48 research outputs found
Dipolar thermocapillary motor and swimmer
We present a theoretical description and an experimental realization of a
thermocapillary dipole induced in a Hele-Shaw cell under a steady temperature
gradient. We demonstrate experimentally how several dipoles can be superposed
in order to create various 2D flow patterns, and how a confined dipole can act
as a thermocapillary motor for driving fluids in microfluidic circuits. In
addition, we show how the principles behind the thermocapillary dipole can be
applied in order to drive thermocapillary swimmers on fluid-liquid interfaces
Flow of Power-Law Liquids in a Hele-Shaw Cell Driven by Non-Uniform Electroosmotic Slip in the Case of Strong Depletion
We analyze flow of non-Newtonian fluids in a Hele-Shaw cell, subjected to
spatially non-uniform electroosmotic slip. Motivated by their potential use for
increasing the characteristic pressure fields, we specifically focus on
power-law fluids with wall depletion properties. We derive a p-Poisson equation
governing the pressure field, as well as a set of linearized equations
representing its asymptotic approximation for weakly non-Newtonian behavior. To
investigate the effect of non-Newtonian properties on the resulting fluidic
pressure and velocity, we consider several configurations in one- and
two-dimensions, and calculate both exact and approximate solutions. We show
that the asymptotic approximation is in good agreement with exact solutions
even for fluids with significant non-Newtonian behavior, allowing its use in
the analysis and design of microfluidic systems involving electro-kinetic
transport of such fluids.Comment: 20 pages, 7 figure
Elastic deformations driven by non-uniform lubrication flows
The ability to create dynamic deformations of micron-sized structures is
relevant to a wide variety of applications such as adaptable optics, soft
robotics, and reconfigurable microfluidic devices. In this work we examine
non-uniform lubrication flow as a mechanism to create complex deformation
fields in an elastic plate. We consider a Kirchoff-Love elasticity model for
the plate and Hele-Shaw flow in a narrow gap between the plate and a parallel
rigid surface. Based on linearization of the Reynolds equation, we obtain a
governing equation which relates elastic deformations to gradients in
non-homogenous physical properties of the fluid (e.g. body forces, viscosity,
and slip velocity). We then focus on a specific case of non-uniform
Helmholtz-Smoluchowski electroosmotic slip velocity, and provide a method for
determining the zeta-potential distribution necessary to generate arbitrary
static and quasi-static deformations of the elastic plate. Extending the
problem to time-dependent solutions, we analyze transient effects on
asymptotically static solutions, and finally provide a closed form solution for
a Green's function for time periodic actuations.Comment: 25 JFM pages, 10 figure
Real-Time Monitoring of Fluorescence in situ Hybridization (FISH) Kinetics
We present a novel method for real-time monitoring and kinetic analysis of
fluorescence in situ hybridization (FISH). We implement the method using a
vertical microfluidic probe containing a microstructure designed for rapid
switching between a probe solution and a non-fluorescent imaging buffer. The
FISH signal is monitored in real time during the imaging buffer wash, during
which signal associated with unbound probes is removed. We provide a
theoretical description of the method as well as a demonstration of its
applicability using a model system of centromeric probes (Cen17). We
demonstrate the applicability of the method for the characterization of FISH
kinetics under conditions of varying probe concentration, destabilizing agent
(formamide) content, volume exclusion agent (dextran sulfate) content, and
ionic strength. We show that our method can be used to investigate the effect
of each of these variables and provide insight into processes affecting in situ
hybridization, facilitating the design of new assays.Comment: 15 pages, 4 figure
Fluidic Shaping of Optical Components
Current methods for fabricating lenses rely on mechanical processing of the
lens or mold, such as grinding, machining, and polishing. The complexity of
these fabrication processes and the required specialized equipment prohibit
rapid prototyping of optical components. This work presents a simple method,
based on free-energy minimization of liquid volumes, which allows to quickly
shape curable liquids into a wide range of spherical and aspherical optical
components, without the need for any mechanical processing. After the desired
shape is obtained, the liquid can be cured to produce a solid object with
nanometric surface quality. We provide a theoretical model that accurately
predicts the shape of the optical components, and demonstrate rapid fabrication
of all types of spherical lenses (convex, concave, meniscus), cylindrical
lenses, bifocal lenses, toroidal lenses, doublet lenses and aspheric lenses.
The method is inexpensive and can be implemented using a variety of curable
liquids with different optical and mechanical properties. In addition, the
method is scale-invariant and can be used to produce even very large optical
components, without a significant increase in fabrication time. We believe that
the ability to easily and rapidly create high-quality optics, without the need
for complex and expensive infrastructure, will provide researchers with new
affordable tools for fabricating and testing optical designs
Electroosmotic flow in Hele-Shaw configurations with non-uniform surface charge
We present an analytical study, validated by numerical simulations, of
electroosmotic flow in a Hele-Shaw cell with non-uniform surface charge
patterning. Applying the lubrication approximation and assuming thin electric
double layer, we obtain a pair of uncoupled Poisson equations which relate the
pressure and the stream function, respectively, to gradients in the zeta
potential distribution parallel and perpendicular to the applied electric
field. We solve the governing equations for the fundamental case of a disk with
uniform zeta potential and show that the flow-field in the outer region takes
the form of a pure dipole. We illustrate the ability to generate complex
flow-fields around smooth convex regions by superposition of such disks with
uniform zeta potential and a uniform pressure driven flow. This method may be
useful for future on-chip devices, allowing flow control without the need for
mechanical components.Comment: Accepted to Physics of Fluid
Elastohydrodynamics of a pre-stretched finite elastic sheet lubricated by a thin viscous film with application to microfluidic soft actuators
The interaction of a thin viscous film with an elastic sheet results in
coupling of pressure and deformation, which can be utilized as an actuation
mechanism for surface deformations in a wide range of applications, including
microfluidics, optics, and soft robotics. Implementation of such configurations
inherently takes place over finite domains and often requires some
pre-stretching of the sheet. Under the assumptions of strong pre-stretching and
small deformations of the lubricated elastic sheet, we use the linearized
Reynolds and Foppl-von Karman equations to derive closed-form analytical
solutions describing the deformation in a finite domain due to external forces,
accounting for both bending and tension effects. We provide a closed-form
solution for the case of a square-shaped actuation region and present the
effect of pre-stretching on the dynamics of the deformation. We further present
the dependence of the deformation magnitude and timescale on the spatial
wavenumber, as well as the transition between stretching- and bending-dominant
regimes. We also demonstrate the effect of spatial discretization of the
forcing (representing practical actuation elements) on the achievable
resolution of the deformation. Extending the problem to an axisymmetric domain,
we investigate the effects arising from nonlinearity of the Reynolds and
Foppl-von Karman equations and present the deformation behavior as it becomes
comparable to the initial film thickness and dependent on the induced tension.
These results set the theoretical foundation for implementation of microfluidic
soft actuators based on elastohydrodynanmics.Comment: 19 pages, 9 figure
Viscous-elastic dynamics of power-law fluids within an elastic cylinder
In a wide range of applications, microfluidic channels are implemented in
soft substrates. In such configurations, where fluidic inertia and
compressibility are negligible, the propagation of fluids in channels is
governed by a balance between fluid viscosity and elasticity of the surrounding
solid. The viscous-elastic interactions between elastic substrates and
non-Newtonian fluids are particularly of interest due to the dependence of
viscosity on the state of the system. In this work, we study the
fluid-structure interaction dynamics between an incompressible non-Newtonian
fluid and a slender linearly elastic cylinder under the creeping flow regime.
Considering power-law fluids and applying the thin shell approximation for the
elastic cylinder, we obtain a non-homogeneous p-Laplacian equation governing
the viscous-elastic dynamics. We present exact solutions for the pressure and
deformation fields for various initial and boundary conditions for both
shear-thinning and shear-thickening fluids. We show that in contrast to Stokes'
problem where a compactly supported front is obtained for shear-thickening
fluids, here the role of viscosity is inversed and such fronts are obtained for
shear-thinning fluids. Furthermore, we demonstrate that for the case of a step
in inlet pressure, the propagation rate of the front has a
dependence on time (), suggesting the ability to indirectly measure the
power-law index () of shear-thinning liquids through measurements of elastic
deformation.Comment: 27 pages, 5 figure
Fluidic Shaping of Freeform Optical Components
Freeform optical components offer significant compactization of multi-lens
systems, as well as advanced manipulation of light that is not possible with
traditional systems. However, their fabrication relies on machining processes
that are complex, time-consuming, and incompatible with rapid prototyping. This
work presents the ability to shape liquid volumes and solidify them into
desired freeform components, enabling rapid freeform prototyping with high
surface quality. The method is based on controlling the minimum energy state of
the interface between a curable optical liquid and an immersion liquid, by
dictating a geometrical boundary constraint. The boundary shape is modeled as a
cylinder whose arbitrary height is expressed as a Fourier series, allowing for
an analytical solution of the resulting freeform surface as a sum of
Fourier-Bessel functions. Each of these functions represents a different basic
mode, whose superposition creates complex topographies. This solution allows
deterministic design of freeform surfaces by controlling three key parameters -
the volume of the optical liquid, the density of the immersion liquid, and the
shape of the bounding frame. The paper describes a complete workflow for rapid
prototyping of such components, and demonstrates the fabrication of a 35 mm
diameter freeform component with sub-nanometer surface roughness within
minutes.Comment: 4 figures, 14 pages (not including references
Extraction of electrokinetically separated analytes with on-demand encapsulation
Microchip electrokinetic methods are capable of increasing the sensitivity of
molecular assays by enriching and purifying target analytes. However, their use
is currently limited to assays that can be performed under a high external
electric field, as spatial separation and focusing is lost when the electric
field is removed. We present a novel method that uses two-phase encapsulation
to overcome this limitation. The method uses passive filling and pinning of an
oil phase in hydrophobic channels to encapsulate electrokinetically separated
and focused analytes with a brief pressure pulse. The resulting encapsulated
sample droplet maintains its concentration over long periods of time without
requiring an electric field and can be manipulated for further analysis, either
on- or off- chip. We demonstrate the method by encapsulating DNA
oligonucleotides in a 240 pL aqueous segment after isotachophoresis (ITP)
focusing, and show that the concentration remains at 60% of the initial value
for tens of minutes, a 22-fold increase over free diffusion after 20 minutes.
Furthermore, we demonstrate manipulation of a single droplet by selectively
encapsulating amplicon after ITP purification from a polymerase chain reaction
(PCR) mix, and performing parallel off-chip detection reactions using the
droplet. We provide geometrical design guidelines for devices implementing the
encapsulation method, and show how the method can be scaled to multiple analyte
zones