Why is the refill of a marker pen used to write on a whiteboard made of fibers?

The sap in trees,which consists mainly of water in summer,rises in a system of capillaries of radius $r = 2.5 \times 10^{-5} \ m$. The surface tension of sap is $T = 7.28 \times 10^{-2} \ N/m$ and the angle of contact is $0^{\circ}$. Does surface tension alone account for the supply of water to the top of all trees?

In a capillary tube of radius '$R$',a straight thin metal wire of radius '$r$' is inserted symmetrically and one end of the combination is dipped vertically in water such that the lower end of the capillary and the thin wire are at the same level. If '$T$' is the surface tension of water and '$\rho$' is the density of water, then the rise of water in the capillary is:

According to Poiseuille's law,the pressure drop per unit length required to overcome viscous forces is $\Delta P = \frac{8 \eta v}{r^2}$,where $r$ is the radius of the cross-section,$v$ is the fluid velocity,and $\eta$ is the coefficient of viscosity. $A$ capillary tube of radius $a$ is dipped in a liquid of density $\rho$,surface tension $T$,and coefficient of viscosity $\eta$. The liquid starts rising in it so that its height $h(t)$ is a function of time $t$. The resulting rate of change of the momentum of the liquid column in the capillary (taking vertically up to be the positive direction and the contact angle to be close to $0^{\circ}$) is $-\pi a^2 \rho gh + F$. Then $F$ is ($g$ is the acceleration due to gravity):

It is not possible to write directly on blotting paper or newspaper with an ink pen.

When a capillary tube is immersed in water vertically,water rises to a height $h$ inside the tube. If the radius of another capillary tube is $\frac{1}{3}$ that of the previous,the height to which water will rise in this tube is: