Date Published: June 22, 2017
Publisher: John Wiley and Sons Inc.
Author(s): A. Chiolerio, Marco B. Quadrelli.
Organic, inorganic or hybrid devices in the liquid state, kept in a fixed volume by surface tension or by a confining membrane that protects them from a harsh environment, could be used as biologically inspired autonomous robotic systems with unique capabilities. They could change shape according to a specific exogenous command or by means of a fully integrated adaptive system, and provide an innovative solution for many future applications, such as space exploration in extreme or otherwise challenging environments, post‐disaster search and rescue in ground applications, compliant wearable devices, and even in the medical field for in vivo applications. This perspective provides an initial assessment of existing capabilities that could be leveraged to pursue the topic of “Smart Fluid Systems” or “Liquid Engineered Systems”.
Extreme planetary environments represent the next frontier for in‐situ robotic space exploration, where reconnaissance missions will be followed by robotic in‐situ missions, and perhaps later by human exploration. All these missions have one common problem: harsh, extreme environments, where temperature, radiation, and other factors making many of these missions inconceivable at present. Extreme environments, on Earth and other locations in the Solar System, are characterized by low or high temperatures, high‐radiation, high pressure, corrosive and toxic chemicals, etc. As scientific knowledge advances, autonomous missions of increasing complexity are needed to negotiate extreme environments, and these place increasing demands on the corresponding autonomous robotic platform technologies.1
A list of selected keywords is defined here for the sake of the reader:
Liquid is a condensed matter state in the solid state, featuring no shape retention but almost perfect volume retention, if submitted to pressure variation.Fluid is a condensed matter state in which shear forces result in a flow under stress without compromising the continuum of the object; both liquids and gases are fluids.Colloid is a complex condensed matter system lying at the boundary between completely homogeneous systems such as solutions and completely heterogeneous systems such as suspensions. Generally, they are composed of a fine phase dispersed in a solvent in liquid or gas form. They are classified according to the aggregation state of their components (S = solid, L = liquid, G = gaseous): S‐S (solid sol), S‐L (solid emulsion), S‐G (solid foam/aerogel), L‐S (sol), L‐L (emulsion), L‐G (liquid foam), G‐S (solid aerosol), G‐L (liquid aerosol), G‐G, where the first letter refers to the dispersant (carrier phase) and the second to the dispersoid (suspended phase). Colloids feature non‐trivial collective properties and can present two phase transitions: flocculation, occurring when the dispersoids coalesce, and gelation, occurring when the dispersant changes viscosity. Those phase transformation can be reversible, and can also depend on an exogenous input for that reversibility to occur.Filler or dispersoid is a solid (nano‐ or micro‐) particle with a different chemophysical nature with respect to the carrier (liquid solvent or gaseous phase).Smart fluid (SF) is a liquid‐based device with collective properties endowed by the cooperation of the dispersoid and dispersant phases, whose single agents or constituents enable the emergence of smart distributed functionalities such as information processing capabilities, self‐powering, sensing capabilities, and mobility.Ferrofluid (FF) is a colloidal suspension of NPs (nanoparticles) in the superparamagnetic (SPM)/ferromagnetic (FM) state, capable of developing long‐range attractive interactions that are compensated by short range repulsive interactions due to steric hindrance, given by particle functional groups.Magneto(electro)rheological fluid (M(E)RF) is an unstable liquid phase, a ferromagnetic(dielectric) suspension that could be stabilized by the external application of a magnetic(electric) field and formed as a result of magnetization(polarization) forces.Superfluid refers to a quantum mechanical condensed matter state where its atomic constituents behave as bosons in the condensed state, and could be conditioned to have collective properties by an external action.Supercritical fluid (SCF) refers to any liquid above its triple point.
In an attempt to develop an approach to address the SFS feasibility, we make the following scale‐dependent assumptions (Figure1).
To realize SFS, a variety of liquids enabling the function of synthetic cytoplasm could be used: ionic liquids, low melting point metals, polar/nonpolar solvents, inorganic liquids, acting all as solvents to disperse functional fillers. Colloids, in particular FFs and M(E)RFs, would also be important, as well as metal liquid‐like films (MeLLFs).
To provide the required functionalities that would enable practical applications, the SFS could leverage the collective properties of its constituents, relying on the variation of the following physical properties: Shape S variation, Viscosity η variation, Thermal conductivity k variation, Permittivity ε variation, Permeability µ variation, Conductivity σ variation involving charge transfer processes, Magnetization M variation, Absorbance A variation, and Information entropy H variation, among other possible ones.
To summarize and better visualize the observations reported in this study, Table1 collects the the results of a trade‐off among different types of liquid matrices for the SFS. The color code stands for a planetary exploration scenario, providing hints about suitability for space application: small robotic vehicle subject to low gravitational force and no atmosphere (green), Venus‐like robotic vehicle subject to high temperatures and pressures (red), Titan‐like robotic vehicle subject to low temperatures and high pressures (violet), gas/ice giant with huge pressures and supercritical atmosphere (blue). Table2 provides a simplified scheme of the basic and advanced functionalities for a SFS. Finally, Figure11 shows a functional block diagram of the proposed autonomy functions of an SFS, including SFS state sensing and estimation, and feedback of the state to the control function which would enable transduction in a physical environment.
Different scenarios of application would be possible for SFS, and they are discussed next.
Smart Fluid Systems are defined as devices based on organic or inorganic liquid, contained inside a volume by surface tension or by a confining membrane that protects them from a harsh planetary environment. In a biologically inspired vision, SFS could be able of changing shape according to a specific command or by means of a fully passive adaptive system, and provide a solution for innovative space exploration applications in extreme or otherwise challenging environments. Such a liquid robotic system would have the potential of offering innovative solutions to mobility, sensing, energy‐harvesting, and as energy barrier. Until now, research in this area has provided well established proofs of the feasibility of development of a SFS, although no experiment has been done yet in the direction of having a complete autonomous system. The scope of this review was to provide a strong foundational support to build the new field of liquid robotics.
The authors declare no conflict of interest.