Soft matter
Soft matter or soft condensed matter is a type of matter that can be deformed or structurally altered by thermal or mechanical stress which is of similar magnitude to thermal fluctuations.
The science of soft matter is a subfield of condensed matter physics. Soft materials include liquids, colloids, polymers, foams, gels, granular materials, liquid crystals, flesh, and a number of biomaterials. These materials share an important common feature in that predominant physical behaviors occur at an energy scale comparable with room temperature thermal energy, and that entropy is considered the dominant factor. At these temperatures, quantum aspects are generally unimportant. When soft materials interact favorably with surfaces, they become squashed without an external compressive force.
Pierre-Gilles de Gennes, who has been called the "founding father of soft matter," received the Nobel Prize in Physics in 1991 for discovering that methods developed for studying order phenomena in simple systems can be generalized to the more complex cases found in soft matter, in particular, to the behaviors of liquid crystals and polymers.
History
The current understanding of soft matter grew from Albert Einstein's work on Brownian motion, understanding that a particle suspended in a fluid must have a similar thermal energy to the fluid itself. This work built on established research into systems that would now be considered colloids.The crystalline optical properties of liquid crystals and their ability to flow were first described by Friedrich Reinitzer in 1888, and further characterized by Otto Lehmann in 1889. The experimental setup that Lehmann used to investigate the two melting points of cholesteryl benzoate are still used in the research of liquid crystals as of about 2019.
In 1920, Hermann Staudinger, recipient of the 1953 Nobel Prize in Chemistry, was the first person to suggest that polymers are formed through covalent bonds that link smaller molecules together. The idea of a macromolecule was unheard of at the time, with the scientific consensus being that the recorded high molecular weights of compounds like natural rubber were instead due to particle aggregation.
The use of hydrogel in the biomedical field was pioneered in 1960 by Drahoslav Lím and Otto Wichterle. Together, they postulated that the chemical stability, ease of deformation, and permeability of certain polymer networks in aqueous environments would have a significant impact on medicine, and were the inventors of the soft contact lens.
These seemingly separate fields were dramatically influenced and brought together by Pierre-Gilles de Gennes. The work of de Gennes across different forms of soft matter was key to understanding its universality, where material properties are not based on the chemistry of the underlying structure, more so on the mesoscopic structures the underlying chemistry creates. He extended the understanding of phase changes in liquid crystals, introduced the idea of reptation regarding the relaxation of polymer systems, and successfully mapped polymer behavior to that of the Ising model.
Distinctive physics
Interesting behaviors arise from soft matter in ways that cannot be predicted, or are difficult to predict, directly from its atomic or molecular constituents. Materials termed soft matter exhibit this property due to a shared propensity of these materials to self-organize into mesoscopic physical structures. The assembly of the mesoscale structures that form the macroscale material is governed by low energies, and these low energy associations allow for the thermal and mechanical deformation of the material. By way of contrast, in hard condensed matter physics it is often possible to predict the overall behavior of a material because the molecules are organized into a crystalline lattice with no changes in the pattern at any mesoscopic scale. Unlike hard materials, where only small distortions occur from thermal or mechanical agitation, soft matter can undergo local rearrangements of the microscopic building blocks.A defining characteristic of soft matter is the mesoscopic scale of physical structures. The structures are much larger than the microscopic scale, and yet are much smaller than the macroscopic scale of the material. The properties and interactions of these mesoscopic structures may determine the macroscopic behavior of the material. The large number of constituents forming these mesoscopic structures, and the large degrees of freedom this causes, results in a general disorder between the large-scale structures. This disorder leads to the loss of long-range order that is characteristic of hard matter.
For example, the turbulent vortices that naturally occur within a flowing liquid are much smaller than the overall quantity of liquid and yet much larger than its individual molecules, and the emergence of these vortices controls the overall flowing behavior of the material. Also, the bubbles that compose a foam are mesoscopic because they individually consist of a vast number of molecules, and yet the foam itself consists of a great number of these bubbles, and the overall mechanical stiffness of the foam emerges from the combined interactions of the bubbles.
Typical bond energies in soft matter structures are of similar scale to thermal energies. Therefore the structures are constantly affected by thermal fluctuations and undergo Brownian motion. The ease of deformation and influence of low energy interactions regularly result in slow dynamics of the mesoscopic structures which allows some systems to remain out of equilibrium in metastable states. This characteristic can allow for recovery of initial state through an external stimulus, which is often exploited in research.
Self-assembly is an inherent characteristic of soft matter systems. The characteristic complex behavior and hierarchical structures arise spontaneously as a system evolves towards equilibrium. Self-assembly can be classified as static when the resulting structure is due to a free energy minimum, or dynamic when the system is caught in a metastable state. Dynamic self-assembly can be utilized in the functional design of soft materials with these metastable states through kinetic trapping.
Soft materials often exhibit both elasticity and viscous responses to external stimuli such as shear induced flow or phase transitions. However, excessive external stimuli often result in nonlinear responses. Soft matter becomes highly deformed before crack propagation, which differs significantly from the general fracture mechanics formulation. Rheology, the study of deformation under stress, is often used to investigate the bulk properties of soft matter.