Recently, Prof. Hans-Jürgen Butt, Member of EU Academy of Sciences, delivered an online lecture combining with his latest research findings, entitled "Wetting Phenomena: In Search for the Missing Force on Moving Drops" for GTIIT teachers and students.

Before the lecture, Prof. Hans-Jürgen Butt briefly introduced the 2021 Top Ten Emerging Technologies in Chemistry released by the International Union of Pure and Applied Chemistry (IUPAC): blockchain technology, semisynthetic life, superwettability, artificial humic matter from biomass, chemical synthesis of RNA and DNA, sonochemical coatings, chemiluminescence for biological use, sustainable production of ammonia, targeted protein degradation and single-cell metabolomics.

When the liquid and solid contact, the liquid adhesion layer will extend along the solid surface and form a contact angle, and when the contact angle is acute, it is called the liquid wetting solid. "The wetting phenomena have been studied for more than 200 years. Still, many fundamental questions are not understood. Therefore, it has huge potential to be studied further," Prof. Hans-Jürgen Butt said.

With the question "What is the friction of a sliding drop and how does it depend on the velocity", Prof. Hans-Jürgen Butt showed that contact angle hysteresis is essential for the resistance to sliding and also showed the lateral adhesion of a drop. With the comparison of the ideal and real outcome, he concluded that the surface of the solid from which the liquid has just receded has a surface tension higher than that of the original dry surface.

The friction depends on the nanoscopic process at the contact line, so Prof. Hans-Jürgen Butt further analyzed the factor influencing contact line, adaptation processes and charging of surfaces by moving water drops. In the end, he raised the conclusion, "Fundamental issues in dynamic wetting are still not understood. The practical outcome is far away from the predicting drop speed. Adaptation (spontaneous change of the surface in the presence of the liquid) can play a significant role. Electrostatic forces on insulting and low-permittivity surface are significant."

Prof. Hans-Jürgen Butt also showed some valuable questions remaining to be solved. "As you can see, humans are still far away from the essence of wetting phenomena. Since wetting is important for many applications such as coating, painting and flotation, we must keep on researching. This is not only my job, but also yours, the future scientists!"

Text: Liu Huayi, GTIIT News & Public Affairs

Photos: Yan Rundong, GTIIT News & Public Affairs