High-resolution, high-sensitivity measurements of adhesion forces have the potential to transform many applications. A fiber-tip microforce sensor (FTMS) newly developed by Prof. Yiping Wang and colleagues from the Guangdong-Hong Kong Joint Research Center for Fiber Optic Sensors at Shenzhen University, displays unprecedented levels of micronewton sensitivity to measure adhesion forces. Their findings are published in the latest issue of Multidisciplinary Institute for Digital Publishing (MDPI) – Biosensors.
What is adhesive strength?
Adhesive force describes how two interfaces are pulled and held together under general environmental conditions. For example, when water is poured into a glass, natural molecular mechanisms act between the surface of the glass and the water molecules.
The adhesion force is a collective effect constituted by the attractive electrostatic force, the chemical bonds between the hydrogen atoms, the van der Waals force and the capillary force.
The importance of understanding adhesion forces in depth has increased due to the rapid growth of nanotechnology. At nano-interfaces, the surface/volume ratio increases. A better knowledge of the physics underlying these nano-interfaces will help to develop better nanotechnologies.
What is adhesive force used for?
Adhesive forces are advantageously used in many industries. The molecular dynamics at the interface of two materials determines the critical structural behavior in civil and automotive engineering. Biomedicine uses adhesive forces in drug development. Materials science, nano, electromechanics and the aerospace industry study interface interactions to develop valuable tools.
What technology is used to measure adhesion forces?
Atomic force microscopy (AFM) is currently the main technique used to measure forces between interfaces. AFMs use a probe to scan the surface under study and record any deviation using a feedback loop.
Adhesion force detection was performed routinely using AFM. While slight variations of AFMs successfully revealed subtle adhesion forces, several drawbacks were encountered.
AFMs have mainly been used in specialized laboratories due to their large dimensions and complicated operational procedures.
In addition, the rigidity of the various materials used as micro-cantilever limits the detection sensitivity. AFM tips resonate or fluctuate in the presence of external forces. The detection threshold for subtle variations is low for weak forces, such as adhesion forces, and the AFM feedback mechanism was not successful.
Fiber optic based sensing schemes are another method used for force sensing at interfaces. Optical fibers provide shielding against electromagnetic interference and are compact. However, the elasticity associated with silica, a major component of fiber technology, has proven to be too high to measure adhesion forces, which are of the order of micro-newtons.
Fiber Tip Microforce Sensor
The research efforts of Professor Yiping Wang’s group have resulted in the development of a fiber-tip microforce sensor (FTMS) capable of detecting very weak forces. The objective of the FTMS was to measure the adhesion forces between interfaces at the micro and nanoscale.
Femtosecond laser two-photon polymerization (TPP) nanolithography is a commonly used 3D fabrication method in nanotechnology. The FTMS in Professor Wang’s lab was built with ultra-high precision using femtosecond TPP.
A micron-sized fixed-beam probe was 3D printed on single-mode fiber (SMF). As described in the MDPI biosensors publication, the spacing between the fixed-beam probe and the face of the fiber created a Fabry-Perot microcavity. This unique arrangement made the FTMS capable of detecting weak forces.
Any forces encountered by the tight beam probe will change the length of the Fabry-Perot cavity. This can be any non-contact force such as electric fields, or an external element can slightly push the fixed beam probe. The change in Fabry-Perot length acts as an optical signal amplifier to detect the weak force.
FTMS was used to detect adhesion forces to the surface of a hydrophilic hydrogel. The measured data was fitted with a finite element analysis model. The contributions of the electrostatic force and the capillary force have been studied. The FTMS was found to have a force sensitivity of 1.05 nm/μN and a force detection resolution of 19 nN. Another non-contact adhesion strength test of adult female hair has also been successfully measured by FTMS.
The results of Wang’s group present the FTMS as a flexible and compact microscale force sensor for adhesion forces acting on micro/nano scale structures.
Due to the high force sensitivity displayed by FTMS, it is positioned to have a large impact on the evaluation of adhesion forces. Besides the increased imaging potential in biological materials, the small size, flexible use and all-optical operation are attractive features in other industries. AFM microscopes can easily integrate the FTMS into its range of capabilities.
Zou, Mengqiang, Changrui Liao, Yanping Chen, Zongsong Gan, Shen Liu, Dejun Liu, Li Liu and Yiping Wang. (2022) Measurement of interfacial adhesion force with a 3D-printed fiber-tip Microforce sensor. Biosensors, 12, no. 8: 629. www.mdpi.com/2079-6374/12/8/629/htm