Later on this website, during my sojourn on ink, I mention (jovially) that it is just dirty water. This means, most of what we know about the surface tension of water also applies to ink.
Surface tension is a property/ability of the surface of a liquid to form a separating sheet at the boundary with surrounding material, may it be solid or gaseous. Through electrochemical potentials, certain chemical components of the liquid accumulate and line up on this surface layer, thus its chemical, as well as physical properties, differ from those inside the liquid.
Without surface tension, insects could not walk on water or one liquid sit on top of solid matter, like raindrops on the surface of a leaf shown in photo 1. The waxy surface of the leaf (succulent) is non-wettable, and the area where the water touches it contracts, minimising the contact area. You can observe that depending on the size and location of the drop, the contact angle differs. And of course, there are inconsistencies in the surface qualities of the leaf.
The surface tension of the water determines the form, style, and degree of contact with other materials and is the cause of capillary action.
Surface tension is the cause for liquids forming drops in free fall, sometimes perfect spheres. A good example is a pellet or shot tower where the shot for guns is made of lead, photo 2. The diameter of those spheres depends on a material’s characteristics and environmental conditions.
Drops sitting on a surface attain a shape depending on the characteristics of both, the liquid as well as the surface. The quality of a (mostly hard) surface, which describes its way of “welcoming” a liquid, is called wettability.
Photos 3 and 4 demonstrate two examples of wettability:
Photo 3 shows water on an angled leaf where size and gravity cause variations of the contact angle. Even the small drops still behave as expected. This is a non-wettable surface. An important point to be considered in the design of fountain pen feeds; the ink doesn’t come in drops.
Photo 4 shows water on stainless steel which had been polished some time ago and the wax has been washed off at places where the puddles have formed, and the contact angle is flatter.
These photos demonstrate the interaction of the attributes of the fluid and the solid matter. This will be explained more in-depth in the chapter Feeds made of Plastic. Further to that, there is the third partner in the surface tension trio, namely the surrounding gas which I have ignored because, under the normal living conditions of a fountain pen, its impact is insignificant.
I am not going to elaborate on the general aspects of physics and chemistry on surface tension. There are excellent articles on the net. I like to refer to Wikipedia.
Let’s look at the physics, as far as it concerns the feed and ink. In drawing 1, you see the above photos expressed in their physics terms. They describe what is called surface reaction, how a liquid reacts when it comes in contact with any other liquid or a gas or a solid. This behaviour is caused by the characteristics of the gas, liquid and solid corresponding with each other. The wettability is expressed by the contact angle θ, pronounced theta.
The upper sketch of drawing 1 describes the situation in photo 3, the dew-drop on the leaf. The more non-wettable a surface is (gas or solid), the more a drop contracts into a sphere, the larger the contact-angle θ is resulting in the reduction of the contact area; the two materials don’t like each other, they are hydrophobic.
Vice versa, as shown in the lower sketch of drawing 1, the more wettable, hydrophilic a surface is, the more a drop spreads itself onto the surface, and the smaller the contact angle θ is. This describes the situation in photo 4.
Be aware, the contact-angle θ is measured on the inside of the liquid. Some explanations of the contact-angle will show it as the exterior angle called the “wetting angle”. To confuse matters, the expressions are mixed up. Since we know about this now, we don’t worry about it.
Photo 5 shows both arrangements on a piece of weathered timber where the varnish has peeled off on the right side of the photo while still protecting the timber on the left. You can also observe the water pulled into the grooves of the wood grain and progressing along away from the puddle.
As shown in photo 3, the surface reaction also exists on vertical or sloped surfaces. This spreading or contracting of the liquid’s skin causes the rise or suppression of liquid between two plates with a narrow enough distance but also in pipes (round, square or rectangular) of small enough cross-section. This is called the capillary action, which I will describe in the next chapter on Capillaries.
For a bit of appetite wetting I include drawing 2, the two pairs of lines you can regard as parallel surfaces or tubes. The material of the left pair A is hydrophilic (small θ). The pair on the right B is hydrophobic (large θ). The container’s surface is neutral (θ = 90º). Thus the same liquid (grey colour) rises/lowers to different levels depending on the distance of the plates or tube diameter and the materials’ surface reactant attributes. Trust me.
Take a moment and evaluate the image. It is good to develop a sense for this, so you do not need to think about it later when we talk about the design of the feed.
In a fountain pen, we have at least three different materials in contact with the ink: the tank or cartridge, the feed and the nib. Therefore, their surface reaction behaviour will vary, sometimes to our advantage, sometimes not.
After this introduction on surface tension, we advance to the next chapter which expands more on Capillaries