My research encompasses experimental investigations on the physical origins of natural phenomena existing from mesoscopic to geophysical scales. Many of the systems are far from equilibrium, where intriguing phenomena are likely to occur such as the coffee ring stains in a drying coffee drop, hyperuniform structures in disordered systems, star-shaped oscillations of Leidenfrost drops, buckling/wrinkling morphologies in plants, and polygonal terrain cracks. My purpose is to utilize the hands-on tools in laboratory to uncover the underlying linear/nonlinear mechanisms behind these natural phenomena. Additionally, I am also interested in biomechanics especially the biology-inspired applications for instance self-cleaning, self-propulsion, and drag-reduction.
The concept of hyperuniform states which are now considered as a new state of matter has drawn extensive attention since its original proposal a decade ago. These states are characterized by the anomalous suppression of long-wavelength density fluctuations like crystals and quasi-crystals, yet are statistically isotropic like liquids and glasses, and can be found in a large varieties of fields such as physics, mathematics, chemistry, biology, engineering, material sciences containing disordered states. Such amorphous states provide a new classification of states regarding the long-range order in disordered structures, and are desirable due to their astonishing optical, mechanical, and hydrodynamic behaviors in a broad class of disordered systems like granular and porous media, glasses, colloidal suspensions, and photonic materials. Experimentally, these states are essentially realized through highly-nonequilibrium pathways for instance self-assembly, self-organization, shearing, packing, and sedimentation. In particular, I am interested in the correlations between the hyperuniformity, long-range hydrodynamics, and the geometries and heterogeneities of sedimenting objects in the regime of low Reynolds number. Thus more technically, my purpose can be summarized as "looking for the hidden order" that (seemingly) is not apparent on large length scales.
Leidenfrost effect describes the phenomenon that a liquid drop levitating above a hot surface survives for a significantly long lifetime owing to the presence of a thermally insulating vapor layer fed by its own evaporation beneath the drop. In this levitated state, the drops are free to undergo vibrations due to the absence of liquid-solid contact. Interestingly, large Leidenfrost drops can form large-amplitude star-shaped oscillations with a characteristic frequency in a self-organized fashion. We find that the star oscillations (≈ 13 Hz) are parametrically induced by the pressure oscillations (≈ 26 Hz) in the vapor for water drops. The pressure oscillations are inherently driven by the capillary waves of a characteristic wavelength beneath the drop traveling from the drop center to the edge, and the waves are generated by a large shear stress by the rapid vapor flow, suggesting a purely hydrodynamic nature of such patterned oscillations. Additionally, Leidenfrost drops also show potential opportunities for self-propulsion as endowing the substrate with asymmetric geometries, which is likely to result from the modulation of the drop curvature and thereby the Laplace pressure gradient, or the Marangoni stress. My purpose is to understand the linear and nonlinear dynamics involved in the Leidenfrost effect, and furthermore to optimize the protocols for manufacturing processes, for instance glass lenses and self-propelling fluidic devices, soft engines (elastic Leidenfrost effect), in industries.
Crack patterns are ubiquitous in nature, for example the craquelure cracks in old paintings, the T/Y-shaped cracks in dried mud, the columnar joint cracks and the polygonal terrain cracks. My motivation can be twofold: (1) to understand the fundamental selecting mechanism of crack patterns under various scenarios (symmetry breaking? flaw pre-existence?), (2) to figure out the dominant principles for the onset of cracking and the propagation of cracks (stick-slip? straight or curved?), which have not reached a consensus hitherto. Drying colloidal (Brownian and non-Brownian) suspensions has been widely employed as a model system for answering the aforementioned questions. Basically, as a thin colloidal film is deposited on a substrate, evaporation occurs at the free surface, then numerous menisci are formed at the liquid/vapor interface, giving rise to a negative Laplace pressure. Consequently, the atmospheric pressure will compress the film in order to form a more densely-packed network with particles connected by liquid bridges. As evaporation proceeds, the concomitant in-plane tensile stress builds up, and once it reaches a critical value cracking occurs in order to release stress (Griffith criterion). In laboratory, we use a conventional microscopy and additional characterizing techniques to track down the dynamics of the particle networks in order to gain a better understanding of these nonequilibrium processes that construct the building blocks of nature.
Nature has been acting as a gigantic school for researchers and engineers. In particular, I am interested in the emergent applications based on biological organisms. To name a few, the self-cleaning of lotus leaves, run-tumble/collective behavior of bacteria, water-walking of water striders, capillary feeding of phalaropes, and the drag-reduction of sharks. Those characteristics are basically endowed by the unique physical/physicochemical properties of the organisms. For instance, The self-cleaning of lotus leaves basically originates from the fractal or hierarchical structures combined with special chemical compositions of the leaves. Water-walking and drag-reduction are also closely related to the unique nano/micro textures of the organisms in addition to their special hydrodynamic strategies for propulsions. The bacteria exhibit run-tumble locomotion when the concentration is small, and show collective motions they are closely packed. More interestingly, phalaropes feed by generating vortexes, what a super-smart strategy! I am interested in how exactly these astonishing inherent strategies are developed, i.e., how the structures interact with surrounding flows, and the hydrodynamic, statistical & mechanical properties of those "active matter". We hope a deep understanding of the underlying mechanisms behind these strategies could provide guide for the design of artificial smart materials, swimmers, and transporters.