실리콘 나노선 내장형 시스템의 비젖음성 계면 특성과 연속흐름 다상 공정

Abstract

본 논문은 초소수성 및 발유성을 동시에 지닌 실리콘 나노와이어를 미세유체 시스템 내부에 도입하여 기체 및 액체, 더 나아가서 고체 촉매까지 포함하는 다상(multiphase) 시스템을 구축하여 다양한 화학적 응용을 수행한 연구에 대한 결과이다. 이 다상 시스템은 높은 효율성을 지닌 것으로 이미 알려진 미세유체 채널 시스템의 활용도를 더욱 끌어올려 오염 물질 분해, 이차원 물질의 박리(exfoliation), 기체 및 액체 반응물을 통한 화학 합성 수행, 기체 및 액체 화합물의 분리, 계면 촉매를 활용한 이산화탄소의 선택적 분리, 포집 및 활용 등의 다양한 과학적, 공학적 응용들을 도출해낼 수 있었다. 본 논문의 각 장(chapter)은 구축된 시스템이 갖는 비젖음성(non-wetting)의 정도에 따라 크게 두 가지로 구분되었고, 각각은 초소수성 성질을 갖는 구조체를 포함한 시스템을 활용한 응용에 관한 장과 초소수성 및 발유성 성질을 동시에 갖는 구조체를 포함한 시스템을 활용한 응용에 관한 장이다. 첫 번째 장은, 초소수성 미세유체 시스템을 통해 정적인 공기 포켓과 수용성 유체 흐름 사이의 이상(two-phase) 계면을 활용한 응용을 다루었다. 초소수성 특성을 갖는 수직 배열 실리콘 나노와이어 다발이 미세유체 채널 내부에 도입되었고, 수용성 유체 흐름은 초소수성 나노와이어 구조 사이를 투과하지 못하고 구조 위를 미끄러지듯 흐르게 된다. 이때 나노와이어 구조 사이에 포집된 공기 포켓은 외부에서 주어진 초음파 환경에 의해 공동화 기포 붕괴(cavitation bubble collapse)를 유발하게 되고, 이는 순간적으로 섭씨 5,000도 및 500 기압에 달하는 환경을 조성한다. 이러한 극단적인 환경은 화학적 관점에서 수산화 라디칼을 생성하여 물속에 존재하는 오염 물질을 효과적으로 분해할 수도 있으며, 물리적 관점에서 이차원 물질을 박리하여 효과적으로 수 층 규모의 층상 구조로 만들 수 있다. 두 번째 장에서는, 초소수성 및 발유성을 동시에 갖는 미세유체 시스템을 통해 기체 흐름과 액체 흐름 사이의 안정적인 이상 계면을 형성하고, 궁극적으로는 이 계면에 고체 촉매를 위치하게 하여 다상 계면까지 구축하게 된다. 특히, 단순히 수용성 유체뿐 아니라 지용성 유체의 흐름까지 기체 흐름과 함께 안정적으로 구현하였단 점이 주목할 만하다. 이러한 표면 구조 특성을 구현하기 위해 수직 배열된 실리콘 나노와이어 다발에 추가적으로 수백 나노미터 크기의 실리카 나노입자를 도포함으로써 계층화 구조를 형성하고, 표면 불소화 처리를 통하여 표면 에너지를 낮추는 등의 공학적 기법이 도입되었다. 결과적으로 만들어진 시스템을 통하여 우선적으로 기체 및 액체 상의 반응물을 효과적으로 반응하는 무막(membrane-free) 반응 시스템을 구현하였다. 이후 반대로 기체와 액체가 섞여 흐르는 세그먼트 흐름(segmented flow)에서 기체만 따로 분리, 포집하기 위해 액체에 대한 반발성을 갖는 실리콘 나노와이어가 활용되었다. 궁극적으로 이러한 실리콘 나노와이어의 상단 부분에 기체와 액체 계면에서 작용할 수 있는 계면 촉매를 고정하였고, 이를 통해 이산화탄소를 포함하는 기체에서 이산화탄소만 선택적으로 포집 및 분리한 후 액체 반응물과의 반응을 통해 부가가치를 지닌 화합물을 합성하는 데 성공하였다. 이러한 시스템은 기존의 다상 반응 시스템의 단점을 효과적으로 극복하고, 기존에 불가능한 새로운 형태의 반응도 확장 가능케하는 플랫폼으로서 그 가치를 가진다.

Engineering the wettability of surfaces to be superhydrophobic, superoleophobic, and superamphiphobic has attracted considerable interest due to their unique wetting properties, leading to various applications such as oil-water separation, anti-fouling, self-cleaning, etc. The surfaces with a high apparent contact angle and low contact angle hysteresis, which make the liquids to exhibit the Cassie-Baxter state, are mostly constructed with hierarchical surface textures and low surface energy. The surface forms a composite gas-liquid-solid interface by trapping an air film between the liquid and the surface textures. Surfaces with vertically aligned micro-scale bundles of silicon nanowires (SiNWs) either with superhydrophobic or superamphiphobic natures possess enough space to entrap and transport gaseous compounds between the spikes (i.e. interconnected air pockets). The applicability of these surfaces is broadened with a combination of a dynamic stream of liquid compounds above the structures. Using the principle, microfluidic systems for gas-liquid two-phase reactions were designed with built-in patterns of SiNWs array. The systems which enable direct contact between the two streams of gaseous compounds and liquid solution without the need of a gas-permeable membrane (i.e. ‘membrane-free’) are proposed to overcome the challenges encountered during the utilization of conventional reaction systems for gas-liquid two-phase organic synthesis. In Chapter 2, super-hydro-phobic microfluidic systems with a two-phase interface between static air pockets and water stream equipped with built-in SiNWs bundles with superhydrophobic nature are demonstrated as micro-sonolysis reactors. The superhydrophobic SiNWs have an ability to hold up the water flow above themselves while retaining air between themselves. The SiNWs are also at the heart of the sonolysis system, which effectively generates and stabilize the air-water interfaces around the tip of themselves. Under ultra-sonication, the interconnected air pockets between the SiNWs have an ability to create nuclei for cavitation bubbles. The cavitation bubble collapse instantaneously induces high temperatures up to 5,000 K and local pressure up to 500 atm. The applications in SiNW-based microfluidic systems lead to an enhancement of energy efficiency which is many times higher than those of the other processes reported to date as well as the ‘greenness’ of processes. In Chapter 2-1, the utilization of the cavitation bubble collapse to degrade pollutant compounds from water in the micro- and the milli-fluidic system is demonstrated. The cavitation bubble collapse instantaneously inducing high temperature and pressure can generate OH radicals from water. The promotion of OH radicals is by the structure itself without any additional energy inputs (e.g. UV, ozone, and heat) or additives (e.g. catalyst). The superhydrophobic SiNW-embedded sonolysis system developed has been demonstrated to deliver a performance that far exceeds the existing advanced oxidation processes (AOPs) in terms of fast and efficient treatment of polluted water. Pollutants including 4-chlorophenol, bisphenol A, carbamazepine, and propranolol were used as model pollutants that were completely removed within 12 min, as opposed to hours needed by other AOPs. In Chapter 2-2, bulk black phosphorus (BP) was exfoliated into few-layer flakes, both in continuous flow condition. Atomically thin layered BP has been regarded as a successor to other 2D materials with great potential in various energy-related applications. The exfoliation of 2D materials under ultrasonic treatment was attributed to the energetic physical effects such as micro-turbulence and shockwaves. These physical effects originated from the cavitation bubble collapse activated by the superhydrophobic SiNWs. The cavitation effect can be strategically tailored by changing the power of ultra-sonication between a strong and a weak level. Strong ultra-sonication could be sufficient to effectively fragment a bulk BP into smaller bulk counterpart. Weak ultra-sonication contributes to delaminate the fragmented BP into few-layer BP flakes. Very rapid exfoliation of BP was feasible in the SiNW micro-sonolysis system. This could reduce the degradation of BP which is usual in prolonged sonication treatment in water. In Chapter 3, super-amphi-phobic microfluidic systems with multiphase interfaces with gas and liquid streams and solid catalysts in which SiNWs with superamphiphobic nature are incorporated are demonstrated. For these researches, the surface wettability of SiNWs was engineered to be superamphiphobic which means streams of both water and organic solvent can be repelled by the structures. Moreover, the utilization of a dynamic stream of gas was additionally investigated in this chapter. The liquid stream flows in contact with an underlying gas stream with an identical direction to basically perform a gas-liquid two-phase chemical synthesis. Owing to their superamphiphobic nature, the SiNWs can physically divide the routes for these liquids and gas streams while maintaining the laminar contact between them. In Chapter 3-1, Gas-liquid two-phase reactions were firstly conducted in the SiNW-based microfluidic system. Oxygen and an organic solution were used as model reactants which were infused into the system with separated inlets. The stable and membrane-free interface between the reactants results in significant enhancements in the inter-phasic mass transfer which were proved by a superior reactivity compared to the conventional dual-channel membrane system with identical design. For the model reaction, the conversion reached 79 % within a residence time of 3 min, as opposed to only 56 % of the dual-channel membrane system of the same residence time. In Chapter 3-2, the built-in SiNWs bundles were utilized to separate a gas from the gas-liquid segmented flow within a totally enveloped microfluidic system to deal with carcinogenic compounds. The superamphiphobic SiNWs have the ability to capture a gas bubble under the liquid. This property was fully utilized to separate the gas-phase chloromethyl methyl ether (CMME) from a byproduct liquid. Despite the toxicity and carcinogenicity of CMME, value-added chemicals can be synthesized from it. The exposure of the CMME could be prevented during the utilization process by being enveloped in continuous-flow microfluidic systems. With less harmful precursors, the CMME gas was firstly generated inside a microfluidic system while generating a liquid-phase byproduct. The segmented flow composed of the CMME gas and the byproduct liquid was then separated by system equipped with the superamphiphobic SiNWs. With the repellent nature against the liquid, SiNWs could successfully capture the CMME gas from the byproduct liquid and transport it to a separated outlet connected with the following reaction step for the generation of value-added products. In Chapter 3-3, the SiNWs-based gas-liquid-catalyst multiphasic microfluidic reactor was ultimately developed by immobilizing an ionic liquid catalyst on the tip of the SiNWs. Around the tip of the SiNWs, gas and liquid streams realized a laminar flow with an aid of the superamphiphobic nature of SiNWs and the catalysts were positioned at the very interface between the gas and liquid streams by being immobilized on the tip of the SiNWs. Multiphasic reaction with gas, liquid, and catalyst was feasible with the strategy. For a model reaction, the utilization of pure carbon dioxide (CO2) from the CO2-containing flue gas was performed. Pure CO2 was captured and separated from the flue gas with the ionic liquid catalyst and underwent reaction with liquid reactants to produce high-valued specialty chemicals with up to 97 % yield under mild conditions.