2001
Xie, Xuming
Tanveer, Saleh
This dissertation concerns existence of a steadily translating
finger solution in a Hele-Shaw cell for small but non-zero
surface tension ($\epsilon^2 $).Though there are numerous
numerical and formal asymptotic results for this problem, we
know of no mathematically rigorous results to date. By showing
equivalence of the problem to a certain nonlinear integro-differential
equation in a complex plane strip, we show that
analytic symmetric finger solutions exist for sufficiently
small surface tension for a discrete set of values of $(\lambda
-\frac{1}{2})/\epsilon^{\frac{4}{3}}$, where $\lambda$ is the
relative finger width. The methodology consists of proving
existence and uniqueness of analytic solutions for a weaker
half strip problem for any $\lambda$ where the symmetric condition
is relaxed. This is followed by analysis near appropriate
complex turning points that determine exponentially small terms
in $\epsilon$,as $\epsilon \to 0^+$. We prove that for
$\lambda-\frac{1}{2}$ small the symmetry condition is satisfied
if only if $(\lambda -\frac{1}{2})/\epsilon^{\frac{4}{3}} $
attains a discrete set of values. When the symmetry condition is
satisfied,the weaker half strip problem is shown to be
equivalent to the original steady finger problem.