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Title: Emergent surface tension in boiling granular media
Author: Clewett, James
Awarding Body: University of Nottingham
Current Institution: University of Nottingham
Date of Award: 2013
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In this thesis we present findings from an experimental and numerical study of loosely confined, dry granular gases subject to vertical vibration. We found that the system phase separates into a high-density, liquid-like phase and a low-density gas-like phase. The phase separation was shown to occur at a critical driving amplitude but is independent of frequency. To introduce our work, we give an overview of phase separation in driven granular gases. Ofpatiicular interest are: a solid-liquid-like phase separation in tightly confined, dry granular mono layers and a liquid-gas-like phase separation in loosely confined, wet granular gases. Our system differs from the above examples in two significant ways: our cell is deeper than that used to tightly confine the granular mono layers, so that we avoid the formation of a solid-like phase; our patiicles are dry and as such there are no cohesive forces between the particles. The liquid-gas phase separation is a useful system in which to study far-from-equilibrium phenomena because the particles are easily homogenised and then quenched into the phase-separating state. The system also allows us to smoothly approach the phase-transition boundaries. The phase separation was shown to be spinodal driven, with a region of negative compressibility due to an excess in the granular temperature of the particles in the dilute phase. The origin of the excess temperature was traced to the coherent motion of particles above a critical driving amplitude. By switching to a frequency modulated driving signal the phase separation was suppressed, demonstrating the requirement for coherent motion. The experiment shows the importance of using realistic driving motion in simulations. The phase-separation coarsening dynamics were shown to be similar to that of thermodynamic systems evolving under curvature driven diffusion (model B). Using the Cahn-Hilliard equation we accurately predict the dominant length scale in the early-time dynamics. In thelIDodynamics the Cahn-Hilliard equation desclibes the minimisation of an excess interfacial energy. This suggests that we might define an effective free energy for our granular system, however, as yet it is not clear what is meant by free energy in the context of a far-fi·om-equilibrium system. Finally, by studying the surface tension of quasi-2D liquid-like droplets in the steady state, we found behaviour consistent with Laplace's equation, demonstrating that the surface tension is real. Detailed measurements of the pressure in the interfacial region show that the surface tension results predominantly from an unexpected anisotropy in the kinetic energy part of the pressure tensor, in contrast to thelIDodynamic systems where surface tension arises from either the attractive interaction between pmticles or entropit considerations. The general nature of our argument for the Oligin of the surface tension means that it should apply to other granular phase separations and segregations in granular rnixhlres. As such this might be a new general mechanism in far-from-equiliblium thermodynamics. Throughout the thesis we use numerical simulations, configured with a geometry matching that of the experimental cell. To simulate the two million particles required we created a bespoke molecular dynamics code to execute using GPGPU hardware. The peliormance of our simulations was comparable to the state of the art in the literature, approximately twenty times faster than simulations on modern CPU processors.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available