Browsing by Author "Hwang, Dohgyu"

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    All-graphene-based open fluidics for pumpless, small-scale fluid transport via laser-controlled wettability patterning
    Hall, Lucas S.; Hwang, Dohgyu; Chen, Bolin; Van Belle, Bryan; Johnson, Zachary T.; Hondred, John A.; Gomes, Carmen L.; Bartlett, Michael D.; Claussen, Jonathan C. (2021-01)
    Open microfluidics have emerged as a low-cost, pumpless alternative strategy to conventional microfluidics for delivery of fluid for a wide variety of applications including rapid biochemical analysis and medical diagnosis. However, creating openmicrofluidics by tuning the wettability of surfaces typically requires sophisticated cleanroom processes that are unamenable to scalable manufacturing. Herein, we present a simple approach to develop open microfluidic platforms by manipulating the surface wettability of spin-coated graphene ink films on flexible polyethylene terephthalate via laser-controlled patterning. Wedge-shaped hydrophilic tracks surrounded by super-hydrophobic walls are created within the graphene films by scribing micron-sized grooves into the graphene with a CO2 laser. This scribing process is used to make superhydrophobic walls (water contact angle B1608) that delineate hydrophilic tracks (created through an oxygen plasma pretreatment) on the graphene for fluid transport. These allgraphene open microfluidic tracks are capable of transporting liquid droplets with a velocity of 20 mm s(-1) on a level surface and uphill at elevation angles of 78 as well as transporting fluid in bifurcating cross and tree branches. The all-graphene open microfluidic manufacturing technique is rapid and amenable to scalable manufacturing, and consequently offers an alternative pumpless strategy to conventional microfluidics and creates possibilities for diverse applications in fluid transport.
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    A Kirigami Approach for Controlling Properties of Adhesives and Composites
    Hwang, Dohgyu (Virginia Tech, 2022-02-25)
    Controlling the layout of elasticity in materials provides new opportunities for generating various functionalities such as shape-morphing capability, large stretchability, and elastic softening for aeronautics, drug delivery, soft robotics, and stretchable electronics applications. Recently, techniques building upon kirigami principles, the Japanese art of paper cutting, have been considered an effective strategy to control stiffness and deformation of materials by systemically integrating cut patterns into inextensible sheets. The performance of kirigami-inspired materials relies primarily on geometric features defined by cut patterns rather than chemistry of constituents, which can enable high compatibility with diverse material sets across a wide range of length scales. However, kirigami has been relatively unexplored to control adhesion and current challenges such as the intrinsic trade-off between high deformability and load-bearing capacity limits applications that require large shape change and structural strength. This thesis demonstrates that the kirigami approach is a powerful tool to control interfacial properties of adhesive films, and that composite approaches in kirigami-inspired material can overcome the deformation-strength trade-off. The kirigami principle is applied to adhesives to control adhesion through arrays of linear cut patterns (Chapter 2). The spatial layout of elasticity in the kirigami-inspired adhesive enhances adhesion over homogeneous adhesive systems and generates anisotropic adhesion. The utility of the proposed adhesive design criteria is further extended to complex non-linear cut patterns (Chapter 3). These non-linear patterns significantly enhance adhesion relative to linear patterns in adhesives and unpatterned films, while also enabling easy release and spatial control of adhesion across a sheet. The enhancement enabled by cut geometry remains effective in diverse adhesives, on various surfaces, and in wet and dry conditions. The adhesion dependence on cut geometry is further investigated to understand how arrays of sub-patterns adjacent to primary non-linear patterns affect adhesion performance (Chapter 4). Kirigami composites are also developed to overcome the trade-off between large deformability and load-bearing capacity (Chapter 5). A composite architecture is developed consisting of low melting point metal alloys incorporated into patterned elastomeric layers. This composite approach shows the ability to rapidly morph into complex, load-bearing shapes, while achieving reversibility and self-healing capability through phase change driven by embedded heaters. The utility of the multi-functional composite is demonstrated through a multimodal morphing drone which transforms from a ground to air vehicle and an underwater morphing machine which can be reversibly deployed to collect cargo. This thesis is then summarized by discussing key findings, contributions, and future perspectives (Chapter 6).