Different Evolution Behaviors of Adhesion Force with Relative Humidity at Silica/Silica and Silica/Graphene Interfaces Studied using Atomic Force Microscopy.

The influence of relative humidity (RH) on adhesion forces demands clarification. Adhesion forces at silica/silica and silica/graphene interfaces were measured on an atomic force microscope to investigate the evolution behaviors with RH and the contact time dependence at a certain RH. For the silica/silica interface, the adhesion force at a location by decreasing RH is independent of RH, but increases as a whole with RH both at a location and in the force volume mode by increasing RH. However, for the silica/graphene interface at a location, the adhesion force remains unchanged with RH as a whole by reducing RH and tends to decrease as a whole by increasing RH. In the force volume mode, the adhesion force at the silica/graphene interface is independent of RH. For the silica/silica interface, the adhesion force increases logarithmically with dwell time at a low RH and remains unchanged at a high RH. However, for the silica/graphene interface, the force is not dependent on RH at low and high RHs. The results can serve to further understand the mechanisms and behaviors of adhesion forces and promote the anti-adhesion design for small-scale silicon-based structures.

Influence of Lateral Movement on Level Behavior of Adhesion Force Measured Repeatedly by an Atomic Force Microscope (AFM) Colloid Probe in Dry Conditions.

An atomic force microscope (AFM) was operated to repeatedly measure the adhesion forces between a polystyrene colloid probe and a gold film, with and without lateral movement in dry conditions. Experimental results show that the adhesion force shows a level behavior without lateral movement and with a small scan distance: the data points are grouped into several levels, and the adhesion force jumps between different levels frequently. This was attributed to the fact that when the cantilever pulls off the sample, the contact area of the sample is not exactly the same between successive contacts and jumps randomly from one to another. Both lateral velocity and material wear have little influence on level behavior. However, with a medium scan distance, level behavior is observed only for some measurements, and adhesion forces are randomly distributed for the other measurements. With a large scan distance, adhesion forces are randomly distributed for all measurements. This was attributed to the fact that the cantilever pulls off the sample in many different contact areas on the scanning path for large distances. These results may help understand the influence of lateral movement and imply the contribution of asperities to adhesion force.

Direct Evidence of a Radius of Collection Area for Thin Film Flow in Liquid Bridge Formation by Repeated Contacts Using AFM.

A liquid bridge in a nanoscale gap is of considerable significance in lots of scientific and industrial fields. However, the formation mechanism is not well understood, leading to many contradictory experimental results. In this work, contact experiments were carried out between tipless cantilevers coated with potassium hydroxide and a silica surface on an atomic force microscope under different relative humidities (RHs). Results show that capillary condensation is dominant and thin film flow is difficult or even impossible at low RHs (31-37%). However, at high RHs (62-82%), thin film flow is dominant and materials were collected with a domed three-dimensional feature in the contact zone. There was a circle centered at the feature with a radius of collection area (can be as large as ∼23.6 µm), inside which all of the liquid seems to flow into the water bridge. The radius of collection area is used as direct evidence and as a parameter to reflect the efficiency of thin film flow. This fabrication technique of a domed feature may be viewed as a promising additive manufacturing in the microscale, and this work may also shed some light on the study of the controversial RH dependence of capillary force and other related research works.

Time-Dependent Dynamic Behaviors of a Confined Liquid To Achieve Tailored Adhesion Force with Repeated Contacts Revealed by Atomic Force Microscopy.

The adhesion forces between two silica surfaces were measured by using an atomic force microscope with different experimental parameters in air to investigate the dynamic behavior of a confined liquid. Results show that the adhesion force is time-dependent and increases at first sharply and then slightly with dwell time until saturation is reached, with a long equilibrium time. This behavior is well explained by a dynamic meniscus model, in which a liquid bridge grows gradually because of liquid film flow with a large viscosity. Also, the large viscosity was attributed to the formation of orthosilicic acid and subsequent polymerization. With repeated contacts, the liquid bridge changes into two droplets on both surfaces after separation. The liquid in both forms can be controlled to flow into or out of the contact zone by the experimental parameters to achieve tailored adhesion forces. If the liquid of previous contact remains in the contact zone, the adhesion force increases with repeated contacts and then reaches saturation, which can also be explained by the model qualitatively. However, if the liquid droplets vanish before the next contact, the adhesion force usually decreases or remains unchanged. More liquid will be collected with larger contact times. Meanwhile, the droplets remaining on the surfaces get smaller until they vanish without a contact. Moreover, both piezo velocity and scan distance can be used to control the proportion of contact time. In addition, a viscous force should be considered with a large retraction velocity. The changing trend and magnitude of adhesion force depend on the experimental parameters and their coupling effects. The results may facilitate the anti-adhesion design of small-scale silicon-based systems.