The class of materials known as ‘soft matter’ includes a wide variety of products and foods from the everyday life, such as shampoo, toothpaste, shaving foam, milk, yogurt, mayonnaise, as well as the majority of biological fluids and tissues.
The hallmark of these materials consists of unique mechanical properties, which are in between the liquid and solid state. Such a hybrid nature, which is macroscopically manifested, arises from the way particle move and rearrange at the microscopic scale. This process, known as ‘structural relaxation’, displays complex features, often due to the emergence of collective and heterogeneous dynamics: unlike simple liquids, soft matter forming particles tend to move cooperatively, as the displacement of a single particle requires the rearrangement of many other neighbours.
A recent collaboration among Cnr, University of Naples Federico II (Unina), University of Cincinnati (Ohio, USA) and the Procter and Gamble Co. (P&G) led to the Differential Variance Analysis (DVA) a novel method to measure and visualize the ‘relaxation process’ in soft materials. The work, now published in Scientific Reports (Nature Publishing Group), was performed across Naples (Italy) and the P&G Labs of Cincinnati (Ohio, USA) by Raffaele Pastore (Spin-Cnr), Giuseppe Pesce (Dept. of Physics, University of Naples) and Marco Caggioni (P&G, Cincinnati).
By now, experimental characterization of soft material relaxation is typically performed by highly specialized groups, due to the complexity of the current techniques. A quite common method consists in tracking the position of each particle (single particle tracking), so as to indirectly reconstruct the global relaxation. However, single particle tracking can be used only if particles are clearly resolved by the microscope, and, even so, it may be arduous and not statistically meaningful, due to the limited number of successfully tracked particles. Other sophisticated techniques, based on Dynamic Light Scattering, are a robust approach to quantitatively monitor the relaxation, but do not provide any direct visualization. DVA dramatically simplifies these measurements, since it is directly applicable to digital videos, without tracking single particle positions. The new method focuses on the variance of the differential frames, obtained subtracting digital images at different lag-times. In this way, it is not only possible to quantitatively monitor the relaxation process, but also to obtain a direct visualization of its heterogeneous and cooperative nature. DVA can be performed in any laboratory equipped with a simple optical microscope or other acquisition set-ups providing digital videos of a sample. The simplicity of this technique meets the demand of a broad scientific community, which is potentially interested in investigating the dynamics of soft materials but bumped into the complicated experimental set-ups available until now.
“The dynamics of many soft materials is similar to that of crowded places: imagine, for example, being in the middle of a subway train during rush hours, aimed at getting off: you can succeed in getting off only if other people move cooperatively to facilitate your passage. The study of this relaxation process is crucial to understand the behaviour of soft materials and tailor their mechanical properties”, Raffaele Pastore (Spin-Cnr) says.
Potential application are widespread in material science and biology: as telling example, recent results show that the features of the relaxation process highlight and allow rationalizing pathological conditions in epithelial cell tissues, such as asthma and the propensity of cancerous tissues to trigger metastasis.
Moreover, from an industrial point of view, DVA opens the way to novel and simple approaches to investigate the temporal evolution of fast-moving consumer goods, such as detergents and fabric enhancers, which are tailored to have a long-term stability and to preserve their performance during the overall supply chain.
“The collaboration with P&G shows that basic research and industry could form part of a single scientific community, whose challenges are shared by different disciplines, from physics to biology. Finally, I would like to thank Vincenzo Guida (P&G), Roberto Cerbino (University of Milan) and the Joint Laboratory Cnr-Ntu Singapore for supporting this project”, R. Pastore concludes.
Rome, April 26th, 2017
Who: Spin-Cnr (Superconductors, oxides and other innovative materials and devices), Naples Uos
What: experimental method to measure and visualize the ‘relaxation process’ in soft materials: ‘Differential Variance Analysis: a direct method to quantify and visualize dynamic heterogeneities’, Scientific Reports 7, 43496 (2017). Authors: R. Pastore (Cnr-Spin Uos Napoli), G. Pesce (Dip.to di Fisica, Unina) and M. Caggioni (P&G, Cincinnati). Link: http://www.nature.com/articles/srep43496
For further information: Raffaele Pastore (Spin-Cnr, Naples Uos), email: firstname.lastname@example.org, tel. +39/081/676804,
1. From left to right, two frames separated by a 10s time-lag, and the resulting differential frame. Particles moved significantly during this interval give rise to coupled spots of low (blu) and high (red) intensity. The investigated system is a popular model of soft material: a dense colloidal suspension of micron sized hard-sphere beads in water.
2. a)Differential frames at different time-lags. b) Intensity variance as a function of the time-lag. As the time passes, more and more particles move, giving raise to variance increase. When the system has definitely relaxed, the variance attains a plateau.