Hydrodynamics of Writing with Ink

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Original entry: Tamas Szalay (APPHY225 2012)

"Hydrodynamics of Writing with Ink"

Jungchul Kim, Myoung-Woon Moon, Kwang-Ryeol Lee, L. Mahadevan, and Ho-Young Kim

Phys. Rev. Lett., 107, 264501 (2011)


In this paper, the authors examine the hydrodynamics of writing with ink on paper with a capillary source, eg. a fountain pen. The experiment is performed with a model system of superhydrophilic silicon micropillars to create the rough surface, and a simple glass capillary tube with various solutions inside to simulate the pen. In this model system, they calculate the expected rate of spreading for a stationary pen, and use this to derive the line width for a moving pen, which they then compare to experimental results. The analysis is then extended to a system using actual ink and paper.

Soft matter keywords

Wetting, Laplace pressure, surface tension, spreading, capillary

Physics of Writing

The spreading force caused by the energetic gain of wetting the surface has to be balanced with the viscous shear force holding it back. Letting <math>f</math> be the roughness (total surface area over projected area, <math>f \ge 1</math>), the effective driving force (as a function of r) due to the surface energies is <math>F_d = 2 \pi \gamma (f-1) r</math>

This can be balanced with the resisting force due to viscous shear, which scales as <math>F_r \sim \mu U (r^2 - R^2)f/h</math>, where <math>R</math> is the radius of the droplet-surface contact (roughly the radius of the tube itself), and <math>U = \frac{dr}{dt}</math>. Solving and integrating for <math>U</math> (at late times and small tube radius) gives:

<math>r \sim (\phi \frac{\gamma}{\mu}h)^{1/2}t^{1/2}</math>

with <math>\phi \equiv \frac{(f-1)}{f}</math>.

One of the key considerations for spreading, however, is the depletion of the liquid from the pen/tube as time passes. The Laplace pressure inside the capillary sets up an equilibrium rise height, and as the liquid flows out of the tube, this difference sets up a pressure drop <math>\Delta p_t = p_0 - p_t = 2\gamma/R - \rho g H</math>, where <math>H</math> is the difference in height from equilibrium. There is a minimum roughness for the liquid to spread on its own, from balancing the driving force to get past the tube radius and the resulting pressure drop:

<math>f_{min} \approx 1 + 2h/R - Hh/l_c^2 \in (1.04-1.07)</math>

where the latter numbers are for their particular conditions. The important thing to note is that for a smooth substrate, the liquid does not spread beyond a small radius, due to the additional Laplace pressure that gets set up with the droplet spreading from the capillary tube, as in the figure.

Experimental Results