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Reghan Hill
Soft Matter cover page (Wang & Hill, 2008)
Lab on a Chip cover page (van Heiningen, Mohammadi & Hill, 2010)
We use a balance of theory, computations and experiments to study problems where complex behavior arises from nonlinearities and multiple length and time scales. Our research brings together topics from science and engineering, falling into the general realms of "soft matter" and "nanotechnology", drawing heavily on the core foundations of chemical engineering: applied mathematics, fluid mechanics, transport, and thermodynamics. The projects are often motivated by fundamental questions, with applications in emerging and traditional materials processing and diagnostic technologies.
Micro-fluidic optical-tweezers velocimetry (van Heiningen, Mohammadi & Hill, Lab Chip, 2010).
We have developed optical tweezers for manipulating and characterizing fuzzy colloidal particles, and have advanced this instrument to study electro-microrheological aspects of hydrogel nanocomposites. This program of research includes electro-acoustic diagnostics. Funding comes from the Canada Research Chairs (CRC), Canada Foundation for Innovation (CFI), and NSERC Research Tools and Instruments (RTI) programs.
In another area, which is supported by NSERC through the Research Tools and Instruments (RTI) program, we have an continue to study the organization of macromolecules in phospholipid-bilayer membranes using novel, quantitative methods of imaging, analysis and modelling, including quantitative fluorescence recovery after photobleaching (self-diffusion), post-electrophoresis relaxation (gradient diffusion), and electrophoresis (electrophoretic mobility).
Colloidal crystals synthesized by graduate student M Li.
Research funded by NSERC through the SENTINEL bio-active paper network of Canada is also underway. This supports studies of biologically active membranes (reinforced hydrogels) synthesized from cellulose, Nature's most abundant and renewable biopolymer.
In 2010, an extensive nationwide research program funded by the NSERC Green Fibre Network was initiated. This seeks to develop innovative and environmentally friendly technologies based on cellulose fibers from Canadian wood fibres. Our interests here are in seeking a fundamental understanding of---and practical control over---the barrier properties of nano- and micro-structured cellulose wood fibre networks.
Electrical microrheology of hydrogel-colloid composites (Wang & Hill, Soft Matter, 2008).
We are applying electro-acoustic diagnostics to hydrogels and other soft, complex fluids. These experiments are guided by extensive theoretical calculations undertaken in our group. Experimental tools are generously provided by NSERC through the RTI program.
With support from NSERC and Ovivo Water Technologies, we are working with Professor Frigon and his group to optimize electro-osmotic dewatering for wastewater sludge disinfection. Our group is developing mathematical models to guide experiments that will ultimately minimize energy consumption and achieve high degrees of microbial disinfection. This work is guided by the extensive fundamental research we have undertaken in electrokinetic, microhydrodynamic, and electrical-elastic-fluid coupling phenomena.
Many graduate students are generously supported by McGill Engineering Doctoral Awards (MEDA). Other members have received funding from the McGill Tomlinson and Schlumberger Ltd Scholarships. The sections below give a brief overview of present and past research projects.
Phospholipid bilayer membranes
Organization of polymer-derivatized phospholipid bilayers (e.g., Zhang & Hill, J. R. Soc. Interface, 2010). Experiments and theory are adopted to study kinetic and thermodynamic aspects of tethered polymers and phase equilibrium in these intriguing two-dimensional complex fluids.
Image acquisition photobleaching of DSPE-PEG2k-CF in DOPC supported lipid bilayers with continuous recovery by self-diffusion (Zhang & Hill, J. R. Soc. Interface, 2010)
Scanning confocal microscopy of supported phospholipid bilayer membrane synthesized by PhD candidate H. Zhang.
Nanotube micromechanics with optical tweezers
Optical tweezers are used with hydrodynamic flow to bend thin-walled polymeric nanotubes (Huang et al., Soft Matter, 2011) and, thus, measure the elastic modulus of the 11 nm thick wall, which comprises several alternating layers of SMA and PEI. These experiments (Huang et al., Soft Matter, 2011) furnish a modulus of about 1 GPa for the lamellar composite, whereas neither polymer component can form water-stable structures on its own.
Optical tweezers micromechanics from the PhD graduate students B Huang and JA van Heiningen (Soft Matter, 2011)
Single-particle microelectrophoresis with optical tweezers
Optical tweezers are used to perform single particle micro-electrophoresis experiments (e.g., van Heiningen, Mohammadi & Hill, Lab Chip, 2010) and colloidal assembly.
Micro-fluidic optical-tweezers velocimetry developed by PhD candidate J van Heiningen.
Optical tweezers built by PhD candidate J van Heiningen for maneuvering individual microspheres.
Nanocomposite membranes and polymer melts
This type of analytical calculation (Wang & Hill, Soft Matter, 2009) provides the first quantitative interpretation of the anomalous bulk viscosity of nanocomposite polymer melts (with molecular weight above the entanglement value) reported experimentally by Mackay & coworkers (Nature Materials, 2003). A shell of polymer at the particle-polymer interface adopts the lower Rouse viscosity, while the bulk polymer has the higher entangled melt viscosity. An earlier theoretical paper (Hill, Phys. Rev. Lett., 2008) quantifies how free-volume at the particle-polymer interface endows glassy polymeric nanocomposite membranes with enhanced permeability and reverse selectivity to small penetrant molecules, as first reported experimentally by Merkel & coworkers (Science, 2002).
Hydrogel-colloid composites
Electrical microrheology of hydrogel nanocomposites (upper panel from Wang & Hill, Soft Matter, 2008). Theory is used to calculate the static and dynamic electrical susceptibility of colloidal inclusions in hydrogels (e.g., Mohammadi & Hill, Proc. R. Soc. A, 2010), which, in turn, is guiding experiments to probe physicochemical aspects of the microstructure.
The incremental pore mobility for dilute dispersions of charged colloidal particles immobilized in uncharged hydrogel skeletons (from Hill, J. Colloid Interface. Sci., 2007). Solid lines are numerically exact solutions of the full electrokinetic model (see Hill, J. Fluid. Mech., 2006), and dashed lines are analytical approximate theories for asymptotically thin and thick diffuse double layers (e.g., Hill, Phys. Fluids, 2006).
Microfibrillar cellulose gels
Theoretical interpretation of the bulk elastic modulus of microfibrillar cellulose hydrogels (Hill, Biomacromolecules, 2008).
Theoretical interpretation (lines) of the bulk elastic modulus of microfibrillar cellulose gels (circles) measured by Paakko et al. (Biomacromolecules, 2007). Classical theory (e.g., de Gennes and Doi & Edwards) predicts a power-law exponent 2.25, whereas this scaling theory (Hill, Biomacromolecules, 2008) is the first to capture the experimentally observed transition from exponent 11/3 to 7 with increasing fiber concentration.
Electrokinetics of polymer-coated colloids
Polymer coatings provide an economical means of tailoring off-the-shelf `bare' colloidal particles for traditional applications of colloid chemistry and emerging (micro- and nano-scale) technologies. Hill, Saville and Russel (2003) recently developed theory and computations to guide and interpret electrokinetic experiments: e.g., electrophoresis, dielectric spectroscopy and electrokinetic-sonic-amplitude (ESA).
A computer package that implements their work (MPEK) provides numerically 'exact' steady and dynamic electrophoretic mobilities and polarizabilities, drag coefficients, and other single-particle and dilute-suspension properties, for particles with neutral and charged coatings. See Hill & Saville (2005) for extensive comparisons of the full model with approximate theories and experiments (mobility of PEO-coated latices, red blood cells and polyelectrolyte micelles). Hill (2004) examines the electrophoretic mobility using a self-consistent mean-field description of PEG chains grafted to micron-sized liposomes.
Electrophoretic mobility of red blood cells with various polyelectrolyte-layer thicknesses, but fixed amount of polymer and charge) (Hill & Saville, 2005). Solid lines are numerically exact solutions of the full electrokinetic model for soft colloidal spheres (Hill, Saville & Russsel, J. Colloid Interface Sci., 2003), and broken lines are analytical approximate theories from the literature that neglect polarization and relaxation (e.g., Ohshima, 1989).
Monte Carlo representation of `fuzzy' nanoparticles (Bamotra & Hill, 2005)
Computational fluid dynamics
Theory, lattice-Boltzmann simulations and experimental studies of finite-Reynolds-number flows in ordered and random arrays of spheres have helped to elucidate how fluid inertia affects fluid flow and transport in porous media for chemical engineering unit operations. In fixed beds of particles, the complex fluid motions that occur at moderate Reynolds numbers can be used to help understand much more complex systems in which the particles are free to move, such as in fluidized beds and suspensions. Most recently, the lattice-Boltzmann methodology has been applied to compute gas dynamics in the pores of bubbly rock samples.
Weakly turbulent flow in a porous medium (Hill & Koch, J. Fluid Mech., 2002)
Gas transport dynamics in Stromboli scoria (with Prof. D. Baker's group, Earth and Planetary Sciences, McGill University, 2008). X-ray tomographic imaging of synthetic and natural scorias is used with pore-scale lattice-Boltzmann simulations of flow to elucidate how gas bubble dynamics in magma ducts control the nature of volcanic eruptions.