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We use colloidal model systems to investigate glasses. Glasses in a physical sense are formed when the cooling rate of a liquid upon phase transition to a solid does not allow crystallization. The formed system shows a microscopic structure like a liquid, but exhibits at a same time properties of a solid like elasticity. One key characteristic of glasses is that they are not in equilibrium and exhibit relaxations lowering the overall energy of the system. This process is known as aging.
Understanding the physics of glass has been referred to as one of the major remaining scientific challenges and much research has recently been done in this field. Our research focuses on characterising the degree of disorder in a glass and the associated aging process. Recent theories have suggested the concept of an effective temperature in glasses that has all properties of a thermodynamic temperature. According to the theories a glass in contact with a bath exhibits two temperatures: one temperature equals the temperature of the bath, the other temperature (effective temperature) is larger than the bath temperature. It is proposed that a glass immediately after it is formed (quenched to bath temperature) possesses an effective temperature larger than the bath temperature. Upon aging the effective temperature cools down and approaches the bath temperature.
We have established a method in order to experimentally test the concept of an effective temperature. So far only few experiments have been done and the results remain inconclusive. Our method uses an optical tweezer to measure the thermal fluctuations of a trapped probe particle. The probe particle is embedded in a colloidal glass. Our measurements of the disk-like colloidal glass laponite indicate an elevated temperature of about two to three times the bath temperature.