Xiao Li Zhao

AFM for Preparing Si Masters in Soft Lithography

Atom Force Microscopy (AFM) can be employed to create surfaces in Si substrate with recessed features. The resulting patterns can serve as masters to make the required elastomeric stamps for soft lithography. Morphology analysis of patterned features on Si substrate and polydimethylsiloxane (PDMS) stamp by AFM imaging confirms that pattern can be successfully transferred from Si substrates to PDMS stamps. It is shown that this method for creating masters can be performed with an AFM, making this method particularly straightforward, economical and accessible to a large technical community that are provided with AFM for measurement.

Transient dynamic analysis of high power density gear transmission

In this paper, the dynamic problem of high power density gear transmission in transient state is studied based on flexible multi-body model. First, the flexible gear transmission multi-body model is built with the detailed formula of contact force and component modes synthesis method. Second, the transient time as dynamic external excitation is undertaken for high power density gear transmission, which has larger flexibility than others. Numerical and theoretical results confirm a significant influence of transient state on the dynamic force and deformation. Furthermore, the reasonable time of gear transmission transient state can be avoided the great impact force, reduce vibration and noise in gear system. Consequently, the transient time of gear transmission is an innegligible excitation factor for gear transmission.

Fabrication of composite structures based on microwrinkles

The composite structures were fabricated by wrinkle formation and optical lithography. Complex and ordered wrinkle patterns are spontaneously created on metal thin films thermally deposited onto elastomeric polymers owing to mismatch of thermal expansion. In the experiments, a thin film of Au with 20-50 nm thick was deposited by ion sputtering on the surface of polydimethylsiloxane (PDMS) substrates. To modulate wrinkle patterns regularly and uniformly, optical lithography was used to fabricate given structures in Silicon wafers, and these structures were transferred onto PDMS substrates by replica molding. In the process of wrinkle formation, the configuration on the PDMS substrates could effect or regulate wrinkle formation, and so the composite structures effectively integrate the lithographic and wrinkle patterns. These unique structures have potential application in optical devices, sensors and actuators.

Analysis and Evaluation of Interface Mechanics for Thin Films using the Method of Image

The evaluation of interface mechanics was very useful for the design of thin solid film. According to the method of image, interface stresses and deformations of coating-substrate system were gained. Using this mathematical calculation model, the indentation test data was carried out and the results were analysed. Based on the mechanical model and test, the critical state of interface stresses from elastic deformation to plastic yield was evaluated. From experimental and calculating results, interface shear stress and normal stress had different affecting level for the coating failure under the same surface load. And a fitting relationship equation for the two stresses was also gained to determining the failure value in designing. The method of this paper could be used for thin film interface mechanics design and evaluation.

Experimental Research on Influence of Temperature and Compression on Wrinkle Evolution of Thin Films on Compliant Substrates

Wrinkle evolution under temperature and compression was experimentally explored in a metal film deposited on a compliant polymer. After wrinkle formation, the samples undergo thermal annealing in different temperature circumstances. In the range of 75-150°C, the wavelength and amplitude of wrinkles became both smaller as the temperature increased, indicating that annealing effectively releases residual stress and promote material modification. In contrast, compression only changed wrinkle rearrangement, and did not alter the wavelength and amplitude. These results suggest that external stimulation can transform wrinkled surfaces. This work provides the guidance for service conditions of surface wrinkling.

Effect of Wrinkled Microstructure on Surface Wettability of Ginkgo Leaves

In this work, the effect of wrinkled microstructures on surface wettbility of ginkgo leaf was investigated. The upper and lower layers of the ginkgo leaf were comparatively measured by Scanning Electron Microscopy (SEM) for contrast of morphological characters. Measurement results show that the upper epidermis is mainly composed of regularly long-rangle stripes of cellular structures with gently smooth cell wall. On the other hand, the lower one is made up of chaotically isolated cellular structures with wrinkled cell wall. The comparative measurement of contact angle (CA) and roll-off angle in the upper and lower epidermis with deionized water was conducted. Wetting behaviors indicate that the lower has stronger hydrophobicity and weaker adhesion than the upper. Through theoretical analysis, the difference in wettability and adhesion was the result of isolated micro-bumps made by hierarchical wrinkles on the lower epidermis, leading to form solid-air-liquid interface between water and wrinkled surface. This research explains the difference of ginkgo leaf in wettability, and has important reference value in the design and preparation of the surface for the biomimetic materials.

Nanomechanical Properties and Surface Wettability of Carbon Films Prepared by Magnetron Sputtering

In this work, carbon films were deposited by magnetron sputtering on silicon substrate. The effect of sputtering time on the surface wettability and mechanical properties of carbon films was investigated. Contact angle measurement was used to analyse surface wettability, and the nanomechanical properties were characterized by nanoindentation. In experiments, the sputtering time was 45 min, 60 min, 75 min and 90 min. The measurement results show that the maximum film hardness was achieved for sputtering time 90 min, with a value of 2.34 GPa. Longer sputtering time resulted in preferable mechanical properties. It was analyzed that the size of the crystal grains on the substrate surface and thickness of the films were increased with the increment of sputtering time. The surface roughness decreased with the increase of sputtering time. Moreover, Youngs modulus increased with sputtering time and the maximum value was 16.94 GPa. The contact angle measurement results show that the prepared films take on the hydrophilicity. The minimum contact angle was achieved for sputtering time 45 min with a value of 54o.

Wettability of Polymeric Bionic Surface Replicated from Ginkgo Leaves

Ginkgo is one of the oldest extant seed plants through hundreds of millions of years of evolution. Ginkgo biloba has many unique properties and applications such as drug development and drinking tea. In recent years hydrophobic surfaces with bionic structures have attracted increasing interest for fundamental research and practical applications. As we all know, the Ginkgo leaf has remarkable texturing surface. In this manuscript, wettability of the bionic surface replicated from Ginkgo leaves was explored. The Ginkgo leaves were used as the original mold, from which microstructures were replicated into the surface of polydimethylsiloxane (PDMS). Compared with the topography of Ginkgo leaves, the topographical surface of PDMS was investigated by optical microscopy and scanning electron microscopy. By measuring the contact angle of polymeric bionic surfaces, there is the increase of ~20 degree than flat PDMS surfaces. Mechanical compression was applied on the polymeric bionic surfaces in one dimension, with the real-time measurement of the contact angle. The experimental results reveal that the wetting behavior of the surface can be reversibly tuned by applied mechanical stress, which induces the change in micro-scale topography. This research provides a guide for fabricating and tuning hydrophobic surfaces for various surface engineering applications.

Nanomechanical properties and surface wettability of TiO2 films prepared by magnetron sputtering

In this work, TiO2 films were deposited by magnetron sputtering on glass substrate. The effect of sputtering time on the surface wettability and mechanical properties of TiO2 films was investigated. Contact angle measurement was used to analyse surface wettability, and the nanomechanical properties were characterized by nanoindentation. In order to better understand the mechanical properties of prepared TiO2 films, surface morphology of the films was carried out by using atomic force microscopy (AFM). In experiments, the sputtering time varied from 30 to 150 min. The measurement results show that the maximum film hardness was achieved for sputtering time 150 min, with a value of 7.41 GPa. Longer sputtering time resulted in preferable mechanical properties. It was analyzed that the size of the crystal grains on the substrate surface and thickness of the films were increased with the increment of sputtering time, which can be explained the increasing trend of hardness with sputtering time between 30 min and 120 min. The surface roughness decreased with the increase of sputtering time. Moreover, Young’s modulus increased with sputtering time and the maximum value was 72.74 GPa. Hardness and modulus of elasticity of the prepared films were higher than these of glass substrate. This demonstrates that the mechanical properties of the prepared films reached the required for optical applications. The contact angle measurement results show that the prepared films have hydrophilic properties. The maximum contact angle was achieved for sputtering time 120 min with a value of 89°.

Microstructure evolution and FT-IR spectra of silicon induced by pulsed laser irradiation

Microstructures of silicon induced by pulsed laser irradiation were investigated. Microstructure evolution induced by two different laser configurations (lasers with wave length of 355 nm and 532 nm) was characterized by scanning electron microscopy. The measurement results show that periodic structure and villous structure were fabricated on silicon surface by varying the intensity of the applied lasers under atmospheric conditions. Regardless of the laser configurations used, it is found that the used laser intensity plays an important role in the microstructure formation. At low laser intensity, the main character of the irradiated surface is periodic ripples and the period of ripple structure is about 400 nm. Villus structure is the main feature formed with the increase of laser intensity. The mechanism of the villus formation induced by laser irradiation was proposed. Based on the experimental results, the clusters induced by laser irradiation recombine with each other and grow under the increase of laser intensity. During irradiation, the induced plasma expanded upward from the target surface. The clusters and ions in the plume collided with each other. Besides, the collision also happened between molecules of air and the clusters originated from target. As a result of numerous collisions, neutral molecules were dissociated into polarized nanoclusters. The induced nanoclusters redeposited on the target surface. The clusters recombine with each other and adhere to protuberance on the surface. This process results in the formation of cluster-assembled villous microstructure. The optical character of the irradiated samples was measured by FT-IR spectroscopy, and the transmission spectra were analyzed.