By Jan Gorgol
Humidity has huge effects on fiber and particle adhesion and micro-manipulating them in a broad variety of areas. Just a few of the areas where researchers have looked at such effects include manipulation of wood fibers , micro encapsulation of perfumes and drugs, effects of humidity on electrospun replacement tissue for damaged ear membranes and resistivity effects in carbon fiber polymer-matrix composites.
Micromanipulation of Wood Fibers and Humidity Effects
Grasping and releasing micro sized objects are two important and difficult processes in micro-assembly and micro-fabrication. Testing the mechanical properties of such micro and nanoscale materials has experienced a significant increase in importance with the recent development and industrialization of micro and nano devices with the enhancement of such testing devices interest in characterizing biological materials at micro- and nanoscales has also increased.
Professor Pasi Kallio’s Micro- and Nanosystems Research Group at Tampere University of Technology (TUT) are experts in developing such systems for manipulating, treating and characterizing micro- and nanoscale biological and industrial process samples. One part of their work focuses on manipulation and characterization of wood fibers which have a high importance in the paper making industry in seeking to lower production costs. 
By combining a GenRH-A humidity generator with a transparent box sealed around their robotic platform to provide a humidity controlled environment they could study the humidity related effects at 25, 55 and 85% RH on fiber adhesion and release in their micro-gripper systems. The test results clearly indicate that higher humidity values result in more difficulties during the releasing process when manipulating natural fibrous materials. As expected, humidity has a significant influence on the adhesion force between the fiber and micro-grippers. The release mechanism was considerably improved using a vacuum pressure approach combined with end-effector vibration, allowing release of adhered fibers from micro-grippers in higher humidity environments.
In general, the adhesion forces can be classified into three main categories including electrostatic forces, van der Waals forces and surface tension/capillary forces. It’s possible that at relative humidity conditions higher than a transition point, the tip-surface adhesive force may be the sum of the tip-surface interaction in water and the capillary force, due to capillary condensation. At low relative humidity levels, this surface adhesive force is dominated by interaction in water vapour alone, since the capillary condensation cannot be trigged from insufficient water. If the humidity value exceeds the transition point, the adhesive forces at the micro-gripper-fiber interface could be increased by capillary condensation. In the future, adding humidity generation control to AFM may help clarify the mechanism.