Weird Quantum Force Finds Classical Equivalent

The Casimir effect, as it is known, represents the attraction force exerted between parallel conducting surfaces placed into vacuum. On macroscopic scale, it is virtually undetectable, but on distances as small as 10 nanometers, about 100 times the size of an average atom, the Casimir effect can produce forces as powerful as the equivalent of the pressure of one Earth atmosphere. Its strength falls rapidly with the increase in distance and the force is triggered by electromagnetic quantum fluctuations in the fabric of space-time.

However, the equivalent of the Casimir effect, the so-called 'critical Casimir effect', working on the same basic principle, usually takes place into liquid mediums characterized by a critical point of temperature and pressure, when the distinction between the gas and liquid phase is impossible. The critical point of a substance is experienced when the basic constituents gradually start to separate each other and cause fluctuations similar to the electromagnetic quantum fluctuations in the Casimir effect, due to the different sizes and shapes of the constituent particles.

As a result, two close parallel surfaces submerged in such a substance would experience an attraction force towards each other. A recent new experiment made in Germany, at the University of Stuttgart, demonstrated the effect, first predicted by Michael Fisher and Pierre-Giller de Gennes in 1978. Furthermore, the experiment led by Clemens Bechinger showed that not only does the force draw the surfaces towards each other, but they also can be made to repel each other.

The theory is that two close conducting surfaces can only allow a limited number of field frequencies between each other, the so-called 'boundary conditions'. Only wave fluctuations having the wavelength a multiple of that between the surfaces are considered resonating frequencies, while the non-resonating ones are being suppressed, meaning that the field inside the two conductors is not enough to create a pressure that would equal the one outside the system, so the two are pushed towards each other.

The experiment conducted at the University of Stuttgart involved the usage of a 3 micrometer polystyrene sphere inside a glass filled with a mixture of water and oil, brought to a critical temperature of 34 degrees Celsius. Shining the system with laser light, the team was able to accurately determine the distance between the polystyrene ball and the glass wall.

Thermal agitation constantly bombarded the small sphere with liquid molecules, thus the critical Casimir effect could only be detected through statistical calculation, which revealed a force of about 600 femto-Newtons.

The attraction force between the two can only be experienced when both are coated with either water or oil, and can be turned into repulsion force by switching one of the preferred coatings. Although it may not seem like a very interesting effect, the truth is that it may have a series of applications in the MEMS field, reducing the interactions of the quantum Casimir effect between nanoelectromechanical systems. However, the dependency of the effect on critical temperatures represents a disadvantage.