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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

Contact-line speed and interface configuration at the onset of entrainment.(a) Schematic of the slip velocity profile, u(x, z=0), close to the contact line. The fluid velocity decays at distances comparable to the contact-line slip length, lD. (b) Interface configuration at the critical speed in a 14-μm-thick channel, just before drop emission. (c) Schematic representation of the interface configuration close to the contact line (amplified from frame b). The shape of the interface is characterized by the local curvature κ=1/R, where the radius of curvature is measured at the tip of the front, the bending angle θ and the microscopic contact angle θm. The surface tension and Laplace pressure forces arising from the deformation of the interface are shown as light arrows. The contact line and front speeds, V and U, are indicated as dark arrows. (d) Critical curvature of the interface as a function of the channel thickness. The curvature was measured from the experimental interface profiles by fitting a circular arc at the tip of the front. Scale bar in b, 10 μm. Error bars correspond to the s.d. of the sample.
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f4: Contact-line speed and interface configuration at the onset of entrainment.(a) Schematic of the slip velocity profile, u(x, z=0), close to the contact line. The fluid velocity decays at distances comparable to the contact-line slip length, lD. (b) Interface configuration at the critical speed in a 14-μm-thick channel, just before drop emission. (c) Schematic representation of the interface configuration close to the contact line (amplified from frame b). The shape of the interface is characterized by the local curvature κ=1/R, where the radius of curvature is measured at the tip of the front, the bending angle θ and the microscopic contact angle θm. The surface tension and Laplace pressure forces arising from the deformation of the interface are shown as light arrows. The contact line and front speeds, V and U, are indicated as dark arrows. (d) Critical curvature of the interface as a function of the channel thickness. The curvature was measured from the experimental interface profiles by fitting a circular arc at the tip of the front. Scale bar in b, 10 μm. Error bars correspond to the s.d. of the sample.

Mentions: Using a lattice-Boltzmann numerical algorithm (see Methods), we integrated equations (1) and (2) for a front forced between two parallel plates (see Fig. 3). We resort to two-dimensional (2D) simulations that capture the relevant contact-line physics1219. The concentration and velocity fields are shown in Fig. 3a. The scale of the interface is clearly visible; to mimic the experimental conditions we fixed its value to be comparable to the separation between the plates, H. In such a regime, the simulations show that the velocity field close to the interface is homogeneous and does not vary appreciably over length scales comparable to the channel thickness. The slip velocity, which corresponds to the tangential component of the velocity field to the wall, deviates from the stick boundary condition over length scales comparable to the channel thickness (Figs 3b and 4a). This sharply contrasts with macroscopic systems, where such deviations occur over length scales much smaller than the channel thickness and are thus not expected to contribute dominantly to the overall fluid dynamics. In the superconfined regime this picture changes. Figure 3b shows the velocity profile close to the wall. The velocity peaks at the contact line and decays over a length scale lD, which is larger than the interface thickness (indicated in the figure by the width of the gradient of the concentration field). From the simulations, the ratio of the two length scales was found to be ξ/lD≈0.42. The local deviation from the stick boundary condition arises from the imbalance in the chemical potential caused by the deformation of the interface, which allows the contact line to move by virtue of diffusive transport1112. The chemical potential deviations from equilibrium decay over the same length scale lD as shown in Fig. 3c.


Superconfinement tailors fluid flow at microscales.

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

Contact-line speed and interface configuration at the onset of entrainment.(a) Schematic of the slip velocity profile, u(x, z=0), close to the contact line. The fluid velocity decays at distances comparable to the contact-line slip length, lD. (b) Interface configuration at the critical speed in a 14-μm-thick channel, just before drop emission. (c) Schematic representation of the interface configuration close to the contact line (amplified from frame b). The shape of the interface is characterized by the local curvature κ=1/R, where the radius of curvature is measured at the tip of the front, the bending angle θ and the microscopic contact angle θm. The surface tension and Laplace pressure forces arising from the deformation of the interface are shown as light arrows. The contact line and front speeds, V and U, are indicated as dark arrows. (d) Critical curvature of the interface as a function of the channel thickness. The curvature was measured from the experimental interface profiles by fitting a circular arc at the tip of the front. Scale bar in b, 10 μm. Error bars correspond to the s.d. of the sample.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f4: Contact-line speed and interface configuration at the onset of entrainment.(a) Schematic of the slip velocity profile, u(x, z=0), close to the contact line. The fluid velocity decays at distances comparable to the contact-line slip length, lD. (b) Interface configuration at the critical speed in a 14-μm-thick channel, just before drop emission. (c) Schematic representation of the interface configuration close to the contact line (amplified from frame b). The shape of the interface is characterized by the local curvature κ=1/R, where the radius of curvature is measured at the tip of the front, the bending angle θ and the microscopic contact angle θm. The surface tension and Laplace pressure forces arising from the deformation of the interface are shown as light arrows. The contact line and front speeds, V and U, are indicated as dark arrows. (d) Critical curvature of the interface as a function of the channel thickness. The curvature was measured from the experimental interface profiles by fitting a circular arc at the tip of the front. Scale bar in b, 10 μm. Error bars correspond to the s.d. of the sample.
Mentions: Using a lattice-Boltzmann numerical algorithm (see Methods), we integrated equations (1) and (2) for a front forced between two parallel plates (see Fig. 3). We resort to two-dimensional (2D) simulations that capture the relevant contact-line physics1219. The concentration and velocity fields are shown in Fig. 3a. The scale of the interface is clearly visible; to mimic the experimental conditions we fixed its value to be comparable to the separation between the plates, H. In such a regime, the simulations show that the velocity field close to the interface is homogeneous and does not vary appreciably over length scales comparable to the channel thickness. The slip velocity, which corresponds to the tangential component of the velocity field to the wall, deviates from the stick boundary condition over length scales comparable to the channel thickness (Figs 3b and 4a). This sharply contrasts with macroscopic systems, where such deviations occur over length scales much smaller than the channel thickness and are thus not expected to contribute dominantly to the overall fluid dynamics. In the superconfined regime this picture changes. Figure 3b shows the velocity profile close to the wall. The velocity peaks at the contact line and decays over a length scale lD, which is larger than the interface thickness (indicated in the figure by the width of the gradient of the concentration field). From the simulations, the ratio of the two length scales was found to be ξ/lD≈0.42. The local deviation from the stick boundary condition arises from the imbalance in the chemical potential caused by the deformation of the interface, which allows the contact line to move by virtue of diffusive transport1112. The chemical potential deviations from equilibrium decay over the same length scale lD as shown in Fig. 3c.

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