Hele-Shaw flow

Hele-Shaw flow is defined as flow taking place between two parallel flat plates separated by a narrow gap satisfying certain conditions, named after Henry Selby Hele-Shaw, who studied the problem in 1898.^[1][2] Various problems in fluid mechanics can be approximated to Hele-Shaw flows and thus the research of these flows is of importance. Approximation to Hele-Shaw flow is specifically important to micro-flows. This is due to manufacturing techniques, which creates shallow planar configurations, and the typically low Reynolds numbers of micro-flows.

The conditions that needs to be satisfied are

${\displaystyle {\frac {h}{l}}\ll 1,\qquad {\frac {Uh}{\nu }}{\frac {h}{l}}\ll 1}$

where ${\displaystyle h}$ is the gap width between the plates, ${\displaystyle U}$ is the characteristic velocity scale, ${\displaystyle l}$ is the characteristic length scale in directions parallel to the plate and ${\displaystyle \nu }$ is the kinematic viscosity. Specifically, the Reynolds number ${\displaystyle Re=Uh/\nu }$ need not always be small, but can be order unity or greater as long as it satisfies the condition ${\displaystyle Re(h/l)\ll 1.}$ In terms of the Reynolds number ${\displaystyle Re_{l}=Ul/\nu }$ based on ${\displaystyle l}$, the condition becomes ${\displaystyle Re_{l}(h/l)^{2}\ll 1.}$

The governing equation of Hele-Shaw flows is identical to that of the inviscid potential flow and to the flow of fluid through a porous medium (Darcy's law). It thus permits visualization of this kind of flow in two dimensions.^[3][4][5]

Let ${\displaystyle x}$, ${\displaystyle y}$ be the directions parallel to the flat plates, and ${\displaystyle z}$ the perpendicular direction, with ${\displaystyle h}$ being the gap between the plates (at ${\displaystyle z=0,h}$) and ${\displaystyle l}$ be the relevant characteristic length scale in the ${\displaystyle xy}$-directions. Under the limits mentioned above, the incompressible Navier–Stokes equations, in the first approximation becomes^[6]

${\displaystyle {\begin{aligned}{\frac {\partial p}{\partial x}}=\mu {\frac {\partial ^{2}v_{x}}{\partial z^{2}}},\quad {\frac {\partial p}{\partial y}}&=\mu {\frac {\partial ^{2}v_{y}}{\partial z^{2}}},\quad {\frac {\partial p}{\partial z}}=0,\\{\frac {\partial v_{x}}{\partial x}}+{\frac {\partial v_{y}}{\partial y}}+{\frac {\partial v_{z}}{\partial z}}&=0,\\\end{aligned}}}$

where ${\displaystyle \mu }$ is the viscosity. These equations are similar to boundary layer equations, except that there are no non-linear terms. In the first approximation, we then have, after imposing the non-slip boundary conditions at ${\displaystyle z=0,h}$,

${\displaystyle {\begin{aligned}p&=p(x,y),\\v_{x}&=-{\frac {1}{2\mu }}{\frac {\partial p}{\partial x}}z(h-z),\\v_{y}&=-{\frac {1}{2\mu }}{\frac {\partial p}{\partial y}}z(h-z)\end{aligned}}}$

The equation for ${\displaystyle p}$ is obtained from the continuity equation. Integrating the continuity equation from across the channel and imposing no-penetration boundary conditions at the walls, we have

${\displaystyle \int _{0}^{h}\left({\frac {\partial v_{x}}{\partial x}}+{\frac {\partial v_{y}}{\partial y}}\right)dz=0,}$

which leads to the Laplace Equation:

${\displaystyle {\frac {\partial ^{2}p}{\partial x^{2}}}+{\frac {\partial ^{2}p}{\partial y^{2}}}=0.}$

This equation is supplemented by appropriate boundary conditions. For example, no-penetration boundary conditions on the side walls become: ${\displaystyle {\mathbf {\nabla } }p\cdot \mathbf {n} =0}$, where ${\displaystyle \mathbf {n} }$ is a unit vector perpendicular to the side wall (note that on the side walls, non-slip boundary conditions cannot be imposed). The boundaries may also be regions exposed to constant pressure in which case a Dirichlet boundary condition for ${\displaystyle p}$ is appropriate. Similarly, periodic boundary conditions can also be used. It can also be noted that the vertical velocity component in the first approximation is

${\displaystyle v_{z}=0}$

that follows from the continuity equation. While the velocity magnitude ${\displaystyle {\sqrt {v_{x}^{2}+v_{y}^{2}}}}$ varies in the ${\displaystyle z}$ direction, the velocity-vector direction ${\displaystyle \tan ^{-1}(v_{y}/v_{x})}$ is independent of ${\displaystyle z}$ direction, that is to say, streamline patterns at each level are similar. The vorticity vector ${\displaystyle {\boldsymbol {\omega }}}$ has the components^[6]

${\displaystyle \omega _{x}={\frac {1}{2\mu }}{\frac {\partial p}{\partial y}}(h-2z),\quad \omega _{y}=-{\frac {1}{2\mu }}{\frac {\partial p}{\partial x}}(h-2z),\quad \omega _{z}=0.}$

Since ${\displaystyle \omega _{z}=0}$, the streamline patterns in the ${\displaystyle xy}$-plane thus correspond to potential flow (irrotational flow). Unlike potential flow, here the circulation ${\displaystyle \Gamma }$ around any closed contour ${\displaystyle C}$ (parallel to the ${\displaystyle xy}$-plane), whether it encloses a solid object or not, is zero,

${\displaystyle \Gamma =\oint _{C}v_{x}dx+v_{y}dy=-{\frac {1}{2\mu }}z(h-z)\oint _{C}\left({\frac {\partial p}{\partial x}}dx+{\frac {\partial p}{\partial y}}dy\right)=0}$

where the last integral is set to zero because ${\displaystyle p}$ is a single-valued function and the integration is done over a closed contour.

In a Hele-Shaw channel, one can define the depth-averaged version of any physical quantity, say ${\displaystyle \varphi }$ by

${\displaystyle \langle \varphi \rangle \equiv {\frac {1}{h}}\int _{0}^{h}\varphi dz.}$

Then the two-dimensional depth-averaged velocity vector ${\displaystyle \mathbf {u} \equiv \langle \mathbf {v} _{xy}\rangle }$, where ${\displaystyle \mathbf {v} _{xy}=(v_{x},v_{y})}$, satisfies the Darcy's law,

${\displaystyle -{\frac {12\mu }{h^{2}}}\mathbf {u} =\nabla p\quad {\text{with}}\quad \nabla \cdot \mathbf {u} =0.}$

Further, ${\displaystyle \langle {\boldsymbol {\omega }}\rangle =0.}$

The term Hele-Shaw cell is commonly used for cases in which a fluid is injected into the shallow geometry from above or below the geometry, and when the fluid is bounded by another liquid or gas.^[7] For such flows the boundary conditions are defined by pressures and surface tensions.

• Diffusion-limited aggregation
• Lubrication theory
• Thin-film equation
• Hele-Shaw clutch