Effects of rotation on granular jamming: a study inspired by self-burrowing seeds

학위논문 (박사)-- 서울대학교 대학원 : 기계항공공학부, 2017. 2. 김호영.In the presented thesis, we conducted experimental and mathematical analysis of the effects of rotation on granular jamming, which is inspired by self-burrowing rotary seeds. Based on the results of the drag reduction by seed's spinning in granular media, we experimentally revealed that rotational motion impede the formation of force chain network near the intruder by inducing local slip motion of grains, leading the reduction of the effective area where granular hydrostatic acts. In addition, we found a semi-empirical model for the drag reduction depending on the relative slip velocity of grains by rotation, which showed a good agreement with the experimental results. To validate this physical model, we newly provided a noble and effective technique to investigate the internal jamming process in granular medium using the electrically conductive particles. This study can provide an advanced insight into how grain rearrangements in a localized region can significantly weaken the drag force in granular media. We first introduced the self-burrowing rotary seeds, which is a motivation of this study. We present the results of a combined experimental and theoretical investigation of the mechanics of self-burial of some plant seeds whose morphologies respond to environmental changes in humidity. The seeds of Erodium and Pelargonium have hygroscopically responsive awns that play a critical role in their self-burial into soil. The awn, initially coiled in a dry state, uncoils to stretch linearly under highly humid condition because of a tilted arrangement of cellulose microfibrils in one of the layers of the awn's bilayered structure. By measuring the mechanical characteristics of the awns of Pelargonium carnosum, we found that the extensional force of the awn can be aptly modeled by the theory of elasticity for a coiled spring. We further showed that although the resistance to the seed-head penetrating relatively coarse soils without spinning is large enough to block the digging seed, the rotation of the seed greatly reduces the soil's resistance down to a level the awn can easily overcome. Our mechanical analysis reveals that the self-burial of the seed is a sophisticated outcome of the helically coiled configuration of the awn. Next, we studied the drag force for the slowly moving intruder whose speed is comparable with the penetrating speed of the self-burrowing seeds in soil. The drag force acting on the intruder is determined by the inertial number I, which can be estimated by the importance of the dynamic friction relative to the static friction in granular media. In quasi-static flow regime I < 10^-3, the static friction resulted from the granular hydrostatic pressure mainly responsible for the drag force. We measured the drag forces for the vertically penetrating intruders in granular medium, and found that the granular hydrostatic pressure normally acts on the intruder. In addition, we investigated the effects of the grain size polydispersity and the relative size of grain to the intruder on the granular drag in quasi-static flow regime. We next presented quantitative measurements and mathematical analysis of the granular drag reduction by rotation, as motivated by self-digging of Erodium and Pelargonium seeds. The seeds create an extensional motion with rotation to dig into soil before germination using their moisture-responsive awns, which are originally helical shaped but reversibly deform to a linear configuration in a humid environment. We showed that the rotation greatly lowers the resistance of soil against penetration because grain rearrangements near the intruder change the force chain network. We found a general correlation for the drag reduction by relative slip of grains, leading to a mathematical model for the granular drag of a rotating intruder. In addition to shedding light on the mechanics of rotating body in granular media, this work can guide us to design robots working in granular media with enhanced maneuverability. To validate the rearrangement of the force chains by rotation of the intruder, we provided a noble technique to investigate the internal jamming in granular media. Granular jamming is determined by the force chain which consists of contacts between neighboring grains. Under jamming, the electrically conductive particles forms the electrical current paths along with the force chains. Based on the intuitive consideration, we experimentally measured the electrical resistance of granular medium consisting of the electrically conductive particles, solder balls, while trusting an intruder with rotation and without rotation. We found that the electrical resistance decreases with the burial depth, and the reduction behavior of the electrical resistance is delayed as increasing the rotational speed indicating that the effective area, which is supported by the granular force chains, is reduced by rotation. This work brings the complicated granular jamming down to a comprehensible level and gives a new shortcut to fabricate sustainable digging robots.1 Introduction 1 1.1 Outline 1 1.2 Backgrounds 2 1.2.1 Hygroscopic botanical motion 2 1.2.2 Drag force in granular materials 3 2 Self-burial mechanics of hygroscopically responsive awn 7 2.1 Introduction 7 2.2 Mechanisms and geometry of botanical movements 10 2.3 Theory of the force generated by self-burrowing seeds 12 2.4 Materials and methods 16 2.4.1 The seeds of Pelargonium carnosum 16 2.4.2 The measurement of forces generated by the awn 17 2.4.3 The measurement of forces required for digging 18 2.5 Experimental results 18 2.6 Conclusions 22 3 Quasi-static drag force in shallow granular media 25 3.1 Introduction 25 3.2 Materials and methods 26 3.3 Results and discussion 27 3.3.1 Vertical drag in quasi-static flow regime 27 3.3.2 Effects of polydisperse grains on granular drag 31 3.3.3 Effects of particle size on stick-slip fluctuation 32 3.4 Conclusion 35 4 Reduction of granular drag inspired by self-burrowing rotary seeds 36 4.1 Introduction 36 4.2 Results and discussion 38 4.2.1 Vertical granular drag in quasi-static flow regime 38 4.2.2 Reduction of granular drag by rotation 41 4.2.3 Relative slip motion of grains induced by rotation 42 4.3 Summary 44 5 Effects of rotation on granular jamming: force-dependent electrical resistance in granular medium 46 5.1 Introduction 46 5.2 Materials and methods 47 5.2.1 The measurements of the granular drag in glass beads 47 5.2.2 The measurements of the granular drag in solder balls 48 5.2.3 The measurements of the electrical resistance of granular medium 50 5.3 Results and discussion 50 5.3.1 Reduction of granular drag by rotation 50 5.3.2 Granular drag in solder balls 54 5.3.3 Electrical resistance of granular medium 54 5.3.4 Effects of rotation on electrical resistance of granular medium 56 5.4 Conclusions 57 6 Concluding remarks 60 6.1 Conclusions 60 6.2 Future work 63 References 64 Abstract (in Korean) 73Docto

Development of Ti6Al4V and Pure Tungsten 3D printing process using DED

MasterThis thesis presents experimental investigations of 3D printing Ti6Al4V and pure tungsten structures by Directed Energy Deposition (DED). Experiments were conducted using an in-house DED printing system. First, the effect of build direction, wall thickness and heat treatment on the tensile strength and elongation were investigated. The strength of as-built samples were high enough to meet the ASTM requirements. However, only the samples built in the transverse direction met the requirement of elongation. Heat treatment made the mechanical properties isotropic and enhanced the elongation up to ~10 %. As the thermal condition was different depending on the wall thickness, it affected mechanical properties, especially samples with short cooling time between subsequent line scans. Second, this work reports successful fabrication of pure tungsten structures by DED, revealing the required process conditions. The effect of laser power, scan speed, powder feed rate and carrier gas velocity on the stability and properties of the structures is first analyzed, based on which the proper process condition for effective 3D printing of tungsten parts is proposed. Analyses of the fabricated samples show that the density and the hardness can be as high as 18.9 g/cm3 (98.4 % of the theoretical value) and 3.9 GPa, respectively. The results indicate that the optimal condition for 3D printing of tungsten is 400 ~ 530 J/mm2 in terms of specific energy and that high-speed or high-mass injection of powder can induce waviness on the surface

콜로이드 자기 조립 소자를 위한 금속-반도체 혼성 클러스터의 제조

학위논문(석사) - 한국과학기술원 : 생명화학공학과, 2008.2, [ vi, 70 p. ]The aggregates of small number of colloidal particles, colloidal clusters, have been studied from many researchers for understanding of nanoscopic matters as using atomic model and photonic materials. These colloidal clusters provide various possibilities of constructing more complex colloidal structures than can be realized by simple spherical particles. To form compact packing clusters, emulsion templates are used as confined geometry and evaporation of emulsion induce the colloidal clusters that have the unique configuration according to the number of constituent particles. Especially, binary colloidal clusters have selective interactable regions than are uncovered with small colloidal particles. The morphology of the binary colloidal clusters inspires the self-organized colloidal devices. In order to function as a device, colloidal clusters should have active components that can be activated by light, electricity, etc.. We selected semiconductor quantum dots as the active components in self-organized colloidal clusters. Recently, nanometer-sized structures with at least one dimension smaller than the critical size for a given property of a material have attracted considerable attention, mainly due to their size-dependent properties and flexible processing chemistry. Finally, we succeeded to fabricate the self-organized colloidal devices with CdSe semiconductor quantum dots and Au shell particles. Our exploration is expected to bring forward new fields or conceptual paradigm in fabricating nanoscopic devices.한국과학기술원 : 생명화학공학과