Focus: Small, Attractive Bubbles
Salad dressing, whipped cream, and mayonnaise all depend on microscopic particles of oil or fat remaining mixed in water–each would be useless if the particles clumped up and settled out of solution. Understanding the forces between such “colloid” particles is also critical for controlling their properties in many other products, and in the 15 June PRL an Australian team claims to show the best evidence yet to explain a long-standing controversy about one of these forces. They say the mysterious long-range attraction between colloid particles comes from small bubbles that can attach to the particles and pull them together; but not all experts are convinced.
In the classic view of colloid particles, there are two main forces between them: Van der Waals attraction acts within a few nanometers, while the similar electric charges of the particles creates a repulsion that can extend to 100 nm or so. Based on this simple view, the particles remain dispersed as long as the repulsion is strong enough compared with the thermal motions of the particles. In the past three decades, several other types of forces have been described which complicate this picture, but none has been as puzzling and controversial as the long-range (roughly 100 nm) attraction measured between the hydrophobic surfaces of oil-like particles in water.
According to Phil Attard, of the Ian Wark Research Institute, in The Levels, Australia, there are three main competing theories that try to explain the force, but two of them depend on some type of cooperation of water molecules over distances of 1000 molecular diameters–a type of long-range order not normally seen in liquids. Even though Attard helped to develop these theories, he prefers a third one: that a single, submicrometer bubble can attach to both surfaces and pull the particles together. With the bubble theory, the range of the force comes from the bubble size and is variable, consistent with the experiments on the force that often don’t agree on its range, or even its existence. On the other hand, some basic thermodynamics calculations suggest that such small bubbles are unstable.
In their latest work, Attard and his colleagues used an atomic force microscope in water to measure the delicate forces between a 20-µm-diameter hydrophobized glass bead and a flat hydrophobic surface at separations of 0 to 250 nm. As they eased the particle toward the surface, they found a steep repulsion just before the attraction, a sign that the particle was nearing a surface before being pulled in by the mystery force. Also supporting the bubble hypothesis, the team observed a sudden onset of the force, rather than a force gradually increasing with the approach of the surfaces, the way most conventional forces behave. Using an optical microscope, the team saw long-lasting submicrometer bubbles attached to the two surfaces away from the contact region, suggesting that fairly small bubbles are stable enough to last for the length of an experiment. Those bubbles, however, were still larger than the ones proposed to account for the force.
Jacob Israelachvili, of the University of California at Santa Barbara, points out some problems with the work. He says that when other researchers have attempted to remove the gas from their experiments, the results were not consistent–some claimed an effect, others did not. Besides, he says, the bubble theory only moves the mystery from the cause of the force to the source of the bubbles. Rudolf Podgornik, of the Jozef Stefan Institute in Ljubljana, Slovenia, finds the Australian group’s interpretation “reassuring,” but he also points to inconsistencies in previous data.