Denture quality and patient satisfaction in elderly Chinese in Hong Kong

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Dynamics of Oppositely Charged Emulsion Droplets

Talk #29Junior Scientist, Postdoc and Student WorkshopsTwo droplets are typically expected to coalesce upon contact due to the minimization of surface energy. However, when droplets are placed under an electric field, the presence of electric stress can lead to even more intriguing dynamics. For example, in a significantly strong electric field, the droplets bounce away from each other upon contact instead of coalescing. In this work, we investigate the dynamics of two oppositely charged droplets. We characterize the dynamics of emulsion droplets by a state diagram using an electrocapillary number and relative separation number. A phenomenon of periodic contact and separation of two oppositely charged droplets, which we term periodic contact, is demonstrated and studied. Two qualitatively different types of periodic contact are identified: “fuse-and-split” and periodic non-coalescence. In regime of “fuse-and-split”, the droplets first coalesce into a jet that remains stable for tens of milliseconds; afterwards, it breaks up into droplets again. In regime of “periodic non-coalescence”, the droplets contact periodically without coalescence. We show this periodic contact occurs because of the interaction between electric stress and surface tension and only exists when the electric conductivity of droplets is relatively high. When droplets are not in contact, the electric stress deforms the droplets against the surface tension and leads to their approach. Upon contact, the electric stress is relieved and surface tension starts to dominate during the evolution of droplets’ interface. By analyzing the surface energy evolution, we show that “fuse-and-split” which enables fluid exchange between the droplets represents a way for droplets to reach a state of minimized surface energy. The periodic non-coalescence which prevents the fluid exchange represents an energy barrier to stop the droplets from approaching the minimized surface energy state. Also, droplets in the regime of “fuse-and-split” will eventually remain separated or transition to the periodic non-coalescence with the change of the droplet shape. Our work enriches the understanding of dynamics associated with charged emulsion droplets as well as other research problems which involve the interplay of electric stress and surface tension.published_or_final_versio

Fabrication of uniform multi-compartment particles using microfludic electrospray technology for cell co-culture studies

Footnote in article: Paper submitted as part of the 3rd European Conference on Microfluidics ... 2012In this work, we demonstrate a robust and reliable approach to fabricate multi-compartment particles for cell co-culture studies. By taking advantage of the laminar flow within our microfluidic nozzle, multiple parallel streams of liquids flow towards the nozzle without significant mixing. Afterwards, the multiple parallel streams merge into a single stream, which is sprayed into air, forming monodisperse droplets under an electric field with a high field strength. The resultant multi-compartment droplets are subsequently cross-linked in a calcium chloride solution to form calcium alginate micro-particles with multiple compartments. Each compartment of the particles can be used for encapsulating different types of cells or biological cell factors. These hydrogel particles with cross-linked alginate chains show similarity in the physical and mechanical environment as the extracellular matrix of biological cells. Thus, the multi-compartment particles provide a promising platform for cell studies and co-culture of different cells. In our study, cells are encapsulated in the multi-compartment particles and the viability of cells is quantified using a fluorescence microscope after the cells are stained for a live/dead assay. The high cell viability after encapsulation indicates the cytocompatibility and feasibility of our technique. Our multi-compartment particles have great potential as a platform for studying cell-cell interactions as well as interactions of cells with extracellular factors. © 2013 AIP Publishing LLC.published_or_final_versio

Multicompartment polymersome gel for encapsulation

We introduce an approach that combines the concepts of emulsion-templating and dewetting for fabricating polymersomes with a large number of compartments. The resultant polymersome gel behaves as a gel-like solid, but is a true vesicle suspended in an aqueous environment. Due to the thin membranes that separate the compartments, the polymersome gels have a high volume fraction of internal phase for encapsulation of hydrophilic actives; they also provide a large surface area of diblock copolymer membrane for encapsulation of lipophilic actives. Multiple actives can also be encapsulated in the gel without cross-contamination. Our technique represents a simple and versatile bulk approach for fabricating polymersome gels; it does not require the use of any specialized equipment or subsequent polymerization steps to solidify the gel. The resultant polymersome gel is promising as an encapsulating structure as well as a scaffold for tissue engineering. © 2011 The Royal Society of Chemistry.postprin

Demonstration of syringe-pump-induced disturbance in microfluidic system with low interfacial tension

Talk #21Junior Scientist, Postdoc and Student WorkshopsSyringe pump provides precise and constant flow rates so it is widely used in microfluidic research and applications. Most syringe pumps are mechanically driven and introduce fluctuations or pulses to the inlet flow and thus affect the steadiness of the flow. However, to the best of our knowledge, no evidences confirmed that these are really induced by syringe pumps. Here we introduce a robust visual detection of the unsteadiness induced by the stepping motor in a syringe pump, in form of ripples on the interface of an aqueous two-phase system which has low interfacial tension. We use a typical glass capillary device to generate a co-flow of two immiscible phases in our experiments [1]. The ripples are found to exhibit the same frequency as that delivered by the stepping motor of the syringe pump which drives the inner fluid, named as fpump, for various flow rates Q, syringe diameters D and advancing step sizes s, according to fpump = 4Q/(πD2s). The experimental results suggest that the low interfacial tension system can reflect the disturbance aroused from the inner pump, thus give an insight into understanding the fluctuation that syringe pumps induces and provide a way to test whether the unsteadiness in microfluidic system is related to syringe pump or not.published_or_final_versio

Capillary micromechanics of cell-encapsulated microgels

Junior Scientist, Postdoc and Student WorkshopsTalk #33Cell behaviors, such as adhesion, proliferation, migration and growth, are closely affected by their surrounding environment [1–5]. For example, cells respond to the elasticity of their substrate by changing their stiffness and morphologies [6–12]; stem cell proliferates into different cell types depending on the material properties of substrates including wettability, surface chemistry and elasticity [5,13–15]. Thus, the cell-material interaction is of great importance in understanding the cell responses to signals within their immediate environments [1,2]. Due to its tunable elasticity, microgel is frequently employed to mimic tissues or as synthetic substrates to study cell behavior such as adhesion [9]. Microgels are also extensively used as delivery vehicles in vivo [21], or as scaffolds for 3-D cell culture [22]. For these applications, the interaction between the soft hydrogel materials and cells dictate the fate of delivery vehicles or the cultured cells [21,22]. Among the physical and chemical properties of cell/material interfaces, the mechanics between cell and materials receive wide attention [5–9,15,22,24]. It is shown that kidney epithelial and 3T3 fibroblastic cells can feel and respond to the elasticity of substrates by changes in their reduced spreading and increased motility [8]. Other parameters such as cell type and shape also have more pronounced effect on cell stiffness [6]. These correlations enrich our understanding the interaction between cell behavior and the mechanical properties of their adhesive substrates. However, most of the results are obtained in substrates of 2-D geometry while neglecting the possible influence from fluids in the surroundings [25], which is present in most biological systems. Recent advance of microfluidics enables the encapsulation of cells in 3-D microgels with well-defined physiochemical properties. Moreover, a recently developed technique Capillary Micromechanics allows the comprehensive characterization of mechanical properties of microgels at single particle level [26,27]. Thus, Capillary Micromechanics is promising platform to investigate cell response in well-defined microgels with surrounding shearing flow. In this work, we encapsulate single cell in a hydrogel microparticle with different stiffness. By using Capillary Micromechanics, we monitor the subsequent development of the cells and correlate their development with the stiffness of the hydrogel matrices. Moreover, the cell proliferation in response to different pressure exerted by surrounding fluids, which is present in vivo, is also investigated. We IAS Program on Frontiers of Soft Matter Physics: from Non-equilibrium Dynamics to Active Matter | 2-29 Jan 2014 demonstrate our approach can be used for studying cell growth in well-controlled environment systematically. Our understandings can provide important guideline when designing hydrogel carriers for biological materials, such as cells and embryos.published_or_final_versio

The dripping-to-jetting transition in a co-axial flow of aqueous two-phase systems with low interfacial tension

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Fluctuation-induced dynamics of multiphase liquid jets with ultra-low interfacial tension

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Three-dimensional printing-based electro-millifluidic devices for fabricating multi-compartment particles

In this work, we demonstrate the use of stereolithographic 3D printing to fabricate millifluidic devices, which are used to engineer particles with multiple compartments. As the 3D design is directly transferred to the actual prototype, this method accommodates 3D millimeter-scaled features that are difficult to achieve by either lithographic-based microfabrication or traditional macrofabrication techniques. We exploit this approach to produce millifluidic networks to deliver multiple fluidic components. By taking advantage of the laminar flow, the fluidic components can form liquid jets with distinct patterns, and each pattern has clear boundaries between the liquid phases. Afterwards, droplets with controlled size are fabricated by spraying the liquid jet in an electric field, and subsequently converted to particles after a solidification step. As a demonstration, we fabricate calcium alginate particles with structures of (1) slice-by-slice multiple lamellae, (2) concentric core-shells, and (3) petals surrounding the particle centers. Furthermore, distinct hybrid particles combining two or more of the above structures are also obtained. These compartmentalized particles impart spatially dependent functionalities and properties. To show their applicability, various ingredients, including fruit juices, drugs, and magnetic nanoparticles are encapsulated in the different compartments as proof-of-concepts for applications, including food, drug delivery, and bioassays. Our 3D printed electro-millifluidic approach represents a convenient and robust method to extend the range of structures of functional particles.published_or_final_versio

Breakup dynamics and dripping-to-jetting transition in a Newtonian/shear-thinning multiphase microsystem

The breakup dynamics in non-Newtonian multiphase microsystems is associated with a variety of industrial applications such as food production and biomedical engineering. In this study, we numerically and experimentally characterize the dripping-to-jetting transition under various flow conditions in a Newtonian/shear-thinning multiphase microsystem. Our work can help to predict the formation of undesirable satellite droplets, which is one of the challenges in dispensing non-Newtonian fluids. We also demonstrate the variations in breakup dynamics between shear-thinning and Newtonian fluids under the same flow conditions. For shear-thinning fluids, the droplet size increases when the capillary number is smaller than a critical value, while it decreases when the capillary number is beyond the critical value. The variations highlight the importance of rheological effects in flows with a non-Newtonian fluid. The viscosity of shear-thinning fluids significantly affects the control over the droplet size, therefore necessitating the manipulation of the shear rate through adjusting the flow rate and the dimensions of the nozzle. Consequently, the droplet size can be tuned in a controlled manner. Our findings can guide the design of novel microdevices for generating droplets of shear-thinning fluids with a predetermined droplet size. This enhances the ability to fabricate functional particles using an emulsion-templated approach. Moreover, elastic effects are also investigated experimentally using a model shear-thinning fluid that also exhibits elastic behaviors: droplets are increasingly deformed with increasing elasticity of the continuous phase. The overall understanding in the model multiphase microsystem will facilitate the use of a droplet-based approach for non-Newtonian multiphase applications ranging from energy to biomedical sciences.postprin