Materials both Tough and Soft Jian Ping Gong Science 344, 161 (2014); DOI: 10.1126/science.1252389
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Fiber ends form cavity mirrors Fiber
High rate count at detector
Fiber Laser in
general not available from a single measurement. Rather, a whole series of measurements must be performed to extract the so-called density Low rate count matrix that contains all state at detector populations as well as coherence and correlation properties of the quantum state. For an increasing number of atoms, this task becomes harder because the number of values to be determined scales as 2N × 2N, which becomes a large number even for small numbers of particles (8). Instead, Haas et al. considered only a relevant subpart of the full density matrix spanned by symmetric Dicke-type states with varying numbers of excitations shared. By ﬁrst manipulating the prepared quantum state via microwave ﬁelds and subsequently checking for the presence of excitations, they determined the overlap of the state prepared with one of the Dicke states.
N atoms between mirrors in state |0典 ( ) or |1典 ( )
Weak microwave excitation Fiber ends form cavity mirrors
How darkness sheds light on entanglement. (A) An ensemble of N atoms in their internal ground state (blue) is transparent to a light ﬁeld resonant with the cavity formed by the ends of two ﬁbers (green). As a result, the photon count rate detected through one of the ﬁbers is “high.” (B) Upon irradiation with a weak microwave ﬁeld, exciting atoms to another hyperﬁne state (red), a single excitation in the ensemble renders the resonator opaque. The corresponding photon count rate is “low” and indicates an entangled atomic state in the resonator.
1/√N + + +
where |0冔 denotes a particle in the ground state and |1冔 denotes the excited particle. This state, called the W state in quantum information processing and the Dicke state in quantum optics, is rather robust against decoherence relative to other entangled states, such as Schrödinger’s-cat states or GreenbergerHorne-Zeilinger (GHZ) states, the manyparticle generalization of Einstein-PodolskyRosen pairs. Preparation of indistinguishable particles in a Bose-Einstein condensate is routinely performed in laboratories worldwide, and excitations of the atoms’ hyperﬁne states can be achieved with microwave ﬁelds. The key question, then, is “When has a single excitation entered the system?” Haas et al. tackled this problem using a high-ﬁnesse optical cavity formed by two opposing mirrors at the ends of optical ﬁbers. Photons typically bounce between these mirrors more than 10,000 times. Precisely tuning the resonator length to the wavelength of a weak impinging laser beam leads to light transmission with a small line width. Such a resonator is extremely amenable to even the slightest change of refractive index of its contents. In fact, a single atom in an excited state changes the effective optical path length for the light ﬁeld sufﬁciently to render the resonator opaque, while atoms in other internal states are transparent for the light ﬁeld. Thus, even for many atoms in one internal state, the laser beam is fully transmitted, but if an external microwave ﬁeld injects just a single excitation, the transmission vanishes, heralding the presence of the entangled W state (see the ﬁgure). Full information about a quantum state— even about a single quantum system—is in
Research into larger and other entangled states not only drives the development of emerging quantum applications; it elucidates the fundamental question, “Why does quantum physics explain perfectly everything we know about the microscopic world but is never observed in our everyday macroscopic life?” Only with experiments creating and analyzing larger and larger entangled states will we be able to track, and perhaps steer, the quantum-to-classical transition. References 1. A. Einstein, B. Podolsky, N. Rosen, Phys. Rev. 47, 777 (1935). 2. F. Haas et al., Science 343, 180 (2014); 10.1126/ science.1248905. 3. R. Blatt, D. Wineland, Nature 453, 1008 (2008). 4. T. Monz et al., Phys. Rev. Lett. 106, 130506 (2011). 5. O. Mandel et al., Nature 425, 937 (2003). 6. J. Estève, C. Gross, A. Weller, S. Giovanazzi, M. K. Oberthaler, Nature 455, 1216 (2008). 7. M. F. Riedel et al., Nature 464, 1170 (2010). 8. H. Häffner et al., Nature 438, 643 (2005). 10.1126/science.1251472
Materials both Tough and Soft Jian Ping Gong Tough elastomers are created by adapting an approach previously used for hydrogels.
ydrogels and elastomers are soft materials that have similar network structures but very different afﬁnities to water. Consisting mostly of water, hydrogels resemble biological soft tissues and have great potential for use in biomedical applications; they tend to be very brittle, like fragile jellies. Elastomers are formed of nonhydrated polymer networks and are widely used as load-dispersing and shock-absorbing materials. They are stretchable but break easily along a notch. On page 186 of this issue, Ducrot et al. (1) show that the toughness of elastomers can be improved substantially by Faculty of Advanced Life Science, Hokkaido University, Sapporo, 060-0810, Japan. E-mail: [email protected]
combining two different network materials, an approach previously applied to hydrogels. Double-network hydrogels contain 80 to 90 weight percent (wt %) of water, yet are both hard and strong, with mechanical properties comparable to that of rubbers and cartilages (2, 3). The gels consist of two interpenetrating polymer networks with contrasting mechanical properties. The ﬁrst network is highly stretched and densely cross-linked, making it stiff and brittle. The second network is ﬂexible and sparsely cross-linked, making it soft and stretchable. The toughness of a material is its ability to absorb mechanical energy and deform without fracturing. One deﬁnition of material toughness is the fracture energy, which is the energy per unit area required to make
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a notched crack propagate. 2nd network A 1st network B Double-network gels are tough because the internal fracture of the brittle network dissipates substantial amounts of energy under large deformation, while the elasticity of the second network allows it to return to its original configuration after deformation. The fracture Rigid and brittle Rigid and tough Soft and strechable energy of the double network •Short chain •Long chain G ~ 1000 J/m2 •Dilute •Dense is therefore much larger than those of either of the correG ~ 1 J/m2 G ~ 10 J/m2 sponding single networks. Thus, by sacriﬁcing the rup- Tougher than its parts. (A) By combining different network materials, tough double-network materials can be created. (B) Photo ture of the covalent bonds of a tough double-network hydrogel containing 90 wt % of water (2). The value of the fracture energy G is an indicator of mateof the brittle first network, rial toughness. Ducrot et al. now successfully apply this concept to elastomers. the material gains toughness. The covalent bonds serve as “sacriﬁcial polymers because the second or third net- used block copolymers (6) and Sun et al. bonds,” a term initially used to describe how work is formed from their monomer precur- polyampholytes carrying opposite charges bones fracture (4). sors in the presence of the ﬁrst network. The randomly distributed on the polymer chains In principle, the double-network principle stretched bonds of the ﬁrst network are the (7) to create single-network hydrogels with can be used to toughen other network mate- sacriﬁcial bonds that impart toughness to the dual cross-linking structures. The strong rials, as long as the interpenetrating network elastomers. The authors have also developed bonds in these gels impart elasticity, whereas structure is formed. However, applying this a method to observe the fracture of the sac- the weak bonds rupture during deformation, concept to the nonhydrated elastomers is not riﬁcial bonds in situ by using chemolumi- dissipating energy. The mechanical behavior trivial. The ﬁrst challenge is to form an inter- nescent cross-linking molecules, which emit of these dually cross-linked gels thus resempenetrating double-network structure in which light as they break. This work should stim- bles that of double-network gels. The dual the two networks have contrasting mechani- ulate researchers to develop new classes of cross-linking strategy may also be applied to cal properties. In the case of hydrogels, this tough hydrogels and elastomers and investi- elastomers in the future. structure is synthesized in a two-step process, gate how they fracture. Another goal is to develop tough soft using a polyelectrolyte as the ﬁrst network and Tough materials can dissipate substantial materials with anisotropic mechanical pera neutral polymer as the second network. A amounts of mechanical energy without frac- formance, similar to skin, cartilage, muscle, polyelectrolyte hydrogel swells much more turing. Compared with other tough materials and tendons. For example, self-assembled than a neutral hydrogel, causing the chains to in which noncovalent bonds dissipate energy, molecules such as lipids, rodlike macromolbe highly stretched and stiff as well as highly the covalent sacriﬁcial bonds of brittle net- ecules, or block copolymers may be used to dilute. In the case of elastomers, no poly- works have the advantages of high bond build anisotropic energy-dissipation strucelectrolyte can be used, making it difﬁcult to energy and weak dependence of the stiffness tures. This possibility is demonstrated by an form the interpenetrating network structure. and toughness on temperature and the rate of anisotropic hydrogel consisting of layered Another difﬁculty is that elastomers do not deformation. The disadvantage is that once lipid membranes entrapped in the matrix of a contain solvent. Without solvent, two differ- the brittle network is broken, the covalent neutral network (8). Several different mechent polymers usually do not mix but instead bonds cannot be reformed. The gel therefore anisms for dissipating energy and mainseparate into different phases. To synthesize softens permanently after large deformation. taining elasticity play a role in the design double network elastomers, one must prevent In a recent study, the covalent bonds have of tough soft materials. In a recent review, the phase separation of the two polymers. been replaced with ionic bonds to allow the Zhao provides a guide to how these differDucrot et al. now show how the double- fractured bond to be reformed in double-net- ent mechanisms can be used in the design of network concept can be used to improve the work hydrogels (5). Studies along these lines next-generation tough hydrogels (9). These toughness of elastomers. To obtain a highly have successfully produced tough hydrogels strategies are also applicable to elastomers, stretched and dilute ﬁrst network, they swell that recover after internal rupture. with Ducrot et al.’s study pointing the way to the ﬁrst network elastomer using monomers The double-network concept naturally many exciting materials. of the second network elastomer. They then suggests a general strategy for designReferences polymerize the monomers to complete the ing tough soft materials: incorporation of a 1. E. Ducrot et al., Science 344, 186 (2014). double-network structure. In a further step, mechanically fragile structure to toughen the 2. J. P. Gong et al., Adv. Mater. 15, 1155 (2003). the double-network elastomers can be swelled material as a whole. This strategy is not lim3. J. P. Gong, Soft Matter 6, 2583 (2010). 4. J. B. Thompson et al., Nature 414, 773 (2001). in a third monomer, causing the ﬁrst network ited to double- or multiple-network systems 5. J. Y. Sun et al., Nature 489, 133 (2012). elastomer to be stretched even further; polym- but also applies to single-network systems, 6. K. J. Henderson et al., Macromolecules 43, 6193 (2010). erization of the third monomer leads to a triple as long as they have sacriﬁcial bonds to dis7. T. L. Sun et al., Nat. Mater. 12, 932 (2013). network elastomer. sipate energy and can retain the original con8. M. A. Haque et al., Macromolecules 44, 8916 (2011). 9. X. H. Zhao, Soft Matter 10, 672 (2014). This sequential polymerization method ﬁgurations of the material after large deforprevents phase separation of the different mation. For example, Henderson et al. have 10.1126/science.1252389
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