Physics
http://hdl.handle.net/2262/86
PhysicsMon, 27 May 2019 00:26:19 GMT2019-05-27T00:26:19ZModels of wet two dimensional foams
http://hdl.handle.net/2262/86874
Models of wet two dimensional foams
DUNNE, FRIEDRICH FIONN
In this thesis I used models and computer simulations to investigate the
properties of two dimensional polydisperse foams from the dry limit of zero
liquid fraction to the wet limit of 0.16 liquid fraction.
Initially I used the Plat software, which implements the standard model
of two dimensional foams, to explore the full range of liquid fractions. As
the software becomes increasingly less reliable towards the wet limit, we use
over 500,000 simulations in order to obtain results in this regime. We found
the variation of energy and coordination number with liquid fraction, and
the internal distribution of contacts in the foams.
We then focus on the variation of the coordination number with liquid
fraction close to the wet limit. In particular, we compare the results of the
Plat simulation with those of the Soft Disk model, which is widely used in
the study of foams. The Soft Disk model is widely used due to its simplicity,
but it is approximate, and neglects deformations.
A stark difference between the two models is noted, with the Plat simulation
exhibiting a linear variation of the coordination number with liquid
fraction, and the Soft Disk model exhibiting a square root variation.
We investigate the link between the radial density function (and its analog,
the distribution of separations), and the variation of the coordination
number with liquid fraction in the wet limit. We find a marked difference between the distributions of separations of the two models. This explains
the difference in the variation of the coordination number. It appears to be
due to the fact that the bubbles in the Plat simulation are deformable, while
those in the Soft Disk model are not.
In order to explore the wet limit of two dimensional foams further, we
develop a new model based on the theory of Morse and Witten. This model
is defined for the wet limit, with deformable bubbles. It accurately predicts
the response of a bubble of droplet to small deformations. We develop a
framework and an algorithm for applying this theory to the case of modeling
two dimensional foams.
The new simulation based on this model is tested against the Plat simulation.
It produces comparable foams, with similar variations of the energy
with liquid fraction. It also produces comparable contact changes with
changes in liquid fraction. We propose an extension of this model to the
case of three dimensional foams.
Finally, we demonstrate an additional application of the theory of Morse
and Witten in three dimensions to the calculation of the surface tension of
bubbles and drops. We derive a simple formula, taking two length measurements,
without any free parameters, which predicts the surface tension of
bubbles and drops to a reasonable degree of accuracy (within 2%).
APPROVED
Tue, 01 Jan 2019 00:00:00 GMThttp://hdl.handle.net/2262/868742019-01-01T00:00:00ZThe Effect of Network Formation on the Mechanical Properties of 1D:2D Nano:Nano Composites
http://hdl.handle.net/2262/86865
The Effect of Network Formation on the Mechanical Properties of 1D:2D Nano:Nano Composites
Coleman, Jonathan
Mixtures of 1D carbon nanotubes and 2D nanosheets are important in electrochemical applications where the nanosheets are the active material, while the nanotubes provide electrical conductivity and mechanical reinforcement. While the conductivity of such nano:nano composites has been studied, the mechanical properties have not. Here, we report a detailed study of the structural, electrical, and mechanical properties of composites of MoS2 nanosheets mixed with carbon nanotubes at various volume fractions, ϕ. Microscopic analysis reveals the nanotube network to evolve from a loosely connected structure for ϕ < 1% to a strongly entangled continuous structure for ϕ > 1%. Significantly, while the network consists of low (∼200) aspect-ratio bundles for ϕ < 1 vol %, above this value, entangled ropes predominate, with the aspect ratio rising sharply with increasing ϕ, reaching ∼4700 for the 6.4 vol % sample. While this transition does not affect the electrical properties, it has a significant effect on the mechanical properties. Below ϕ = 1%, both the modulus and failure strain follow short-fiber composite behavior with the modulus increasing as per the rule of mixtures and failure strain falling with ϕ. However, above ϕ = 1%, both parameters increase with ϕ, consistent with continuous-fiber network behavior. The tensile strength also transitions at 1 vol % from a regime limited by the matrix–fiber interfacial strength (ϕ < 1 vol %) to one limited by the strength of the ropes (ϕ > 1 vol %). Similarly, the toughness is constant at low volume fraction but increases strongly for ϕ > 1 vol %, consistent with a model based on percolation theory.
Mon, 01 Jan 2018 00:00:00 GMThttp://hdl.handle.net/2262/868652018-01-01T00:00:00ZExfoliation of 2D materials by high shear mixing
http://hdl.handle.net/2262/86864
Exfoliation of 2D materials by high shear mixing
Coleman, Jonathan
While it has been demonstrated that large scale liquid exfoliation of graphene is possible using high-shear exfoliation, it has not yet been shown to be applicable to a broader range of layered materials. In addition, it would be useful to determine whether the mechanisms reported for shear exfoliation of graphene also apply to other 2D materials. In this work we show that previous models describing high-shear exfoliation of graphene apply to MoS2 and WS2. However, we find the minimum shear rate required to exfoliate MoS2 and WS2 to be ∼3 × 104 s-1, somewhat higher than the value for graphene. We also demonstrate the scalability of shear exfoliation of WS2. By measuring and then optimising the scaling parameters, shear exfoliation of WS2 is shown to be capable of reaching concentrations of 1.82 g l-1 in 6 h and demonstrating a maximum production rate of 0.95 g h-1.
PUBLISHED; cited By 0
Tue, 01 Jan 2019 00:00:00 GMThttp://hdl.handle.net/2262/868642019-01-01T00:00:00ZOptimising composite viscosity leads to high sensitivity electromechancial sensors
http://hdl.handle.net/2262/86863
Optimising composite viscosity leads to high sensitivity electromechancial sensors
Coleman, Jonathan
PUBLISHED; cited By 0
Mon, 01 Jan 2018 00:00:00 GMThttp://hdl.handle.net/2262/868632018-01-01T00:00:00Z