Static response of coated microbubbles:Modeling simulations and parameter estimation

AbstractThe mechanical response of contrast agent microbubbles subject to a static load was investigated in force-deformation curves. Asymptotic relations are fitted with experimental AFM measurements of polymeric microbubbles available in the literature. The elastic modulus and shell thickness are estimated based on the transition from the classical linear (Reissner) to the nonlinear (Pogorelov) regime. The estimated value of the elastic modulus is in the order of GPa and the shell thickness in the order of nm, in good agreement with independent estimates. Numerical simulations recover the above transition and identify a third regime, dominated by the compressibility of the enclosed gas

Rheological State Diagrams for Rough Colloids in Shear Flow

To assess the role of particle roughness in the rheological phenomena of concentrated colloidal suspensions, we develop model colloids with varying surface roughness length scales up to 10% of the particle radius. Increasing surface roughness shifts the onset of both shear thickening and dilatancy towards lower volume fractions and critical stresses. Experimental data are supported by computer simulations of spherical colloids with adjustable friction coefficients, demonstrating that a reduction in the onset stress of thickening and a sign change in the first normal stresses occur when friction competes with lubrication. In the quasi-Newtonian flow regime, roughness increases the effective packing fraction of colloids. As the shear stress increases and suspensions of rough colloids approach jamming, the first normal stresses switch signs and the critical force required to generate contacts is drastically reduced. This is likely a signature of the lubrication films giving way to roughness-induced tangential interactions that bring about load-bearing contacts in the compression axis of flow.National Science Foundation (U.S.) (Award 1232937)National Science Foundation (U.S.) (Award 1602183)United States. Army Research Office (Grant W911NF10-1-0518

Mechanical properties of phospholipid coated microbubbles

Phospholipid coated, inert gas filled microbubbles (MBs) are currently in widespread use in medical applications for the enhancement of diagnostic ultrasound images, and they are promising candidates for use in the area of targeted drug/gene delivery and uptake. As phospholipid coated MBs were developed for use with diagnostic ultrasound, their behaviour under acoustic loading is well investigated, however much less is known about their response to direct mechanical loading, which will potentially prove important as the range of uses of MBs expands. This is particularly true of the existing commercially available MB products. In this thesis, atomic force microscopy (AFM) was used to investigate the mechanical behaviour of three types of commercially produced phospholipid coated MBs, Definity®, BR14 and Sonovue®, at small deformations. Force spectroscopy was used to produce force-deformation (F-Δ) curves showing how the MBs deform under mechanical loading. Definity® MBs were deformed with tipless cantilevers at high deformations (though still less than 30% of the initial height of the MB); BR14 and Sonovue® MBs were probed with both tipless and tipped cantilevers to investigate both whole-bubble deformation and also shell indentation. BR14 was limited to low deformations; Sonovue® included both low and high deformations. The F-Δ curves were used to evaluate MB stiffness and also in combination with up to four mechanical models to predict the Young’s modulus of the MBs. The suitability of Reissner, Hertz, Elastic Membrane and De Jong theories for the prediction of the Young’s modulus of the MBs was explored. In the case of Definity® MBs no correlation between MB size and stiffness was observed; however an unexpected size dependence was observed in the Young’s modulus values, possibly due to variations in the thickness of the phospholipid shell. The membrane stretching component of elastic membrane theory was found to be the most applicable model on these MBs in this higher deformation regime. However, in this regime, gas compressibility could play a role and this is not included in the model. We studied the mechanical properties of BR14 MBs at very low deformations using ‘soft’ cantilevers. In this regime, gas compressibility should play a minimal role and there are several mechanical models which may be used. These MBs demonstrated decreasing stiffness with increasing diameter, and little variation in Young’s modulus with diameter. Hertz and De Jong theories showed more realistic Young’s modulus values (compared to other models) with little observable trend. Sonovue® MBs were used for a more comprehensive study of the small and very small deformation regimes using ‘soft’, ‘hard’ and tipped cantilevers. They showed no definitive trend in MB stiffness with MB diameter. Hertz and De Jong theory were again found to be most suitable. Analysis of curves acquired with tipped cantilevers indicated that the stiffness of a localised area of the shell membrane is similar to the overall stiffness of the MB and that the apparent Young’s modulus of the membrane according to the Hertz theory is also similar to that of the MB as a whole. Generally, considering all systems, Reissner theory was found to produce large overestimates of Young’s modulus, exceeding expected values by several orders of magnitude. Hertz and De Jong theories produced underestimates, though by a much smaller margin. Elastic membrane theory worked well and produced realistic Young’s modulus values only at relatively high deformation (the stretching term) in spite of the fact that gas compressibility is not taken into account. The suitability of the models is therefore very dependent on the deformation regime of the experiment. It seems that there is scope for better models at low deformation taking into account the soft shell of the MB and possibly its specific structure. Precise structural information of the MB shells does not exist; it is not trivial to attain and should certainly be a topic of future work with additional instrumentation

The Wetting Behavior of Polymer Droplets: Effects of Droplet Size and Chain Length

Monte Carlo computer simulations were utilized to probe the behavior of homopolymer droplets adsorbed at solid surfaces as a function of the number of chains making up the droplets and varying droplet sizes. The wetting behavior is quantified via the ratio of the perpendicular to the parallel component of the effective radii of gyration of the droplets and is analyzed further in terms of the adsorption behavior of the polymer chains and the monomers that constitute the droplets. This analysis is complemented by an account of the shape of the droplets in terms of the principal moments of the radius of gyration tensor. Single-chain droplets are found to lie flatter and wet the substrate more than chemically identical multichain droplets, which attain a more globular shape and wet the substrate less. The simulation findings are in good agreement with atomic force microscopy experiments. The present investigation illustrates a marked dependence of wetting and adsorption on certain structural arrangements and proposes this dependence as a technique through which polymer wetting may be tuned

Glass transition of polymers in bulk, confined geometries, and near interfaces

When cooled or pressurized, polymer melts exhibit a tremendous reduction in molecular mobility. If the process is performed at a constant rate, the structural relaxation time of the liquid eventually exceeds the time allowed for equilibration. This brings the system out of equilibrium, and the liquid is operationally defined as a glass - a solid lacking long-range order. Despite almost 100 years of research on the (liquid/)glass transition, it is not yet clear which molecular mechanisms are responsible for the unique slow-down in molecular dynamics. In this review, we first introduce the reader to experimental methodologies, theories, and simulations of glassy polymer dynamics and vitrification. We then analyse the impact of connectivity, structure, and chain environment on molecular motion at the length scale of a few monomers, as well as how macromolecular architecture affects the glass transition of non-linear polymers. We then discuss a revised picture of nanoconfinement, going beyond a simple picture based on interfacial interactions and surface/volume ratio. Analysis of a large body of experimental evidence, results from molecular simulations, and predictions from theory supports, instead, a more complex framework where other parameters are relevant. We focus discussion specifically on local order, free volume, irreversible chain adsorption, the Debye-Waller factor of confined and confining media, chain rigidity, and the absolute value of the vitrification temperature. We end by highlighting the molecular origin of distributions in relaxation times and glass transition temperatures which exceed, by far, the size of a chain. Fast relaxation modes, almost universally present at the free surface between polymer and air, are also remarked upon. These modes relax at rates far larger than those characteristic of glassy dynamics in bulk. We speculate on how these may be a signature of unique relaxation processes occurring in confined or heterogeneous polymeric systems.SCOPUS: re.jinfo:eu-repo/semantics/publishe