Soft matter

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Soft matter or soft condensed matter is a subfield of condensed matter comprising a variety of physical systems that are deformed or structurally altered by thermal or mechanical stress of the magnitude of thermal fluctuations. These materials share an important common feature in that predominant physical behaviors occur at an energy scale comparable with room temperature thermal energy (of order of kT), and that entropy is considered the dominant factor. [1] At these temperatures, quantum aspects are generally unimportant. Soft materials include liquids, colloids, polymers, foams, gels, granular materials, liquid crystals, flesh, and a number of biomaterials. When soft materials interact favorably with surfaces, they become squashed without an external compressive force. [2] Pierre-Gilles de Gennes, who has been called the "founding father of soft matter," [3] 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. [4]

Contents

History

The current understanding of soft matter grew from the Albert Einstein's work on Brownian motion, [5] [6] understanding that a particle suspended in a fluid must have a similar thermal energy to the fluid itself (of order of kT ). This work built on established research into systems that would now be considered colloids. [7]

The crystalline optical properties of liquid crystals and their ability to flow were first described by Friedrich Reinitzer in 1888, [8] and further characterized by Otto Lehmann in 1889. [9] The experimental setup that Lehmann used to investigate the two melting points of cholesteryl benzoate are still used in the research of liquid crystals today. [10]

In 1920, Hermann Staudinger, recipient of the 1953 Nobel Prize in Chemistry, [11] was the first person to suggest that polymers are formed through covalent bonds that link smaller molecules together. [12] The idea of a macromolecule was unheard of at the time, with the scientific consensus that the recorded high molecular weights of compounds like natural rubber were instead due to particle aggregation. [13]

The use of hydrogel in the biomedical field was pioneered in 1960 by Drahoslav Lím and Otto Wichterle. [14] 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. [15]

These seemingly separate fields were dramatically influenced and brought together by Pierre-Gilles de Gennes. de Gennes' work across different forms of soft matter were key in 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. [16] de Gennes 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. [16] [17]

Distinctive physics

The self-assembly of individual phospholipids into colloids (Liposome and Micelle) or a membrane (bilayer sheet). Phospholipids aqueous solution structures.svg
The self-assembly of individual phospholipids into colloids (Liposome and Micelle) or a membrane (bilayer sheet).

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. [18] 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. [19]

A defining characteristic of soft matter is the mesoscopic scale of physical structures. The structures are much larger than the microscopic scale (the arrangement of atoms and molecules), and yet are much smaller than the macroscopic (overall) scale of the material. The properties and interactions of these mesoscopic structures may determine the macroscopic behavior of the material. [20] 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. [21] 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 control 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 as thermal energies, therefore, the structures are constantly affected by thermal fluctuations and undergo Brownian motion. [20] 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. [22] [23] This characteristic can allow for recovery of initial state through an external stimuli and is often exploited in research. [24] [25]

Self-assembly is an inherent characteristic of soft matter systems. The characteristic complex behavior and hierarchical structures arise spontaneously as the system evolves towards equilibrium. [20] Self-assembly can be classified as static, where the resulting structure is due to a free energy minimum, or dynamic, which occurs when the system is caught in a metastable state. [26] Dynamic self-assembly can be utilized in the functional design of soft materials with these metastable states through kinetic trapping. [18] [27]

Soft materials often exhibit both elasticity and viscous responses to external stimuli, [22] such as shear induced flow or phase transitions, however, excessive external stimuli often result in nonlinear responses. [1] [28] Soft matter becomes highly deformed before crack propagation, which differs significantly from the general fracture mechanics formulation. [19] Rheology, the study of deformation under stress, is often used to investigate the bulk properties of soft matter. [22]

Classes of soft matter

A portion of the DNA double helix, an example of a biopolymer. DNA animation.gif
A portion of the DNA double helix, an example of a biopolymer.
Host-guest complex of polyethylene glycol oligomer bound within an a-cyclodextrin molecule; a common scaffold used in the formation of gels. The atoms are colored such that red represents oxygen, cyan represents carbon, and white represents hydrogen. Inclusion complex.png
Host-guest complex of polyethylene glycol oligomer bound within an α-cyclodextrin molecule; a common scaffold used in the formation of gels. The atoms are colored such that red represents oxygen, cyan represents carbon, and white represents hydrogen.
Cartoon representation of the molecular order of crystal, liquid crystal, and liquid states. Liquid Crystal.png
Cartoon representation of the molecular order of crystal, liquid crystal, and liquid states.

Soft matter consists of a diverse range of interrelated systems and can be broadly categorized into certain classes. These classes are by no means distinct, as often there are overlaps between two or more groups.

Polymers

Polymers are large molecules composed of repeating subunits whose characteristics are governed by their environment and composition. Polymers encompass synthetic plastics, natural fibers and rubbers, and biological proteins. Polymer research finds applications in nanotechnology, [29] [30] and from materials science and drug delivery to protein crystallization. [24] [31]

Foams

Foams consist of a liquid or solid through which a gas has been dispersed to form cavities. This structure imparts a large surface-area-to-volume ratio on the system. [23] [32] Foams have found applications in insulation and textiles, [32] and are undergoing active research in the biomedical field of drug delivery and tissue engineering. [31] Foams are also used in automotive for water and dust sealing and noise reduction.

Gels

Gels consist of non-solvent-soluble 3D polymer scaffolds, which are covalently or physically cross-linked, that have a high-solvent content ratio. [33] [34] Research into functionalizing gels that are sensitive to mechanical and thermal stress, as well as solvent choice, have given rise to diverse structures with characteristics such as shape-memory, [35] or the ability to bind guest molecules selectively and reversibly. [34]

Colloids

Colloids are non-soluble particles suspended in a medium, such as proteins in an aqueous solution. [36] Research into colloids is primarily focused on understanding the organization of matter, with the large structures of colloids, relative to individual molecules, large enough that they can be readily observed. [37]

Liquid crystals

Liquid crystals can consist of proteins, small molecules, or polymers, that can be manipulated to form cohesive order in a specific direction. [38] They exhibit liquid like behavior in that they can flow, yet they can obtain close-to crystal alignment. One feature of liquid crystals is their ability to spontaneously break symmetry. [39] Liquid crystals have found significant applications in optical devices such as liquid-crystal displays (LCD).

Biological membranes

Biological membranes consist of individual phospholipid molecules that have self-assembled into a bilayer structure due to non-covalent interactions. The localized, low energy associated with the forming of the membrane allow for the elastic deformation of the large-scale structure. [40]

Experimental characterization

Due to the importance of mesoscale structures in the overarching properties of soft matter, experimental work is primarily focused on the bulk properties of the materials. Rheology is often used to investigate the physical changes of the material under stress. [22] Biological systems, such as protein crystallization, are often investigated through X-ray and neutron crystallography, [41] while nuclear magnetic resonance spectroscopy can be used in understanding the average structure and lipid mobility of membranes. [40]

Scattering

Scattering techniques, such as wide-angle X-ray scattering, small-angle X-ray scattering, neutron scattering, and dynamic light scattering can also be used for materials when probing for the average properties of the constituents. These methods can determine particle-size distribution, shape, crystallinity and diffusion of the constituents in the system. [42] [43] There are limitations in the application of scattering techniques to some systems, as they can be more suited to isotropic and dilute samples. [42]

Computational

Computational methods are often employed to understand model soft matter systems, as they have the ability to strictly control the composition and environment of the structures being investigated, as well as span from microscopic to macroscopic length scales. [21] Computational methods are limited, however, by their suitability to the system and must be regularly validated against experimental results to ensure accuracy. [21] The use of informatics in the prediction of soft matter properties is also a growing field in computer science thanks to the large amount of data available for soft matter systems. [44]

Microscopy

Optical microscopy can be used in the study of colloidal systems, however, more advanced methods like transmission electron microscopy (TEM) and atomic force microscopy (AFM) are often used to characterize forms of soft matter due to their applicability to map systems at the nanoscale. [45] [46] These imagining techniques are not universally appropriate to all classes of soft matter and some systems may be more suited to one analysis over the other. For example, there are limited applications in imagining hydrogels with TEM due to the processes required for imaging, however, fluorescence microscopy can be readily applied. [42] Liquid crystals are often probed using polarized light microscopy to determine the ordering of the material under various conditions, such as temperature or electric field. [47]

Applications

Soft materials are important in a wide range of technological applications, and each soft material can often be associated with multiple disciplines. Liquid crystals, for example, were originally discovered in the biological sciences when the botanist and chemist Friedrich Reinitzer was investigating cholesterols. [10] Now, however, liquid crystals have also found applications as liquid-crystal displays, liquid crystal tunable filters, and liquid crystal thermometers. Active liquid crystals are another example of soft materials, where the constituent elements in liquid crystals can self-propel. [48]

Polymers are ubiquitous with soft matter and have found diverse applications, from the natural rubber found in latex gloves to vulcanized rubber found in tires. Polymers encompass a large range of soft matter with applications in material science, an example of this is hydrogel. With the ability to undergo shear thinning, hydrogels are well suited for the development of 3D printing. [27] Due to their stimuli responsive behavior, 3D printing of hydrogels have found applications in a diverse range of fields, such as soft robotics, tissue engineering, and flexible electronics. [49] Polymers also encompass biological molecules such as proteins, where research insights from soft matter research have been applied to better understand topics like protein crystallization. [41]

Foams can naturally occur, such as the head on a beer, or be created with purpose, like fire extinguishers. The range of physical properties available to foams have resulted in applications which can be based on their viscosity. [23] With more rigid and self-supporting forms of foams being used as insulation or cushions, and foams that exhibit the ability to flow being used in the cosmetic industry as shampoos or makeup. [23] Foams have also found biomedical applications in tissue engineering as scaffolds and biosensors. [50]

Historically the problems considered in the early days of soft matter science were those pertaining to the biological sciences. As such, an important application of soft matter research is biophysics with a major goal of the discipline being the reduction of the field of cell biology to the concepts of soft matter physics. [20] Applications of soft matter characteristics are used to understand biologically relevant topics such as membrane mobility, [40] as well as the rheology of blood. [36]

See also

Related Research Articles

<span class="mw-page-title-main">Colloid</span> Mixture of an insoluble substance microscopically dispersed throughout another substance

A colloid is a mixture in which one substance consisting of microscopically dispersed insoluble particles is suspended throughout another substance. Some definitions specify that the particles must be dispersed in a liquid, while others extend the definition to include substances like aerosols and gels. The term colloidal suspension refers unambiguously to the overall mixture. A colloid has a dispersed phase and a continuous phase. The dispersed phase particles have a diameter of approximately 1 nanometre to 1 micrometre.

<span class="mw-page-title-main">Liquid crystal</span> State of matter with properties of both conventional liquids and crystals

Liquid crystal (LC) is a state of matter whose properties are between those of conventional liquids and those of solid crystals. For example, a liquid crystal can flow like a liquid, but its molecules may be oriented in a common direction as in solid. There are many types of LC phases, which can be distinguished by their optical properties. The contrasting textures arise due to molecules within one area of material ("domain") being oriented in the same direction but different areas having different orientations. An LC material may not always be in an LC state of matter.

<span class="mw-page-title-main">Gel</span> Highly viscous liquid exhibiting a kind of semi-solid behavior

A gel is a semi-solid that can have properties ranging from soft and weak to hard and tough. Gels are defined as a substantially dilute cross-linked system, which exhibits no flow when in the steady state, although the liquid phase may still diffuse through this system. A gel has been defined phenomenologically as a soft, solid or solid-like material consisting of two or more components, one of which is a liquid, present in substantial quantity.

<span class="mw-page-title-main">Self-assembly</span> Process in which disordered components form an organized structure or pattern

Self-assembly is a process in which a disordered system of pre-existing components forms an organized structure or pattern as a consequence of specific, local interactions among the components themselves, without external direction. When the constitutive components are molecules, the process is termed molecular self-assembly.

<span class="mw-page-title-main">Pierre-Gilles de Gennes</span> Nobel-laureate physicist

Pierre-Gilles de Gennes was a French physicist and the Nobel Prize laureate in physics in 1991.

<span class="mw-page-title-main">Hydrogel</span> Soft water-rich polymer gel

A hydrogel is a biphasic material, a mixture of porous, permeable solids and at least 10% by weight or volume of interstitial fluid composed completely or mainly by water. In hydrogels the porous permeable solid is a water insoluble three dimensional network of natural or synthetic polymers and a fluid, having absorbed a large amount of water or biological fluids. These properties underpin several applications, especially in the biomedical area. Many hydrogels are synthetic, but some are derived from nature. The term 'hydrogel' was coined in 1894.

<span class="mw-page-title-main">Polyelectrolyte</span> Polymers whose repeating units bear an electrolyte group

Polyelectrolytes are polymers whose repeating units bear an electrolyte group. Polycations and polyanions are polyelectrolytes. These groups dissociate in aqueous solutions (water), making the polymers charged. Polyelectrolyte properties are thus similar to both electrolytes (salts) and polymers and are sometimes called polysalts. Like salts, their solutions are electrically conductive. Like polymers, their solutions are often viscous. Charged molecular chains, commonly present in soft matter systems, play a fundamental role in determining structure, stability and the interactions of various molecular assemblies. Theoretical approaches to describing their statistical properties differ profoundly from those of their electrically neutral counterparts, while technological and industrial fields exploit their unique properties. Many biological molecules are polyelectrolytes. For instance, polypeptides, glycosaminoglycans, and DNA are polyelectrolytes. Both natural and synthetic polyelectrolytes are used in a variety of industries.

In physics, an entropic force acting in a system is an emergent phenomenon resulting from the entire system's statistical tendency to increase its entropy, rather than from a particular underlying force on the atomic scale.

<span class="mw-page-title-main">Biomaterial</span> Any substance that has been engineered to interact with biological systems for a medical purpose

A biomaterial is a substance that has been engineered to interact with biological systems for a medical purpose, either a therapeutic or a diagnostic one. The corresponding field of study, called biomaterials science or biomaterials engineering, is about fifty years old. It has experienced steady and strong growth over its history, with many companies investing large amounts of money into the development of new products. Biomaterials science encompasses elements of medicine, biology, chemistry, tissue engineering and materials science.

Small-angle X-ray scattering (SAXS) is a small-angle scattering technique by which nanoscale density differences in a sample can be quantified. This means that it can determine nanoparticle size distributions, resolve the size and shape of (monodisperse) macromolecules, determine pore sizes, characteristic distances of partially ordered materials, and much more. This is achieved by analyzing the elastic scattering behaviour of X-rays when travelling through the material, recording their scattering at small angles. It belongs to the family of small-angle scattering (SAS) techniques along with small-angle neutron scattering, and is typically done using hard X-rays with a wavelength of 0.07 – 0.2 nm. Depending on the angular range in which a clear scattering signal can be recorded, SAXS is capable of delivering structural information of dimensions between 1 and 100 nm, and of repeat distances in partially ordered systems of up to 150 nm. USAXS can resolve even larger dimensions, as the smaller the recorded angle, the larger the object dimensions that are probed.

<span class="mw-page-title-main">Mesophase</span>

In chemistry and chemical physics, a mesophase or mesomorphic phase is a phase of matter intermediate between solid and liquid. Gelatin is a common example of a partially ordered structure in a mesophase. Further, biological structures such as the lipid bilayers of cell membranes are examples of mesophases. Mesophases with long-range positional order but no orientational order are plastic crystals, whereas those with long-range orientational order but only partial or no positional order are liquid crystals.

<span class="mw-page-title-main">Colloidal crystal</span> Ordered array of colloidal particles

A colloidal crystal is an ordered array of colloidal particles and fine grained materials analogous to a standard crystal whose repeating subunits are atoms or molecules. A natural example of this phenomenon can be found in the gem opal, where spheres of silica assume a close-packed locally periodic structure under moderate compression. Bulk properties of a colloidal crystal depend on composition, particle size, packing arrangement, and degree of regularity. Applications include photonics, materials processing, and the study of self-assembly and phase transitions.

<span class="mw-page-title-main">Nanocellulose</span> Material composed of nanosized cellulose fibrils

Nanocellulose is a term referring to nano-structured cellulose. This may be either cellulose nanocrystal, cellulose nanofibers (CNF) also called nanofibrillated cellulose (NFC), or bacterial nanocellulose, which refers to nano-structured cellulose produced by bacteria.

Equilibrium gel is made from a synthetic clay. Unlike other gels, it maintains the same consistency throughout its structure and is stable, which means it does not separate into sections of solid mass and those of more liquid mass. Equilibrium gel filtration liquid chromatography is a technique used for the quantitation of ligand binding.

<span class="mw-page-title-main">Self-assembly of nanoparticles</span> Physical phenomenon

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Dominique Langevin is a French researcher in physical chemistry. She is research director at the Centre national de la recherche scientifique and leads the liquid interface group in the Laboratory of Solid State Physics at the University of Paris-Sud. She was the Life and Physical Sciences Panel chair for the European Space Sciences Committee of the European Science Foundation from 2013-2021.

<span class="mw-page-title-main">Biomolecular condensate</span> Class of membrane-less organelles within biological cells

In biochemistry, biomolecular condensates are a class of membrane-less organelles and organelle subdomains, which carry out specialized functions within the cell. Unlike many organelles, biomolecular condensate composition is not controlled by a bounding membrane. Instead, condensates can form and maintain organization through a range of different processes, the most well-known of which is phase separation of proteins, RNA and other biopolymers into either colloidal emulsions, gels, liquid crystals, solid crystals or aggregates within cells.

<span class="mw-page-title-main">Aline Miller</span> Professor of Chemistry

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Interfacial rheology is a branch of rheology that studies the flow of matter at the interface between a gas and a liquid or at the interface between two immiscible liquids. The measurement is done while having surfactants, nanoparticles or other surface active compounds present at the interface. Unlike in bulk rheology, the deformation of the bulk phase is not of interest in interfacial rheology and its effect is aimed to be minimized. Instead, the flow of the surface active compounds is of interest..

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