Savvas G. Hatzikiriakos

Patterned superhydrophobic metallic surfaces.

This work shows that after creating certain dual scale roughness structures by femtosecond laser irradiation different metal alloys initially show superhydrophilic behavior with complete wetting of the structured surface. However, over time, these surfaces become nearly superhydrophobic with contact angles in the vicinity of 150 degrees and superhydrophobic with contact angles above 150 degrees. The contact angle hysteresis was found to lie between 2 and 6 degrees. The change in wetting behavior correlates with the amount of carbon on the structured surface. The explanation for the time dependency of the surface wettability lies in the combined effect of surface morphology and surface chemistry.

Role of slip and fracture in the oscillating flow of HDPE in a capillary

Certain polymers exhibit two distinct branches in their capillary flow curves (wall shear stress versus apparent wall shear rate). This gives rise to oscillatory flow in constant‐piston‐speed rheometers and to flow curve hysteresis in controlled‐pressure rheometers. These curious phenomena have attracted considerable interest over a period of many years, but their basic mechanisms are still the subject of debate. Building on previous work we have developed a model that predicts all the essential features of the curves of pressure and flow rate versus time in the oscillatory flow regime. Fluid compressibility and the second branch of the flow curve are necessary features of the model, but fluid elasticity is found not to be an essential element. While our macroscopic measurements do not prove it conclusively, our data lead us to believe that on the high‐flow‐rate branch of the flow curve there is slip along a cylindrical fracture surface near the wall. The jump to the high‐flow branch occurs when this fracture occurs, at an upper critical value of the shear stress, while the jump back to the low‐flow branch occurs when adhesion is established at the fracture surface at a lower critical shear stress.

Rheology of nanocrystalline cellulose aqueous suspensions.

The rheological properties and microstructure of nanocrystalline cellulose (NCC) aqueous suspensions have been investigated at different concentrations. The suspension is isotropic up to 3 wt %, and phase separates to liquid crystalline and isotropic domains at higher concentrations where the samples exhibit a fingerprint texture and the viscosity profile shows a three-region behavior, typical of liquid crystals. The suspension behaves as a rheological gel at even higher concentrations where the viscosity profile shows a single shear thinning behavior over the whole range of shear rates investigated. The effects of ultrasound energy and temperature on the rheological properties and structure of these suspensions were studied using polarized optical microscopy and rheometry. Our results indicate that the amount of applied ultrasound energy affects the microstructure of the suspensions and the pitch of the chiral nematic domains. The viscosity profile is changed significantly at low shear rates, whereas the viscosity of biphasic suspensions at intermediate and high shear rates decreased with increasing temperature. This suggests that, between 30 and 40 °C, structural rearrangement takes place. At higher concentrations of about 10 wt %, the temperature has no significant effect on viscosity; however, a marked increase in viscosity has been observed at around 50 °C. Finally, the Cox-Merz rule was found to fail after a critical concentration, thereby implying significant structural formation. This critical concentration is much higher for sonicated compared to unsonicated suspensions.

Femtosecond laser irradiation of metallic surfaces: effects of laser parameters on superhydrophobicity

This work studies in detail the effect of femtosecond laser irradiation process parameters (fluence and scanning speed) on the hydrophobicity of the resulting micro/nano-patterned morphologies on stainless steel. Depending on the laser parameters, four distinctly different nano-patterns were produced, namely nano-rippled, parabolic-pillared, elongated sinusoidal-pillared and triple roughness nano-structures. All of the produced structures were classified according to a newly defined parameter, the laser intensity factor (LIF); by increasing the LIF, the ablation rate and periodicity of the asperities increase. In order to decrease the surface energy, all of the surfaces were coated with a fluoroalkylsilane agent. Analysis of the wettability revealed enhanced superhydrophobicity for most of these structures, particularly those possessing the triple roughness pattern that also exhibited low contact angle hysteresis. The high permanent superhydrophobicity of this pattern is due to the special micro/nano-structure of the surface that facilitates the Cassie-Baxter state.

Polymer Processing Instabilities: Control and Understanding

“공정 엔지니어를 위한 PRO/II with PROVISION 실무”는 국내의 정유사, 석유화학사와 엔지니어링사를 비롯하여 관련 연구소 및 대학에서 가장 많이 사용되고 있는 범용 모사기 중의 하나인 Simulation Science사의 PRO/II를 이용하여 단위조작 장치에 대한 모사기법에 대한 예제와 그 해결방법들을 모아 놓은 책이다. 이 책의 특징은 국내에서는 최초로 단위조 작 모듈에 대한 공정모사 예제를 실제 화학공정의 조업이나 설계 시에 발생할 수 있는 예제 들을 모아 놓았다는 점이다. 이 책의 저자인 동양대학교 생명화학공학부의 조정호 교수의 다년간의 현장 실무경험을 통하여 축적된 내용들을 예제화하였다는 점에 있어서 화학공정 을 설계하고 조업하는 현장 실무 엔지니어 뿐만 아니라 화학공학을 배우는 3, 4학년 학생 및 대학원생들에게도 좋은 지침서가 될 수 있으리라 사료된다. 이 책의 내용을 살펴보면 1장에서는 PRO/II with PROVISION의 윈도우즈 상에서의 입력 방법과 키워드를 이용한 입력방법을 비교하여 서술하였으며, 2장에서는 평형단 조작 장치의 기본이 되는 각종 플래쉬 문제를 다양하게 소개하였다. 3장에서는 각종 열교환기 관련 예제 를 다루었으며, 4장에서는 펌프, 압축기 및 밸브 등과 같이 압력 변환장치에 관련된 문제들 을 수록하였다. 특히 3장과 4장에서는 단순한 플래쉬 계산이나 압력변환장치의 단위 장치들 에 대한 모사뿐만 아니라 다른 단위조작 장치와 연결된 문제와 엔지니어링 회사에서 실제 설계 시에 부딪히기 쉬운 실제적인 문제들을 위주로 수록하였다. 5장에서는 증류탑, 흡수탑 및 추출탑에 대하여 단순한 이성분계 증류에서부터 hydrocarbon distillation, chemical distillation을 위시하여 sour water stripper, amine absorber 및 방향족 회수를 위한 추출 탑의 모사에 이르기까지 실제 석유화학공장에서 가동되고 있는 증류, 흡수 및 추출탑 위주로 예제를 만들어서 소개한 면이 색다르다. 6장에서는 반응기 형태별 모사 예제들이 수록되었 는데 PFR, CSTR 및 Kinetic Reactor에 대한 예제가 수록되어 있어서 이 책을 공부하는 것 은 단위조작과 반응공학을 동시에 학습하는 효과가 있을 것으로 생각되어진다. 마지막으로 7장에서는 공정모사기의 추가적인 기능으로서 controller, calculator, optimizer 및 case study를 이용하는 방법의 소개와 더불어서 실 예제를 통하여 공정 최적화에 대한 감각도 익 힐 수 있도록 해 놓았다. 2004년에 출간된 공정모사에 관한 지침서와 더불어 지속적으로 출간이 이루어질 공정모 사에 사용하는 열역학 모델 식에 대한 선정 지침서 및 특수증류공정설계, 환경공정설계에 관 한 책들은 화학공학 교과서와 실제 화학공정을 모사기를 통하여 정량적으로 해석하고 모형 화시키는 기법을 소개한다는 면에 있어서 주목할 만하다.

Physics of ice friction

Although the study of friction has a long history, ice friction has only been investigated during the last century. The basic physical concepts underlying the different friction regimes, such as boundary, mixed, and hydrodynamic friction are also relevant to ice friction. However, these friction regimes must be described with respect to the thickness of the lubricating liquidlike layer on ice. In this review the state of knowledge on the physics of ice friction is discussed. Surface melting theories are introduced. These theories attempt to explain the existence and nature of the liquidlike surface layer on ice at any temperature and without any load applied. Pressure melting, as the long-time explanation for the ease of ice friction, is discussed, together with the prevailing theory of frictional heating. The various laboratory setups for ice friction measurements are presented as well as their advantages and disadvantages. The individual influence of the different parameters on the coefficient of ice fr...

INTERFACIAL PHENOMENA IN THE CAPILLARY EXTRUSION OF METALLOCENE POLYETHYLENES

Metallocene catalysts are known to produce homogeneous random polyolefin copolymers with narrow molecular weight distribution and controlled long/short-chain branching. Two such linear low-density polyethylenes were studied by using both constant-stress and capillary rheometry, in order to assess their rheological and processing behavior, as well as to identify critical conditions for the onset of flow instabilities. It was found that these polymers are thermorheologically complex liquids, apparently due to the presence of long-chain branching. Compared with conventional linear low-density polyethylenes, these metallocene polyethylenes exhibit quite unusual behavior in capillary flow, not previously reported to our knowledge. Specifically, we have encountered long transients in start-up of capillary experiments, and in some cases, the capillary reservoir had to be loaded several times before a steady-state pressure was obtained. In addition, we found that these polymers slip at shear stresses higher than ...

The work of adhesion of polymer/wall interfaces and its association with the onset of wall slip

The interfacial characteristics of a variety of polymer/wall interfaces were measured by using the sessile drop method in order to calculate the work of adhesion. Polymers included linear low-density as well high-density polyethylenes, while wall substrates included clean stainless steel and modified stainless steel by applying two different fluoropolymers in order to alter its surface energy. A linear correlation is found between the critical shear stress for the onset of slip and the work of adhesion of the corresponding polymer/wall interface, in agreement with earlier publications of Hill et al. (1991) and Hatzikiriakos et al. (1993). In the present work, the experimental results are interpreted in terms of parameters defined by these two theories. It is suggested that small deviations from the no-slip boundary condition in the case of polymer melt flow are due to a stress-induced chain detachment/desorption of polymer chains from the wall.

Laser-Patterned Super-Hydrophobic Pure Metallic Substrates: Cassie to Wenzel Wetting Transitions

A femtosecond laser was used to create microstructures on very pure metal surfaces. The irradiated samples initially showed super-hydrophilic behavior. With time and exposure to ambient air the contact angle increased to about 160° with very low hysteresis. The surfaces supported the Cassie and Wenzel wetting states, depending on the technique used to deposit the water droplets. The created surface morphologies were idealized with a geometric model that is an assembly of densely packed cylindrical pillars with semispherical caps. Using this geometric model for calculation of the surface roughness, a theoretical Young contact angle of about 99° was calculated for all samples from the Wenzel and Cassie–Baxter equations. While the value of 99° significantly differs from the measured hydrophilic contact angles on the polished pure metallic samples, it indicates that a laser-induced surface reaction must be responsible for the evolution of contact angles to super-hydrophobic ones and that this phenomenon is independent of the type of metal.

Influence of degree of sulfation on the rheology of cellulose nanocrystal suspensions

The rheology and microstructure of two different cellulose nanocrystals (CNC) samples possessing different degrees of sulfation are studied over a broad concentration range of 1 to 15 wt%. CNC suspensions are isotropic at low concentration and experience two different transitions as concentration increases. First, they form chiral nematic liquid crystals above a first critical concentration where the samples exhibit a fingerprint texture and the viscosity profile shows a three-region behavior, typical of liquid crystals. By further increasing the concentration, CNC suspensions form gels above a second critical concentration, where the viscosity profile shows a single shear-thinning behavior over the whole range of shear rates investigated. It has been found that the degree of sulfation of CNC particles has a significant effect on the critical concentrations at which transitions from isotropic to liquid crystal and liquid crystal to gel occur. Rheological properties and microstructure of these suspensions have been studied using polarized optical microscopy combined with rheometry.