We investigate crack formation in deposition films from drying colloidal suspension drops, by varying the roughness and texture of the substrate. The experimental results indicate that the crack number or crack spacing presents a general dependence on the substrate roughness, despite the orientation of the substrate textures. Interestingly, the crack spacing decreases with the increase of the roughness. Two possible mechanisms are proposed to understand the dependence of the cracks on roughness. Firstly, the concentration reduction of the drying suspension due to collecting colloidal particles from the substrate textures decreases the crack spacing. Secondly, stress concentration resulting from the defects (the notches in textures) in the dried deposition enhances crack formation. However, a quantitative estimation by the calculation of the stress concentrating factors reveals that the notch of the substrate textures dominates crack variation. The results here bring forth a practical method for controlling the crack orientation and suppression, and a potential application to crack-free coatings, films and paintings during the drying of complex fluids.

Drying the colloidal suspension leads to the versatile crack patterns, where the microstructure formed during the drying process determines crack patterns. Here, the polymer is introduced into a silica suspension to tune the crack arrangement in the drying deposit. Five dominated types of crack patterns are defined by varying the ratio of nanoparticles to polymer. Two phase separation processes are proposed to explain the spatial characteristics of the crack patterns: the depletion-induced phase separation of particles and polymer in their mixture; and the separation of water and particle (or polymer) clusters by the water drainage.

The Brownian motion of colloidal particles in quasi-two-dimensional (q2D) confinement displays a distinct kinetic character from that in bulk. Here we experimentally report dynamic coupling motion of Brownian particles in a relatively long process (∼100 h), which displays a quasi-equilibrium state in the q2D system. In the quasi-equilibrium state, the q2D confinement results in the coupling of particle motions, which slowly damps the motion and interaction of particles until the final equilibrium state is reached. The process of approaching the equilibrium is a random relaxation of a many-body interaction system of Brownian particles. As the relaxation proceeds for ∼100 h, the system reaches the equilibrium state in which the energy gained by the particles from the stochastic collision in the whole system is counteracted by the dissipative energy resulting from the collision. The relaxation time of this stochastic q2D system is 17.7 h. The theory is developed to explain coupling motions of Brownian particles in q2D confinement.

The morphology of nanospheres is crucial for designing the nanofabrication in the nanosphere lithography. Here, by plasma etching, the controllable tailoring of the nanosphere is realized and its morphology dependence on the initial shape, microscopic roughness, and the etching conditions is investigated quantitatively. The results show that the shape evolution strongly depends on the etching gas, power, and process duration. Particularly, the aspect ratio (diameter/height) significantly increases with violent etching, turning the spherical shape into tiny ellipsoidal nanoparticles. The findings are practical to the protocol of non-uniform etching of nanoobjects and provide the useful design tool for the device fabrication at nanoscale.

Superoleophobicity developed by creating roughness at multiple scales or lowering the surface energy has drawn extensive attention for technological applications. Currently, most methods for fabrication of superoleophobic surfaces employ either the complicated manufacture of micro/nano structures or delicate chemical decorations. Here, the sharp wetting transition from an oleophilic state to an oleophobic one has been practically realized, and the mechanism successfully interpreted. The underwater superoleophobic surfaces are newly realized and successfully controlled by subtly tuning the surface morphology of the alloy by practical corrosion plus a naturally obtained, stable, high-energy inorganic coating layer which induces the sharp wetting transition from oleophilic state to oleophobic state. The contact angle (CA) variations are quantitatively analyzed based on the wetting model by employing a roughness geometrical structure and a corrosion mechanism, which agree well with the measured results. Additionally, the dependence of the roughness on the corrosion procedure is modeled microscopically. Our method uncovers a facile fabrication protocol for optimum underwater superoleophobic surfaces by modifying the microstructures of alloy surfaces in the manufacturing process, which furthermore offers significant insights into the design and creation of other novel antifouling materials.

Curved cracks widely exist in nanoparticle (NP) deposition produced by drying colloidal suspension. Circular cracks, for example, initiate and propagate along a circular trajectory. One feasible theoretical explanation of a circular crack is the Xia-Hutchinson model, in which a preexisting track (flaw loop) in the film is necessary for initiating and propagating the crack on the circular path. Here, we report the first experimental evidence of dried deposition to support this model. Our results indicate that cracks along the circular trajectory can surprisingly "pass" across a 180 mu m air gap. Moreover, two arc-path cracks originate in different areas and propagate to meet, forming a circular trajectory. These unexpected crack initiation and propagation indicate that the crack propagates alone the "preformed" track, experimentally confirming the hypothesis proposed by the Xia-Hutchinson model. The transition of the circular crack to a radial one indicates that the deposition microstructure is the dominant factor for the crack formation.

The fracture mechanics was usually employed to explain the crack propagation in the deposition produced by drying colloidal suspension. However, more complex than conventional fracture, those cracks periodically distribute and make up a unique pattern. Inspired by the concept of spinodal decomposition, here we develop the theory to illustrate the possible mechanism of the spatial arrangement of the cracks. It indicates that before the cracks develop and propagate in the deposition under the law of fracture mechanics, the periodically distributed flaws are generated by the phase separation of colloidal clusters and solvent. Then the cracks originate at the sites of those flaws in terms of fracture mechanics. It concludes that the crack spacing results from the wavelength of the concentration fluctuation during the phase separation, linearly growing with the increase of the deposition thickness and initial particle concentration, which is consistent with experimental results.