Basic Knowledge of Inkjet Droplet and Substrate Interaction
Interaction of Inkjet Droplets with Substrates
1. Ink Droplet Impact and State Change on Paper:
The next challenge to achieving good print quality is ensuring the ink behaves correctly on paper. In many ways, the ink properties required for effective ink droplet ejection (low viscosity, good component stability/solubility, poor wettability outside the print head) are completely opposite to the ink properties needed for optimal performance on paper.
Droplet Impact on Paper:
During the first ∼10 μsec of the drop's impact, the initial spherical droplet spreads out, bulges at the edges, and then recoils. Between 20–80 μs, the droplet reaches its static configuration, with a diameter roughly equivalent to the final dot size. It begins to shrink as the fluid penetrates and reaches the paper, followed by evaporation. The time for these processes largely depends on the type of medium and heating conditions. For example, under normal conditions, a 25 ng ink droplet may take 10-50 ms to fully penetrate porous paper. In contrast, this droplet may take 100-1000 ms to evaporate, meaning that most of the ink is absorbed before it evaporates on porous paper. The ratio of the final dot diameter to the initial droplet diameter is an important parameter, as it quantifies the droplet's coverage capability or "dot gain." Studies show that dot gain increases with Reynolds number, Weber number, and decreasing contact angle. Inkjet droplets typically experience a dot gain of 1.5–3 times.
2. Optical Density:
One of the main goals of printing is to generate vivid images on paper. The parameter used to quantify the darkness of the print is optical density (OD), calculated as OD = -log(I/Io), where Io is the incident light intensity on the print and I is the reflected light intensity. Alternatively, color coordinates (such as L and a) are used to quantify color prints, where L is brightness (lower L → darker), a represents the red/green balance of the color, and b represents the yellow/blue balance. A related quantity, chroma c = a + b, represents the overall intensity of the color; c = 0 indicates neutral gray, and high c indicates vivid colors.
Soluble dyes and dispersible pigments are used as colorants for inkjet inks. The choice of colorant type depends on the application and medium used. For example, on regular copy paper, light reflected from the ink film surface (not passing through the film) tends to scatter in multiple directions due to the irregular paper surface. Minimizing this reflection is key to achieving high black optical density. Therefore, highly structured carbon black pigments are commonly used for non-reflective matte effects on regular paper. In contrast, for color prints on regular paper, unwanted absorption (e.g., magenta dyes absorbing blue and red light) is amplified with each reflection of light through the ink film. Hence, dyes are sometimes more suitable for higher chroma on regular paper since their films typically exhibit less unwanted absorption and scattering compared to pigment films.
3. Ink Absorption and Ink Penetration Control:
Porosity of regular paper ranges from 1-10 μm. Therefore, it may be challenging to keep colorants near the surface of the paper. Various strategies have been adopted to retain colorants on or near the surface, such as using precipitating agents (e.g., acids, salts, and oppositely charged entities) on the paper. Additionally, increasing ink viscosity and reducing wettability (i.e., higher contact angle) may help slow the ink penetration into the medium. Simply adding more colorant to the ink can also improve optical density. However, these strategies must be balanced with medium pretreatment costs, the negative effects of slow ink penetration (longer drying times, severe bleeding), and challenges related to reliably ejecting inks with high solute content.
On glossy media (e.g., photo paper), surface reflection may focus away from the eye, making prints on glossy media typically have higher OD and chroma than prints on regular paper. Inks must be formulated to form smooth thin films on glossy media, which usually requires that the dyes remain soluble or pigments stay stable during the evaporation of water. Gloss uniformity at various colors and ink levels on glossy paper is also important. Dye-based inks are often more convenient for achieving good gloss and uniformity, although pigments with controlled particle size distributions can also achieve excellent gloss performance.
4. Image Durability – Light Fastness:
Over time, exposure to light and gaseous reactants (oxygen, ozone, etc.) can cause inkjet colorants to undergo photochemical reactions, leading to fading and shifts in the original hue. Pigments usually provide better image durability than dyes because the chromophores are bound together, offering some protection against harmful reagents and are more capable of dissipating photon energy. Extensive work has been done to produce slower fading dyes, such as by incorporating transition metal complexes into the chromophores. Output from modern inkjet printers has been proven to last for up to 100 years or longer.
5. Ink Bleeding and Smudging:
During printing, areas of one ink color may contact areas of another ink color. It is best to maintain coherent edges between colors, and prevent colors from “bleeding,” especially when darker ink flows into lighter ink. To prevent bleeding, the ink formulations are often designed so that the ingredients in black ink react with the ingredients in colored inks — for instance, black ink may contain dispersed pigments with carboxyl groups, while yellow ink may contain several weight percent of succinic acid, causing black pigment to lose stability and rapidly flocculate in the presence of yellow ink. In other cases, black colorants are designed to interact with the paper itself, preventing the colorant from diffusing laterally upon contact with the page. Furthermore, the surface tension differences between adjacent inks can drive flow via surface tension gradients (i.e., Marangoni flow), leading to strategies that reduce bleeding by using inks with low wettability, high surface tension, or both.
6. Paper Wrinkling and Curling:
When water penetrates paper, it causes fibers to expand and breaks the hydrogen bonds that hold the paper fibers together. Over short times (0.1-10 seconds), this may cause paper to “cockle,” i.e., localized curling of the initially flat sheet. A simple method to counteract cockling is to add less water on the printing platform. Over longer times (1 minute-7 days), solvents used in the ink may also evaporate. Usually, solvents on the outside of the fibers evaporate first, causing the hydrogen bonds between fibers to reform, while the solvent inside the fibers remains. As the solvent inside the fibers evaporates slowly, the fibers shrink but are unable to slide past each other, causing the sheet to deform. This deformation can be especially prominent if one side of the page is heavily printed while the other side is not, resulting in large-scale curling of the paper. Using ink with low volatility solvent can significantly reduce curling over the practical time scale (weeks to months).
7. Water Resistance and Abrasion Resistance:
Due to the solubility of inkjet colorants (especially dyes), inkjet prints have traditionally been prone to water damage. Recent developments in dye technology have made them more resistant to re-dissolution, with the use of dye "fixers" to improve water resistance. Generally, pigments are more resistant to water damage, though maintaining moderate surface charge on the pigments has been shown to provide excellent water resistance. Abrasion resistance has traditionally been a challenge for inkjet inks (especially pigment-based inks). Adding solution and dispersion polymers to inkjet inks helps improve abrasion resistance and fluorescent pen trailing effects.