Measurement and Control of Slip-Flow Boundary Conditions at Solid-Gas Interfaces

Measurement and Control of Slip-Flow Boundary Conditions at Solid-Gas Interfaces

Dongjin Seo

Department of Chemical Engineering

UC Santa Barbara, Santa Barbara, California

dongjin_seo@ucsb.edu   In traditional Chemical Engineering applications one usually assumes that the no-slip boundary condition for flow applies at the solid-gas interface.  However, it has been known since the time of Maxwell that, even for moderate Knudsen numbers, partial slip could occur, of order the mean free path of gases.  With the growing number of applications for micro- and nano-scale devices and flow systems, it is interesting to quantify more precisely the flow boundary condition. In this work I describe unambiguous measurements where I show that partial slip does occur, that the slip length is a function of both the gas and solid, and that the solid can be altered in-situ to change the slip length.  In-situ changes in slip length could in principal be used to control flow in a variety of important applications.   The main theme of this presentation is the effect of solid surface properties on the flow boundary condition. The effect of water films, organic films, electric fields and the gas species was studied. Water films had a large, but complex effect. On bare hydrophobilic glass, the tangential momentum accommodation coefficient (TMAC) for nitrogen on hydroxyl-terminated silica changed from 0.25 to 0.88 when the humidity changed from 0 to 98 %. On hydrophobized glass, TMAC changed from 0.20 to 0.56 in the same range. The effect of humidity was interpreted with the formation of water film, verified with a quartz crystal microbalance. TMAC on  octadecyltrichlorosilane-coated glass surfaces was examined for five different gases (helium, nitrogen, argon, carbon dioxide, hexafluoride sulfur). A lower TMAC occurred for greater molar mass.   I also developed methods for controlling the flow boundary condition using external stimuli – temperature and electric fields. Using Atomic Force Miscroscopy (AFM), each of these stimuli altered the roughness of the film, so control of the boundary condition was attributed to surface roughness change.  The temperature was used to change the roughness of an octadecyltrichlorosilane film on glass, and the electrical field was used to change the roughness of a 16-mercaptohexadecanethiol film on gold. Considering collisions of simple spheres that conserved momentum and kinetic energy, a surface with periodic triangular grooves had higher TMAC with steeper slopes.   A novel method of measuring gas pressure using an AFM cantilever was presented.  The frequency spectrum of the cantilever vibration in the gas was measured, which was then fit to a continuum description that depends on the density.  Use of the continuum approximation restricts calculation of the pressure to the continuum regime. In the range 0.1 to 2.2 atm, the gauge deviated by less than 5 % compared to a commercial gauge. 

Research Interests:

  Colloids and surface science is my expertise and tool. With this, I am planning on extending my research toward energy engineering and biomedical engineering. I like to propose two research projects for the immediate years that follows. The first project, a novel gas separation technology, is intended for the submission to National Science Foundation (CBET division). This project is intended to prove a new separation principle and separate carbon dioxide from natural gas in the future. The second project, deep drug deliver to lung with aerosol surface modification, was prepared for National Institute of Health R21 funding. This simple modification to aerosol medication would deliver more active ingredient toward alveoli, reducing overdose and cost.  

Separation of Gas Mixtures via Differences in Flow Boundary Conditions

 

The objective of the proposed work is to verify the principle of a novel gas separation method based on difference in flow boundary conditions. The principle to be proved is that different gas species intrinsically have different boundary conditions when their mean free paths are comparable to the characteristic length of the separation system. This will further extended toward the preparation of the prototype gas separation device, which is clean and easy to operate consuming low energy.

 

The specific aims of this project is to 1) implement simple computational simulation and calculations 2) devise a prototype device for continuous process, and 3) build a control system for a batch process. The simulation part will be done with the collaboration with graduate students studying computational fluid dynamics (CFD). The rest will be achieve with glass capillary system coated with surface molecules for passivation and protection. This system would prove the principle found in the previous work can be also applied in tubular systems. For these aims, the following activities will be implemented.

 

Computational Fluid Dynamics: For both methods mentioned above, I will investigate the feasibility of volumetric flow and velocity difference with computer simulation getting help from a graduate student involved in CFD. A simple model would be set up to see the difference velocity of several spheres with different boundary conditions flowing through a cylinder.

 

Devising adequate cylindrical geometry: An experimental equipment would be built. I propose 100 ~ 1000 nm glass capillaries. The interior of the capillaries would be coated with octadecyltrichlorosilane for protection and smoothness. The outlet would be connected to gas chromatography to detect the effluent of different gases

 

Continuous separation experiment: To test this method, a chamber would be built for a binary mixture with oxygen and carbon monoxide. I chose these species because detection equipment is very precise and requires low concentration. Then their concentration would be measure at the outlet.

 

The success of this project would provide clean separation technology with low energy consumption and high throughput without having severe operation conditions. The process will integrate the science and engineering in different fields, surface science, process control, fluid mechanics, and computational calculation, into one field. This will lead to several patents further utilized in commercial separation units.

 

  Roughening of Aerosol for Deep Lung Delivery  

1. Background

 

Chronic Obstructive Pulmonary Diseases (COPDs) are the third leading cause of mortality in the United States  and about 15 million adults have reported they have COPD. Another lung disease, asthma, is even more common for both adults and children; in the United States: one out of 12 adults and one out of 11 children have asthma. Delivery of drugs to treat COPD, asthma, and other pulmonary diseases is achieved through aerosolized (small liquid droplet) medications, primarily delivered via inhalers. These inhalers are used to spray drugs into the lungs via the mouth, which naturally results in unnecessary deposition close to the entrance of lung, rather than deeper into the lung where delivery is required. A longer average travel distance of aerosol particles is therefore desired for better treatment.

 

2. Specific Aims:

 

The aim of this work is to enhance the travel distance of inhalant aerosol medication inside the lungs by increasing the resistance to droplet motion towards airway surfaces (See Fig. 1).  By opposing motion toward the airway surfaces, deposition on the walls is delayed, and the droplets are able to travel to the distal portions of the lungs.

Fig. 1. A stronger lubrication force on rough particles delays approach of particles to lung surfaces, thereby extending travel into the deep lung.

 

Until recently, many chemical engineers assumed the boundary condition for the flow of gases was fixed.  But recent research by our group has shown that very slight (nanoscale) roughening of surfaces can be used to increase the resistance to a particle approaching a solid in air.  The required increase in resistance to motion towards airway surfaces will be achieved through deliberate roughening of the surface of the aerosol droplet through the addition of surfactant. Whether the aerosol particles attempt to make contact with lung passage walls due to gravity, or due to airway curvature, the additional resistance will always push the droplets away from the wall. This allows each droplet to utilize momentum parallel to lung surface, and thereby travel deeper into the lungs.

 

The specific aims are to test the following hypotheses: 1)The addition of surfactants increases the lubrication force acting on particles approaching a wall. 2) These surfactants adsorb to the aerosol – gas (inhaled air) interface, and increase its roughness. 3) An increased lubrication force increases the distance that a droplet travels in a numerical model.4) An increased lubrication force increases the distance that a droplet travels in an experimental model system. 5) Optimum concentrations of surfactants exist and vary by the kind of surfactant and medication.

 

Successful completion of this project will provide a relatively simple, inexpensive, and safe way to enhance delivery of aerosol-based drugs into the entire lung. Most importantly, the increased delivery distance will help to obtain a more even distribution of the drug, which may allow lower doses, and fewer side effects.  Since this enhancement would come by simply adding appropriate FDA-approved surfactants to existing aerosol delivery systems, this additive technology is more likely to be adopted.  The major impact of adoption would be better therapeutic outcomes for both sufferers of pulmonary diseases, and recipients of systematic drugs.

Teaching Interests:

 

I am very interested in interaction with chemical engineering students and helping them become more prepared engineers after graduation. With such interest, I am willing and able to teach general chemical engineering topics in any level, as well as special topics such as surface and colloid chemistry, atomic force microscope (AFM), refinery and chemical plant processes. My diverse research experiences scope through different areas of chemical engineering, physics, mechanical engineering, and material science. In addition to this research activities, my practical experience in a downstream oil company I worked for four years provided me with not only the basic principles of chemical engineering, but also the practical integration of knowledge that is needed for chemical engineers.

 

Teaching Philosophy

My basic teaching philosophy consist of 1) teaching students correct principles and knowledge, 2) encouraging them to utilize the knowledge, 3) getting them involved in teaching, and 4) following up. I believe these principle are needed for students in any fields to learn. As with other teachers, I will lecture with much preparation focusing on how to have them involved during lectures. I will make sure they understand through homework and projects they have to work together and teach together. In my belief, they have to at least attempt to speak or write out what they have learned to their colleagues in their own language. Otherwise, very little can be left with them. Therefore problems and projects will be oriented toward teaching each other. My teaching method will be also tested with exams that require not only calculations but also require interpretation of the calculated results. I have seen from much experience that the students like to produce results. I will help them make one step further by letting them think for physical meaning for the result and the application thereof. This application of knowledge is also important. I will assign a group project with which they can think of the solution to a problem with the knowledge from the courses. I will follow up on their progress by encouraging them to come to office hours to make individual contacts. This way, I can follow up on their progress. This principle will be also true to graduate students. I will brainstorm with them every week to see if they and I can prepare for the next projects they are on by combining results from different projects.

 

Specific Topics of Interest

Among many classes, I like to teach colloids and surface science along with AFM. I like to involve AFM, because it demonstrates the surface forces with visual images. With the advance of nanotechnology and polymer science, the small forces or relatively insignificant properties became important in understanding physical phenomena and chemical engineering processes. I believe chemical engineering students have to see the importance in these features and be able to manipulate the variables to obtain desired results. If this course is not opened, I am happy to prepare the course.

Another course I am very interested in is Unit Operation Laboratory (UO Lab). I loved teaching UO Lab as a graduate student because it gave me more chance of talking to students and finding out not only their knowledge but also their interest in future as engineers. However, the room for improvement I saw among students gave me strong desire to teach this class. I would like to lead this class with evocation of discussion and analysis of the activity and data. One thing I noticed was student tend to focus on the results, and not the meaning. I like to teach them how to produce meaningful conclusion out of data.

 

 

Teaching Experiences

Please refer to my curriculum vitae as well. I have actively engaged in the interaction with undergraduate students by 1) helping them for undergraduate research, 2) lecturing for different classes, and 3) teaching at experimental laboratory classes. These experiences prepared me as an effective teacher. My goal in teaching is helping them independent and I saw such progress in them. I have taught junior, senior, and graduate-level classes as well as lab classes. I also supervised three different undergraduate research projects. In all of them, I had a chance to see them grow toward understanding the problem, finding solution, and being autonomous.

  Outreach Effort   I want to direct my outreach effort mostly toward, but not limited to, K6 to K12 students. Youths, especially girls, around this age lose their interest in science, technology, engineering and mathematics (STEM) area, and especially when they do not make connection between STEM knowledge and the real life. (Ref. 1) Therefore it is important to let them realize that we are living in a world utilizing knowledge from STEM area. I like to make such connection using colloid and surface science. For younger students from K6 to K9, I like let them know daily-life products, shampoo, make-ups, cleaning agents, and even diapers, not only work, but also are better with surface science. This demonstration or a set of simple 5 – 10 minute experiments would be presented during a field trip or as one of outreach experiences for school breaks. During this presentation, each student will be given materials and experience on their own hands how surface science is related to their daily life. For older students in K10 to K12, more advanced project is possible. The surface modification with monolayer molecules, atomic force microscope (AFM) with their own samples, or click-chemistry would be a fun and educating experience for them. A simple monolayer-coating experiment can be done by a group of high school students with much care instruction. This way, they will learn safety issue as well. AFM is hard for them. But they will have fun in seeing the nanometer-sized details of the materials they use every day. Safe click-chemistry experiments that can be produced with house chemicals can be a good start for their scientific journey.   The focus of my outreach is toward school students with easy and fun experiments. This way, they will relate their daily life to science.   Ref. 1 Jeffers, A., Safferman, A., & Safferman, S. (2004). Understanding K-12 engineering outreach programs.

Journal of Professional Issues in Engineering Education and Practice, 130(2), 95-108.