a ramé-hart Model 250 with DROPimage Advanced. We found that grade A 2% homogenized and pasteurized cows milk from the grocery store has a surface tension of 48 mN/m. Fatty acids and proteins are the surface-active agents which lower the surface tension to a value lower than that of water. We also found that the surface tension of milk is inversely proportional to the fat content. Thus, we discovered that a tensiometer can replace a lactometer for measuring the creaminess of milk.

Desde Newsletter Ramé-Hart Agosto, 2016

Why the Cheerios floating on the top of the milk have a tendency to clump together?

This behavior is caused by the surface tension of milk. The molecules at the liquid surface exhibit stronger cohesive bonding than the molecules below the surface. As a result, the surface behaves like a thin film. Since the Cheerios are floating, an upward meniscus forms at the liquid/Cheerio interface. Since the Cheerios are buoyant, they want to move to the highest point - the center of the bowl, to the edge, and to each other.

A contact angle forms where the milk touches the liquid as a result of strong surface energy and poor wetting. The resulting meniscus is upward due to the buoyancy of the Cheerio while the meniscus on of paper clip curves downward due to gravity and its greater density.

The Cheerios effect is useful to understand how self-assembly can work with nanoscopically small parts of uniform shape.

On a related note, while we were studying Cheerios, we became curious about the surface tension of milk. So we took some measurements using a ramé-hart Model 250 with DROPimage Advanced. We found that grade A 2% homogenized and pasteurized cows milk from the grocery store has a surface tension of 48 mN/m. Fatty acids and proteins are the surface-active agents which lower the surface tension to a value lower than that of water. We also found that the surface tension of milk is inversely proportional to the fat content. Thus, we discovered that a tensiometer can replace a lactometer for measuring the creaminess of milk.

 

Bullets

-The Cheerios effect explains how small buoyant objects are attracted to one another due to the contact angle formed by the meniscus at the solid/liquid interface.

-Small light floating objects (e.g., Cheerios) are attracted to one another due to an upward meniscus. Small dense objects (e.g., paper clips) are also attracted to one another due to a downward meniscus. Small light objects and dense objects are mutually repulsed due to opposing menisci.

-The Cheerios effect is evident with small objects (e.g., the size of Cheerios) but is not evident with larger objects (e.g., ships) as the force of surface tension is negligible at large scales.

-The Cheerios effect does not seem to apply to corn flakes due to their irregular shape and the fact that they get soggy quickly in milk. However, the Cheerios effect can be seen with Rice Krispies and is likely to work well with Fruit Loops and possibly Lucky Charms. More testing is required.

-Milk has a lower surface tension than water due to surfactants such as fatty acids and proteins.

-As the fat content in milk is decreased, the surface tension increases. Amazing observations can be made at the breakfast table.

 

 

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