Surface tension and wetting – Experimental approaches and significance in inkjet printing

Surface tension of inkjet inks and the wettability of the printing substrate are important parameters influencing the final printing quality and process reliability. Surface tension and interfacial interactions can be explored with various tensiometry technologies: equilibrium and dynamic surface tension measurements can be utilised in ink formulation and development. In addition to simple surface tension measurements, critical micelle concentration (CMC) can be used to define the point when surface tension becomes independent of surfactant concentration, and interfacial rheology experiments can be used to provide information about interfacial stability. Since the printed image is created by ink droplets, contact angle measurement is a useful tool to demonstrate the ink-substrate interaction. Contact angle measurement also enables the definition of the printing substrate’s surface free energy.

Figure 1. Forces at an interface originate from the fact that the molecules interact differently with surrounding molecules at the interface than those in the bulk. These molecular interactions and interfacial phenomena can be studied with different tensiometry technologies. 

Figure 1. Forces at an interface originate from the fact that the molecules interact differently with surrounding molecules at the interface than those in the bulk. These molecular interactions and interfacial phenomena can be studied with different tensiometry technologies. 

Applying these techniques on real-life samples and processes demands us taking account factors causing contact angle hysteresis as well as the small size of the inkjet droplets. For example, it is well-known that both surface chemistry and roughness influence wetting, and this phenomenon can be understood further by correcting the contact angles for roughness.

Figure 2. Influence of surface roughness on wetting can be taken account in the wetting analysis.

Figure 2. Influence of surface roughness on wetting can be taken account in the wetting analysis.

Since the inkjet ink droplets are very small, penetrative and low viscosity, it sets a high demand on the surface properties of substrates. Contact angle analyses allow the study of droplet spreading and absorption via analysis of the drop dimensions and contact angles with time.

The typical droplet size used in contact angle measurements is between 1 and 10 microlitres. It has been shown that droplet volume has no significant influence on the contact angle with close to-ideal surfaces, such as clean quartz plates. The ideal surface is considered to be smooth, rigid, chemically homogeneous, insoluble, and non-reactive. However, contact angle hysteresis complicates this picture, and the larger the contact angle hysteresis is the greater impact droplet volume has on contact angle.

In recent years, the interest in picolitre droplets has increased due to emerging inkjet technology and the need to analyze small micropatterned and single fibre surfaces. Therefore, discussion about the effects of droplet size on contact angle has broadened to compare microlitre droplets to picolitre droplets. The initial contact angle of picolitre droplets has been demonstrated to correlate best with the contact angle of microlitre droplet. Due to fast evaporation of picolitre droplets, the contact angle decreases as a function of time reaching the receding angle.

Figure 3. Use of a high speed camera enables the measurement of initial pico­litre drop contact angle right after the drop hits the surface and subsequent fast spreading and absorption phenomena.

Figure 3. Use of a high speed camera enables the measurement of initial pico­litre drop contact angle right after the drop hits the surface and subsequent fast spreading and absorption phenomena.

I look forward to discussing this with you further in Lausanne.

Dr. Maiju Pöysti
Product Manager, Attension and KSV NIMA
Biolin Scientific