Classifying and characterizing active materials
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Classifying and characterizing active materials Julia R. S. Bursten1 Received: 27 October 2019 / Accepted: 10 September 2020 © Springer Nature B.V. 2020
Abstract This article examines the distinction between active matter and active materials, and it offers foundational remarks toward a system of classification for active materials. Active matter is typically identified as matter that exhibits two characteristic features: self-propelling parts, and coherent dynamical activity among the parts. These features are exhibited across a wide range of organic and inorganic materials, and they are jointly sufficient for classifying matter as active. Recently, the term “active materials” has entered scientific use as a complement, supplement, and extension of “active matter.” At the same time, new work in the philosophy of science has considered the problem of how to classify the products of synthetic and laboratory processes, and the extent to which the aims of classifying natural kinds compare and contrasts with the aims of classifying these synthetic kinds. In this article, I apply those considerations to the problems of classifying and characterizing active materials. In doing so, I also argue for a conception of active materials’ coherent dynamical activity as multiscale, rather than emergent, and I discuss how the special non-equilibrium status of active materials factors in to classificatory concerns. Keywords Active materials · Classification · Multi-scale modeling · Smart materials · Natural kinds
1 Introduction The principal subject of this discussion is the classification and characterization of active materials. By “active materials” I mean engineered materials that employ principles of active matter in their design. Active matter, in turn, is an umbrella term used to refer to systems whose components are self-propelled and thus out of thermal equilibrium, and which often, as a consequence, are able to generate collective, directed action like flocking and swarming. The movement of a flock of birds is often given as
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Julia R. S. Bursten [email protected] Department of Philosophy, University of Kentucky, 1415 Patterson Office Tower, Lexington, KY 40506, USA
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an example of active matter, and there are numerous biological systems that exhibit self-propelling and collective, directed action, such as microtubules and actin microfilaments. Scientific interest in active materials derives from the desire to mimic these properties of active matter in engineered systems, some organic and others inorganic. One of the aims of this article is to articulate the relationship between three aspects of active materials: (a) the out-of-equilibrium response, (b) the composition of active materials from self-propelled parts, and (c) the collective, directed action characteristic of active-material systems. Achieving this aim will afford the groundwork for the account I then outline, which characterizes active materials as synthetic kinds that exhibit a sustained, multi-scale non-equilibrium response. To charact
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