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Superconfinement tailors fluid flow at microscales.

Setu SA, Dullens RP, Hernández-Machado A, Pagonabarraga I, Aarts DG, Ledesma-Aguilar R - Nat Commun (2015)

Bottom Line: Understanding fluid dynamics under extreme confinement, where device and intrinsic fluid length scales become comparable, is essential to successfully develop the coming generations of fluidic devices.Henceforth, we present a theory that quantifies our experiments in terms of the relevant interfacial length scale, which in our system is the intrinsic contact-line slip length.Our findings show that length-scale overlap can be used as a new fluid-control mechanism in strongly confined systems.

View Article: PubMed Central - PubMed

Affiliation: 1] Department of Chemistry, Physical and Theoretical Chemistry Laboratory, University of Oxford, South Parks Road, Oxford OX1 3QZ, UK [2] Department of Chemistry, Faculty of Science, Universiti Teknologi Malaysia, Johor Bahru, Johor 81310, Malaysia.

ABSTRACT
Understanding fluid dynamics under extreme confinement, where device and intrinsic fluid length scales become comparable, is essential to successfully develop the coming generations of fluidic devices. Here we report measurements of advancing fluid fronts in such a regime, which we dub superconfinement. We find that the strong coupling between contact-line friction and geometric confinement gives rise to a new stability regime where the maximum speed for a stable moving front exhibits a distinctive response to changes in the bounding geometry. Unstable fronts develop into drop-emitting jets controlled by thermal fluctuations. Numerical simulations reveal that the dynamics in superconfined systems is dominated by interfacial forces. Henceforth, we present a theory that quantifies our experiments in terms of the relevant interfacial length scale, which in our system is the intrinsic contact-line slip length. Our findings show that length-scale overlap can be used as a new fluid-control mechanism in strongly confined systems.

No MeSH data available.


Related in: MedlinePlus

Interface dynamics in superconfinement.(a) Two colloid–polymer phases of ultra-low surface tension, γ=30 nN m−1, and different densities (ρL>ρG) and viscosities (ηL<ηG), are forced in a microfluidic channel of fixed width W=110 μm and variable thickness H=8, 10, 14, 17 μm. (b,c) The interface in the plane of the channel develops into a viscous finger (b), which adheres to the bottom plate of the channel to form a thin film (c). (d) Above a threshold driving speed, U*, the front is destabilized; the contact line, of speed V<U*, is unable to follow the rest of the interface, of speed U>U*. This mismatch gives rise to the formation of a fluid jet that releases drops periodically. Scale bars in b–d, 50, 10 and 10 μm, respectively.
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f2: Interface dynamics in superconfinement.(a) Two colloid–polymer phases of ultra-low surface tension, γ=30 nN m−1, and different densities (ρL>ρG) and viscosities (ηL<ηG), are forced in a microfluidic channel of fixed width W=110 μm and variable thickness H=8, 10, 14, 17 μm. (b,c) The interface in the plane of the channel develops into a viscous finger (b), which adheres to the bottom plate of the channel to form a thin film (c). (d) Above a threshold driving speed, U*, the front is destabilized; the contact line, of speed V<U*, is unable to follow the rest of the interface, of speed U>U*. This mismatch gives rise to the formation of a fluid jet that releases drops periodically. Scale bars in b–d, 50, 10 and 10 μm, respectively.

Mentions: Our experimental results are summarized in Fig. 2, where we have probed the dynamics of a superconfined forced front coupled to a contact line using two demixed colloid–polymer phases. The interface thickness of our mixtures, ξ, which is representative of the order of magnitude of the contact-line slip length lD, is of the order of microns (ξ≈1.2 μm). This allows us to access the superconfined regime in micron-sized channels. As opposed to more traditional set-ups, the large interface scale in our experiments offers the additional advantage of directly visualizing the front and the contact line at the scale of thermal fluctuations using confocal microscopy2122. To favour the formation of a fluid front advancing on a solid, we injected two co-existing colloid–polymer phases into a rectangular microfluidic channel, displacing the more viscous polymer-rich, colloid-poor ‘gas' phase with the less viscous polymer-poor colloid-rich ‘liquid' phase (Fig. 2a). The low surface tension of our mixtures, γ=30 nN m−1, sets a comparatively small capillary length, , where Δρ is the density difference between the fluids and g is the acceleration due to gravity. At large enough velocities, the interface develops into a three-dimensional (3D) finger by virtue of the Saffman–Taylor instability22 (Fig. 2b). Because lC is of the order of the channel thickness H, the finger adheres to the bottom plate22 creating a thin film of thickness hf, which occupies roughly half of the channel thickness (Fig. 2c).


Superconfinement tailors fluid flow at microscales.

Setu SA, Dullens RP, Hernández-Machado A, Pagonabarraga I, Aarts DG, Ledesma-Aguilar R - Nat Commun (2015)

Interface dynamics in superconfinement.(a) Two colloid–polymer phases of ultra-low surface tension, γ=30 nN m−1, and different densities (ρL>ρG) and viscosities (ηL<ηG), are forced in a microfluidic channel of fixed width W=110 μm and variable thickness H=8, 10, 14, 17 μm. (b,c) The interface in the plane of the channel develops into a viscous finger (b), which adheres to the bottom plate of the channel to form a thin film (c). (d) Above a threshold driving speed, U*, the front is destabilized; the contact line, of speed V<U*, is unable to follow the rest of the interface, of speed U>U*. This mismatch gives rise to the formation of a fluid jet that releases drops periodically. Scale bars in b–d, 50, 10 and 10 μm, respectively.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4490407&req=5

f2: Interface dynamics in superconfinement.(a) Two colloid–polymer phases of ultra-low surface tension, γ=30 nN m−1, and different densities (ρL>ρG) and viscosities (ηL<ηG), are forced in a microfluidic channel of fixed width W=110 μm and variable thickness H=8, 10, 14, 17 μm. (b,c) The interface in the plane of the channel develops into a viscous finger (b), which adheres to the bottom plate of the channel to form a thin film (c). (d) Above a threshold driving speed, U*, the front is destabilized; the contact line, of speed V<U*, is unable to follow the rest of the interface, of speed U>U*. This mismatch gives rise to the formation of a fluid jet that releases drops periodically. Scale bars in b–d, 50, 10 and 10 μm, respectively.
Mentions: Our experimental results are summarized in Fig. 2, where we have probed the dynamics of a superconfined forced front coupled to a contact line using two demixed colloid–polymer phases. The interface thickness of our mixtures, ξ, which is representative of the order of magnitude of the contact-line slip length lD, is of the order of microns (ξ≈1.2 μm). This allows us to access the superconfined regime in micron-sized channels. As opposed to more traditional set-ups, the large interface scale in our experiments offers the additional advantage of directly visualizing the front and the contact line at the scale of thermal fluctuations using confocal microscopy2122. To favour the formation of a fluid front advancing on a solid, we injected two co-existing colloid–polymer phases into a rectangular microfluidic channel, displacing the more viscous polymer-rich, colloid-poor ‘gas' phase with the less viscous polymer-poor colloid-rich ‘liquid' phase (Fig. 2a). The low surface tension of our mixtures, γ=30 nN m−1, sets a comparatively small capillary length, , where Δρ is the density difference between the fluids and g is the acceleration due to gravity. At large enough velocities, the interface develops into a three-dimensional (3D) finger by virtue of the Saffman–Taylor instability22 (Fig. 2b). Because lC is of the order of the channel thickness H, the finger adheres to the bottom plate22 creating a thin film of thickness hf, which occupies roughly half of the channel thickness (Fig. 2c).

Bottom Line: Understanding fluid dynamics under extreme confinement, where device and intrinsic fluid length scales become comparable, is essential to successfully develop the coming generations of fluidic devices.Henceforth, we present a theory that quantifies our experiments in terms of the relevant interfacial length scale, which in our system is the intrinsic contact-line slip length.Our findings show that length-scale overlap can be used as a new fluid-control mechanism in strongly confined systems.

View Article: PubMed Central - PubMed

Affiliation: 1] Department of Chemistry, Physical and Theoretical Chemistry Laboratory, University of Oxford, South Parks Road, Oxford OX1 3QZ, UK [2] Department of Chemistry, Faculty of Science, Universiti Teknologi Malaysia, Johor Bahru, Johor 81310, Malaysia.

ABSTRACT
Understanding fluid dynamics under extreme confinement, where device and intrinsic fluid length scales become comparable, is essential to successfully develop the coming generations of fluidic devices. Here we report measurements of advancing fluid fronts in such a regime, which we dub superconfinement. We find that the strong coupling between contact-line friction and geometric confinement gives rise to a new stability regime where the maximum speed for a stable moving front exhibits a distinctive response to changes in the bounding geometry. Unstable fronts develop into drop-emitting jets controlled by thermal fluctuations. Numerical simulations reveal that the dynamics in superconfined systems is dominated by interfacial forces. Henceforth, we present a theory that quantifies our experiments in terms of the relevant interfacial length scale, which in our system is the intrinsic contact-line slip length. Our findings show that length-scale overlap can be used as a new fluid-control mechanism in strongly confined systems.

No MeSH data available.


Related in: MedlinePlus