13 research outputs found
Role of uncrosslinked chains in droplets dynamics on silicone elastomers
We report an unexpected behavior in wetting dynamics on soft silicone
substrates: the dynamics of aqueous droplets deposited on vertical plates of
such elastomers exhibits two successive speed regimes. This macroscopic
observation is found to be closely related to microscopic phenomena occurring
at the scale of the polymer network: we show that uncrosslinked chains found in
most widely used commercial silicone elastomers are responsible for this
surprising behavior. A direct visualization of the uncrosslinked oligomers
collected by water droplets is performed, evidencing that a capillarity-induced
phase separation occurs: uncrosslinked oligomers are extracted from the
silicone elastomer network by the water-glycerol mixture droplet. The sharp
speed change is shown to coincide with an abrupt transition in surface tension
of the droplets, when a critical surface concentration in uncrosslinked
oligomer chains is reached. We infer that a droplet shifts to a second regime
with a faster speed when it is completely covered with a homogeneous oil film
Une goutte, deux vitesses : la dynamique surprenante dâune goutte dâeau sur un Ă©lastomĂšre silicone
Une goutte dâeau dĂ©vale gĂ©nĂ©ralement un plan inclinĂ© Ă vitesse constante. NĂ©anmoins, nous avons observĂ© que deux allures successives peuvent exister lorsque la goutte dĂ©vale sur certains Ă©lastomĂšres silicones. Ces matĂ©riaux, trĂšs utilisĂ©s dans lâindustrie et la recherche, sont un outil essentiel en microfluidique par exemple. La plupart des Ă©lastomĂšres silicones commerciaux sont constituĂ©s dâun rĂ©seau de chaĂźnes de polymĂšres rĂ©ticulĂ©es, câest-Ă -dire interconnectĂ©es, mais contiennent Ă©galement une petite proportion de chaĂźnes libres, non rĂ©ticulĂ©es. Notre enquĂȘte pour comprendre lâorigine de la dynamique Ă©tonnante dâune goutte sur un tel matĂ©riau va nous mener sur leurs traces
Extraction of Silicone Uncrosslinked Chains at AirâWaterâPolydimethylsiloxane Triple Lines
International audienceSilicone elastomers such as polydimethylsiloxane (PDMS) are convenient materials routinely used in laboratories that combine ease of preparation, flexibility, transparency, and gas permeability. However, these elastomers are known to contain a small fraction of uncrosslinked low-molecular-weight oligomers, the effects of which are not completely understood, particularly when used in contact with liquids. Here, we show that triple lines involving air, water, and PDMS elastomers are responsible for the contamination of waterâair interfaces by uncrosslinked silicone oligomers through a capillarity-induced extraction mechanism. We investigate both the case of static and moving contact lines and study various geometries ranging from partially immersed PDMS plates to water droplets or air bubbles deposited on PDMS plates, all involving airâwaterâelastomer triple lines. We demonstrate experimentally that the contamination timescale is strikingly shorter for moving contact lines than in the static case. Eventually, we propose a simple poroelastic model capturing the main features of contamination observed in experiments
Elastocapillary deformation of thin elastic ribbons in 2D foam columns
International audienceThe ability of liquid interfaces to shape slender elastic structures provides powerful strategies to control the architecture of mechanical self assemblies. However, elastocapillarity-driven intelligent design remains unexplored in more complex architected liquids - such as foams. Here we propose a model system which combines an assembly of bubbles and a slender elastic structure. Arrangements of soap bubbles in confined environments form well-defined periodic structures, dictated by Plateauâs laws. We consider a 2D foam column formed in a container with square cross-section in which we introduce an elastomer ribbon, leading to architected structures whose geometry is guided by a competition between elasticity and capillarity. In this system, we quantify both experimentally and theoretically the equilibrium shapes, using X-ray micro-tomography and energy minimisation techniques. Beyond the understanding of the amplitude of the wavy elastic ribbon deformation, we provide a detailed analysis of the profile of the ribbon, and show that such setup can be used to grant a shape to a UV-curable composite slender structure, as a foam-forming technique suitable to miniaturisation. In more general terms, this work provides a stepping stone towards an improved understanding of the interactions between liquid foams and slender structures
Capillarity-induced folds fuel extreme shape changes in thin wicked membranes
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Investigating PoreâOpening of Hydrogel Foams at the Scale of Freestanding Thin Films
International audienceControlling the pore connectivity of polymer foams is key for most of their applications, ranging from liquid uptake, mechanics, and acoustic/thermal insulation to tissue engineering. Despite their importance, the scientific phenomena governing the pore-opening processes remain poorly understood, requiring tedious trial-and-error procedures for property optimization. This lack of understanding is partly explained by the high complexity of the different interrelated, multiscale processes which take place as the foam transforms from an initially fluid foam into a solid foam. To progress in this field, this work takes inspiration from long-standing research on liquid foams and thin films to develop model experiments in a microfluidic âThin Film Pressure Balance.â These experiments allow the investigation of isolated thin films under well-controlled environmental conditions reproducing those arising within a foam undergoing cross-linking and drying. Using the example of alginate hydrogel films, the evolution of isolated thin films undergoing gelation and drying is correlated with the evolution of the rheological properties of the same alginate solution in bulk. The overall approach is introduced and a first set of results is presented to propose a starting point for the phenomenological description of the different types of pore-opening processes and the classification of the resulting pore-opening types
Soft, skin-interfaced microfluidic systems with integrated immunoassays, fluorometric sensors and impedance measurement capabilities
International audienceSoft microfluidic systems that capture, store, and perform biomarker analysis of microliter volumes of sweat, in situ, as it emerges from the surface of the skin, represent an emerging class of wearable technology with powerful capabilities that complement those of traditional biophysical sensing devices. Recent work establishes applications in the real-time characterization of sweat dynamics and sweat chemistry in the context of sports performance and healthcare diagnostics. This paper presents a collection of advances in biochemical sensors and microfluidic designs that support multimodal operation in the monitoring of physiological signatures directly correlated to physical and mental stresses. These wireless, battery-free, skin-interfaced devices combine lateral flow immunoassays for cortisol, fluorometric assays for glucose and ascorbic acid (vitamin C), and digital tracking of skin galvanic responses. Systematic benchtop evaluations and field studies on human subjects highlight the key features of this platform for the continuous, noninvasive monitoring of biochemical and biophysical correlates of the stress state
Skin-interfaced soft microfluidic systems with modular and reusable electronics for in situ capacitive sensing of sweat loss, rate and conductivity
International audienceStick-on electrodes capacitively coupled to single-use microfluidic channels enable contactless analysis of sweat in a soft wearable format with real-time wireless data collection