Our research interests center on the physics and chemistry of interfaces through combination of colloid science, polymer chemistry and soft matter physics. We also focus on the developing and applying single-particle force microscopy and microrheology to understand the structures, dynamics and biomolecular interactions of soft materials. Current areas of focus include:
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1. Interaction forces between solid surfaces adsorbed with polymers or biomolecules.
A wide variety of surfaces with switchable properties can be achieved when polymers are physically adsorbed or grafted densely on the solid surface to produce the so-called polymer brushes. These polymer-modified surfaces thereby provide a new perspective to open up numerous applications, including: for example, in promoting the adhesion of micro- and nano-scopic particles or molecules for use as catalysts and sensors or formation of multi-layered assemblies in controlling release of functional molecules. In all the applications mentioned above, the understanding and knowledge of the interaction forces between polymer-modified surfaces in aqueous solutions of varying conditions are critical.
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We use the techniques of Total Internal Reflection Microscopy (TIRM) to study the effect of polymer coatings and adsorbed surfactant layers on the interaction between a colloidal probe particle and a flat substrate. We have investigated the effect of the presence or absence of nanobubbles or microgel particles on the interaction between a free-moving colloidal particle and a flat surface in solution, and found that the existence of nanobubbles or microgel particles in the aqueous solution, like nonadsorbing polymers, can induce depletion attraction between the particle and the flat surface. We have also conducted measurements between two surfaces in the presence of polymers or polyelectrolyte chains, and used TIRM to investigate the influence of different electrolyte environments on the conformational behavior of surface-grafted polyelectrolyte brushes, a study which might find applications in the design of switchable surfaces.
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Currently, we are interesting in the use TIRM to study the effects of adsorbed biomolecules layers on the interaction between a colloidal probe particle and a flat substrate. The interactions between biomacromolecules such as protein-protein, protein-nucleic acid and protein-carbohydrate are controlled by a complex array of intermolecular and intersurface forces. Conventionally, these forces that control biomolecules interactions and their physical chemical origins are typically interfered indirectly from equilibrium binding and kinetic measurements or are calculated with molecular models. Force measurement by TIRM therefore provides a powerful mean of directly quantifying the complex interactions that determine the properties of biological molecules and biomaterials.
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2. Microrheology of soft materials and living cells.
We have recently established a setup that enables the study of the micro-rheological properties and structure of materials near a flat surface. The basic idea is to combine the spatial resolution of total internal reflection microscopy (TIRM) with a magnetic tweezers. In a standard TIRM measurement, an evanescent wave is generated by the total internal reflection of an incident laser light at a solid (glass slide)/liquid interface. The light intensity of such a wave decays exponentially with the distance away from the interface.
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| When a micrometer scale magnetic particle probe is placed within ~102 nm of the interface, it probes all the particle-surface separations that TIRM can resolve. As a result, TIRM is a highly sensitive spatial detector for tracking the vertical motion of the embedded probe particle, and viscoelastic moduli can be quantitatively determined from the effect of confinement. The motion of the embedded probe particle can be controlled by a magnetic driving force, produced by two sets of four electromagnetic pole pieces symmetrically arranged in the upper and lower planes of the sample cell so as to achieve a three-dimensional position control. High precision of the magnetic force, at pN scale, can be achieved by real-time control of the electric current. We are therefore able to oscillate the embedded magnetic particle and monitor the response to infer network viscoelasticity by a combination of the magnetic tweezers and the evanescent wave-scattered particle tracking near a surface. We expect that this technique will have great potential for non-invasive and spatially resolved characterization of the micro-rheological properties of soft materials and living cells. |
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3. Particle-stabilized emulsions: formation and applications.
It has been well documented that solid particles of colloidal size can absorb at liquid-liquid interfaces and provide excellent long-term kinetic stability to emulsions. Such particle-stabilized emulsions, often referred to as “Pickering emulsions”, were described a century ago, and found important for a number of industrial applications including the production of food, cosmetics or coating and petroleum industries. In addition, particle-stabilized emulsions provide a simple and novel template for the production of various functional materials.
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We are particularly interesting in development of stimuli-sensitive microgels as stabilizers for Pickering emulsions. One key advantage of using microgel particles for emulsification lies in the fact that the stability of microgel-stabilized emulsions can be triggered by changing the environmental conditions such as temperature, pH, or magnetic field, offering a novel way to control the emulsion stability. This peculiar tunable stabilizing property is especially desirable in industrial applications such as fuel production and oil transportation processes where the emulsions can be prepared and broken on demand.
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We have recently developed a novel approach to synthesize micron-sized (2-5 micrometer), multi-responsive microgels with different morphologies via a combining of the semi-batch and temperature-programmed surfactant-free precipitation polymerization. Our results have revealed that the crosslinkers and the functional co-monomers could be homogeneously distributed, predominately localized inside the core, or concentrated near the surface of the synthesized microgels. The feasibility to produce larger size microgels will offer the possibility to study the effect of the morphology and spatial distribution of functional groups on the physical properties of microgels with respect to various stimuli in individual, isolated particle. We are elucidating how the morphology and compositional heterogeneity of micorgles contribute to their packing behaviour at the oil-water interface, which in turn affect the mechanism of stabilization of emulsions.
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4. Interparticle interactions at fluid interfaces.
Solid particles adsorbed at fluid-fluid interfaces have been traditionally exploited in emulsification mineral and crude oil recovery as well as food industry, and more recently in a rapidly increasing range of cutting-edge applications, including the creation of nanostructured membrane for filtration and biphasic catalysis, and the fabrication of nanocomposites with tunable electrical or optical properties. For the purpose of supporting these emerging applications, an improved understating of interfacial behavior of particles and their dynamics at a fluid interface is a key issue. Many earlier experiments were carried out on flat, planer fluid interfaces and various forces in interface systems were proposed. However, to date, the behavior of solid particles at interfaces is still a very dynamic area of research with many open challenges that need to be met.
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5. Development of orthopaedic implant biomaterials.
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Biocompatible and biodegradable magnesium (Mg) and its alloys possess excellent potential as orthopaedic implant biomaterials because they have similar mechanical properties to natural bones and magnesium ions (Mg++) are known to promote the growth of new bone. However, because Mg/Mg alloys corrode inside the human body, their mechanical properties deteriorate too quickly and lose their support functions if they are used for bone fracture fixation. Additionally, when the rate of corrosion-induced hydrogen generation is higher than the body’s natural hydrogen adsorption rate, hydrogen bubbles and alkalization can occur. As such, it is important to develop new biocompatible and corrosion-controlled bio-implants made of Mg and its alloys.
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