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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

Drop production in standard and superconfined microfluidic set-ups.(a) Traditional microfluidic set-ups rely on drop geometry to trigger the formation of drops. (b) In superconfinement, the ability of a forced liquid front to cover a microchannel wall can be controlled by varying the thickness of the channel. Our experimental results (symbols) show that drops are produced above a critical driving velocity U*, which can be controlled by varying the thickness of the microchannel, H. The solid line corresponds to the theoretical prediction (see text). Speeds are measured in units of the capillary speed, , where γ is the interfacial tension and  is the mean viscosity of the fluids (see Methods for further information on the experimental set-up and data analysis).
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f1: Drop production in standard and superconfined microfluidic set-ups.(a) Traditional microfluidic set-ups rely on drop geometry to trigger the formation of drops. (b) In superconfinement, the ability of a forced liquid front to cover a microchannel wall can be controlled by varying the thickness of the channel. Our experimental results (symbols) show that drops are produced above a critical driving velocity U*, which can be controlled by varying the thickness of the microchannel, H. The solid line corresponds to the theoretical prediction (see text). Speeds are measured in units of the capillary speed, , where γ is the interfacial tension and is the mean viscosity of the fluids (see Methods for further information on the experimental set-up and data analysis).

Mentions: Current drop-based microfluidic devices rely on nonlinear channel geometries, such as T-junctions and flow focusing constrictions, to induce the pinch-off of droplets from a mother stream (Fig. 1a). A common trait of these set-ups is that they operate in the well-developed hydrodynamic limit. In such a case, the length scale of the confining devices is well above any microscopic length scale of the fluid. As a consequence, one expects that the dominant contributions to the fluid dynamics come from forces acting on the volume of the fluid, such as pressure gradients and viscous friction forces. The limit of sharp length-scale separation breaks down as one enters the nanofluidic regime9, or, equivalently, in microfluidic systems containing complex fluids, such as colloidal mixtures, liquid crystals and bacterial suspensions, where the ‘molecular' size can be of the order of microns. For such strongly confined systems one expects that the dynamics is increasingly dominated by the interaction with confining walls. Specifically, for fluid–fluid–solid systems the range of interfacial forces can be characterized in terms of a contact-line slip length, lD, which is an intrinsic length scale of the solid–fluid system101112. Contact-line slip affects the stability of moving fronts, as it determines the critical speed at which the contact line can move before it lags behind the rest of the fluid front13. Such an entrainment mechanism is determined by the small-scale motion of fluid molecules flowing past the solid near the contact line, which determines how fast it can move14. For large systems, the separation between lD and the typical length scale of the flow has been shown to affect the way in which solid–fluid interactions control the critical speed1516. This has helped to explain complex interface dynamics reported in a variety of experiments1617181920. However, little is known about how multiphase systems respond under extreme confinement, where the system size and the contact-line slip length become comparable.


Superconfinement tailors fluid flow at microscales.

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

Drop production in standard and superconfined microfluidic set-ups.(a) Traditional microfluidic set-ups rely on drop geometry to trigger the formation of drops. (b) In superconfinement, the ability of a forced liquid front to cover a microchannel wall can be controlled by varying the thickness of the channel. Our experimental results (symbols) show that drops are produced above a critical driving velocity U*, which can be controlled by varying the thickness of the microchannel, H. The solid line corresponds to the theoretical prediction (see text). Speeds are measured in units of the capillary speed, , where γ is the interfacial tension and  is the mean viscosity of the fluids (see Methods for further information on the experimental set-up and data analysis).
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f1: Drop production in standard and superconfined microfluidic set-ups.(a) Traditional microfluidic set-ups rely on drop geometry to trigger the formation of drops. (b) In superconfinement, the ability of a forced liquid front to cover a microchannel wall can be controlled by varying the thickness of the channel. Our experimental results (symbols) show that drops are produced above a critical driving velocity U*, which can be controlled by varying the thickness of the microchannel, H. The solid line corresponds to the theoretical prediction (see text). Speeds are measured in units of the capillary speed, , where γ is the interfacial tension and is the mean viscosity of the fluids (see Methods for further information on the experimental set-up and data analysis).
Mentions: Current drop-based microfluidic devices rely on nonlinear channel geometries, such as T-junctions and flow focusing constrictions, to induce the pinch-off of droplets from a mother stream (Fig. 1a). A common trait of these set-ups is that they operate in the well-developed hydrodynamic limit. In such a case, the length scale of the confining devices is well above any microscopic length scale of the fluid. As a consequence, one expects that the dominant contributions to the fluid dynamics come from forces acting on the volume of the fluid, such as pressure gradients and viscous friction forces. The limit of sharp length-scale separation breaks down as one enters the nanofluidic regime9, or, equivalently, in microfluidic systems containing complex fluids, such as colloidal mixtures, liquid crystals and bacterial suspensions, where the ‘molecular' size can be of the order of microns. For such strongly confined systems one expects that the dynamics is increasingly dominated by the interaction with confining walls. Specifically, for fluid–fluid–solid systems the range of interfacial forces can be characterized in terms of a contact-line slip length, lD, which is an intrinsic length scale of the solid–fluid system101112. Contact-line slip affects the stability of moving fronts, as it determines the critical speed at which the contact line can move before it lags behind the rest of the fluid front13. Such an entrainment mechanism is determined by the small-scale motion of fluid molecules flowing past the solid near the contact line, which determines how fast it can move14. For large systems, the separation between lD and the typical length scale of the flow has been shown to affect the way in which solid–fluid interactions control the critical speed1516. This has helped to explain complex interface dynamics reported in a variety of experiments1617181920. However, little is known about how multiphase systems respond under extreme confinement, where the system size and the contact-line slip length become comparable.

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