Modulating patterns of two-phase flow with electric fields

This paper describes the use of electro-hydrodynamic actuation to control the transition between three major flow patterns of an aqueous-oil Newtonian flow in a microchannel: droplets, beads-on-a-string (BOAS), and multi-stream laminar flow. We observed interesting transitional flow patterns between droplets and BOAS as the electric field was modulated. The ability to control flow patterns of a two-phase fluid in a microchannel adds to the microfluidic tool box and improves our understanding of this interesting fluid behavior.Microfluidic technologies have found use in a wide range of applications, from chemical synthesis to biological analysis to materials and energy technologies.^1,2 In recent years, there has been increasing interest in two-phase flow and droplet microfluidics, owing to their potential for providing a high-throughput platform for carrying out chemical and biological analysis and manipulations.^3–8 Although droplets may be generated in many different ways, such as with electric fields or extrusion through a small nozzle,^9–12 the most common microfluidic methods are based on the use of either T-junctions or flow-focusing geometries with which uniform droplets can be formed at high frequency in a steady-state fashion.^13,14 Various operations, such as cell encapsulation, droplet fusion, splitting, mixing, and sorting, have also been developed, and these systems have been demonstrated for a wide range of applications, including cell analysis, protein crystallization, and material synthesis.^1–17In addition to forming discrete droplets, where a disperse phase is completely surrounded by a continuous phase, it is also possible in certain situations to have different phases flow side-by-side. In fact, multi-stream laminar flow, either of the same phase or different phases, has been exploited for both biochemical analysis and microfabrication.^1,2,18–20 Beads-on-a-string (BOAS) is another potential flow pattern, which has been attracting attentions in microfluidics field. BOAS flow, owing to its special flow structures, may be particularly useful in some applications, such as optical-sensor fabrication.²¹ In BOAS flow, queues of droplets are connected by a series of liquid threads, which makes them look like a fluid necklace with regular periods.^21–25 The BOAS pattern is easily found in nature, such as silk beads and cellular protoplasm, and is often encountered in industrial processes as well, such as in electrospinning and anti-misting.^21,22 In general, it is thought that BOAS structure occurs mostly in viscoelastic fluids²² and is an unstable structure, which evolves continually and breaks eventually.^21–29Flow patterns determine the inter-relations of fluids in a microdevice and are an important parameter to control. Common methods for adjusting microfluidic flow patterns include varying the fluid flow rates, fluid properties, and channel geometries. Additionally, the application of an electric field can be a useful supplement for adjusting microfluidic flow patterns, although most work in this area has been focused on droplets and in some cases also on multi-stream laminar flows.^30–33 Here, in addition to forming droplets and two-phase laminar flow with electro-hydrodynamic actuation, we also observed a new stable flow pattern in a non-viscoelastic fluid, BOAS flow. Such flow patterns may find use in controlling the interactions between droplets, such as limited mixing by diffusion between neighboring droplets.To generate droplets, we used the flow-focusing geometry (Figure 1(a)), in which aqueous phase (water) was flown down the middle channel and droplets were pinched off by the oil phase (1-octanol) from the two side channels at the junction; Figure 1(b) shows the droplets formed after the junction. To apply electric field along the main channel where the droplets were formed, we patterned a pair of electrodes upstream and downstream of the junction (Figure 1(a); for experimental details, please see Ref. ³⁴ for supplementary material). The average electric field strength may be calculated from the voltages applied and the distance (1.7 mm) between the two electrodes. When a high voltage was applied along the channel between the two electrodes, the aqueous-oil interface at the flow-focusing junction became charged and behaved like a capacitor. As a result, more negative charges were drawn back upstream towards the positive electrode, and left behind more positive charges at the aqueous-oil interface, which then became encapsulated into the aqueous droplets dispersed in the oil phase.Open in a separate windowFIG. 1.(a) Schematic of the setup. (b) Micrograph showing droplet generation in a flow-focusing junction. The scale bar represents 40 μm.The positively charged aqueous-oil interface was stretched under an applied electric field, and by adjusting the voltage and/or the two-phase flow-rate ratio, we found interestingly that various flow patterns emerged. We tested different combinations of applied voltages and flow-rate ratios and found that most of them resulted in similar flow patterns and transitions between flow patterns.Figure ​Figure22 illustrates the effects of varying the applied voltages on droplets at a fixed liquid flow rate. With increasing electric-field strength and force, we found it was easier for the aqueous phase to overcome interfacial tension and form droplets. For example, as the voltage increased from 0.0 kV to 0.8 kV (average field strength increased from 0 to 0.47 V/μm), droplet-generation frequencies became slightly higher, and the formed droplets were smaller in volume. Additionally, droplets gradually became more spherical in shape at higher voltages.Open in a separate windowFIG. 2.Images showing the effects of applied voltage on droplet shape and flow pattern. Oil-phase flow rate, 0.5 μl/min; aqueous-phase flow rate, 0.2 μl/min. The scale bar represents 40 μm.As the voltage increased further (e.g., up to 1.0 kV in Figure ​Figure3),3), the distance between neighboring droplets became smaller, and the aqueous-oil interface at the junction was stretched further toward the downstream channel. At a threshold voltage (1 kV here with corresponding average field strength of 0.59 V/μm), the tip of the aqueous-oil interface would catch up with the droplet that just formed, and the tip of the interface of this newly captured droplet would in turn catch up with the interface of the droplet that formed before it. Consequently, a series of threads would connect all the droplets flowing between the two electrodes, thus resulting in a BOAS flow pattern.Open in a separate windowFIG. 3.Series of images showing the reversibility and synchronicity of a transitional flow pattern between droplets and BOAS (bead-on-a-string). Voltage applied, 1.00 kV (corresponding field strength of 0.59 V/μm); oil-phase flow rate, 0.5 μl/min; aqueous-phase flow rate, 0.2 μl/min. The scale bar represents 40 μm.At voltages near the threshold value, the flow pattern was not stable, but oscillated between droplets flow and BOAS flow. Figure ​Figure33 is a series of images captured by a high-speed camera that show the flow in this transition region. In Figures 3(a) and 3(b), the string of BOAS became thinner over time, and then the BOAS broke into droplets (Figures 3(c) and 3(d)). The newly formed droplets, however, were not stable either. Thin liquid threads would appear and then connect neighboring droplets, and a new switching period between discrete droplets and BOAS would repeat (Figures 3(e)–3(h)). In addition to this oscillation and reversibility, the flow pattern had a synchronous behavior: all the droplets appeared connected simultaneously by liquid threads or were separated at the same time.When the voltage reached 1.3 kV, which corresponded to an average field strength of 0.76 V/μm, a stable BOAS flow was obtained (Figure 4(a)). BOAS structures are thought to be present mostly in viscoelastic fluids,²² because viscoelasticity is helpful in enhancing the growth of beads and in delaying breakup of the string; thus, the viscoelastic filament has much longer life time than its Newtonian counterpart. Here, with the help of electric field, regular BOAS structures are realized in a non-viscoelastic fluid (water) in microchannels.Open in a separate windowFIG. 4.(a) Micrograph showing BOAS flow in a channel. (b) Profile of the top-half of the BOAS flow recorded continuously at a cross-section (shown in Figure 4(a)) of a channel. Voltage applied, 1.30 kV (corresponding field strength of 0.76 V/μm); oil-phase flow rate, 0.5 μl/min; aqueous-phase flow rate, 0.2 μl/min. The scale bar represents 40 μm.Microenvironment and electric fields alter the common evolution of BOAS structure observed in macroscopic or unbound environments. The BOAS structure formed in our experiments is not a stationary pattern, but a steady-state flowing one. Electric-field force prevents liquid strings from breaking between beads, and thus plays a similar role as elastic force in viscoelastic fluids. Figure 4(b) shows the dynamic BOAS profile, obtained at a fixed plane (shown in Figure 4(a)) perpendicularly across the channel as the BOAS structure passed through it. Droplets and liquid-thread diameters were nearly constant during the sampling time. The longer term experiments (over 3 min) showed there were slight variations of the two diameters in time, but the essential BOAS structure still remained qualitatively the same as a whole.When the voltage was further increased, the string diameter became larger and the droplet diameter became smaller. Because of the low flow-rate ratio (0.4) between the aqueous phase and oil phase used in the experiment depicted in Figure ​Figure4,4, the flow did not further develop into a multi-stream laminar flow, as would be expected at a higher voltage, and instead became unstable and irregular. When the flow-rate ratio was increased to 1.0 and the voltage was adjusted to 3.0 kV (corresponding field strength of 1.76 V/μm), we observed a stable multi-stream laminar flow (Figure ​(Figure5).5). The aqueous stream flowed in the channel center surrounded by the oil phase on the sides. This experiment showed that higher electric-field strengths alone would not give rise to another stable flow pattern (i.e., multi-stream laminar flow), but a suitable flow-rate ratio of aqueous phase to oil phase is required for the formation of stable two-phase laminar flow.Open in a separate windowFIG. 5.Micrograph showing multi-stream two-phase laminar flow in the channel. Voltage applied, 3.00 kV (corresponding field strength of 1.76 V/μm); oil-phase flow rate, 0.5 μl/min; aqueous-phase flow rate, 0.5 μl/min. The scale bar represents 40 μm.The flow patterns we observed may be described by a phase diagram (Figure ​(Figure6),6), which depends on two dimensionless numbers: capillary number, Ca = μ_aU_a/σ, and electric Bond number, Bo_e = E²(εD/σ). Ca and Bo_e describe the ratio of viscous force to interfacial tension force and the ratio of electric-field force to interfacial tension force, respectively. Here, μ_a (1 mPa s), σ (8.5 mN/m), ε (7.1 × 10⁻¹⁰ F/m), E, U_a, and D are, respectively, the aqueous-phase viscosity, aqueous-oil interfacial tension, aqueous-phase permittivity, electric field strength, aqueous-phase velocity, and the hydraulic diameter of the channel at the junction. Figure ​Figure66 shows clearly that at higher Ca, flow pattern changes gradually from droplet to BOAS and to multi-stream laminar flow with increasing Bo_e, which indicates the increasing importance of the electric-field force compared with the interfacial tension force. At lower Ca, flow pattern and transition show similar trend with increasing Bo_e as in the higher Ca case, except that multi-stream laminar flow is not observed. The relatively higher viscous force at higher Ca may be necessary for transitioning to the multi-stream laminar flow regime. In addition, Figure ​Figure66 shows that the BOAS window at the lower Ca is smaller than that at the higher Ca.Open in a separate windowFIG. 6.Phase diagram showing different flow patterns in the Ca and Bo_e space. Hollow symbols: oil-phase flow rate, 0.5 μl/min; aqueous-phase flow rate, 0.5 μl/min. Solid symbols: oil-phase flow rate, 0.5 μl/min; aqueous-phase flow rate, 0.2 μl/min.In summary, we showed the ability to use electric fields to generate and control different flow patterns in two-phase flow. With the aid of an applied field, we were able to generate BOAS flow patterns in a non-viscoelastic fluid, a pattern that typically requires a viscoelastic fluid. The BOAS structure was stable and remained as long as the applied electric field was on. We also report transitional flow patterns, those between droplets and BOAS exhibited both good reversibility as well as synchronicity. And with a suitable flow-rate ratio between the two phases, BOAS flow could be transitioned into a stable two-phase laminar flow by applying a sufficiently high field strength. Finally, a phase diagram was presented to describe quantitatively the flow-pattern regimes using capillary number and electric Bond number. The phenomena we report here on the properties of two-phase flow under an applied electric field may find use in developing a different approach to exert control over droplet based or multi-phase laminar-flow based operations and assays, and also aid in understanding the physics of multi-phase flow.     

Scientific strengths and reported effectiveness: a systematic review of multiliteracies studies

This systematic review is built on the seminal work by the New London Group in 1996. Few endeavours have synthesized findings of empirical studies pertaining to the effects and challenges of multiliteracies practices in various schooling and geographical contexts. Through a five-point Likert scale and a deductive and inductive thematic analysis, we conducted a systematic review of 66 multiliteracies articles from the ProQuest® database. These studies were empirical, qualitative/mixed-method, and ranged from 2006 to 2015. Findings show a burgeoning number of multiliteracies studies occurring in 15 countries, with Canada being the most prominently involved. Our evaluation of the reviewed studies was generally favourable with strengths identified in researchers’ articulation of pertinent theoretical frameworks and connections to existent literature. Our findings refer to insufficient information of data collection and data analysis in a certain number of papers. We also elaborate on major affordances, challenges, and oversights of the multiliteracies practices as reported by the reviewed studies and discuss implications for future multiliteracies research, policies on literacy education, and teacher education in diverse contexts.     

The Imperative of Soft Skill Development in Preventive Conservation Practice and Training

ABSTRACT

Cultural heritage is wonderfully diverse and as heritage preservation professionals, it is our duty to address the preventive conservation of all cultural heritages. However, there is no one set of guidelines, practices or rules that can be applied in all situations. A preventive conservator with strong technical and soft skills is essential in this situation. At the Winterthur/University of Delaware Program in Art Conservation (WUDPAC), preventive conservation has been an identifiable part of the curriculum since the early 1980s. From its establishment, the curriculum has evolved to include the teaching of both current day technical skills for the practice of preventive conservation and soft skills in teamwork, leadership, institutional priorities and goals identification, and written and oral communication. These are taught to all students in their first year and those that select the option of a preventive conservation minor in their second year. Recently, it has become clear that it is not possible to teach adequately both the technical and soft skills needed for the practice of preventive conservation within WUDPAC's existing minor area of study. Additional time is needed to study and develop the complex theories, abilities and requisite skills that characterize the preventive conservation specialty. This paper discusses the work to develop a curriculum for a WUDPAC preventive conservation major, the strong mandate to continue to teach both technical and soft skills, and the surprising resistance to the establishment of a preventive conservation major.     

Railways,rebellions and Rage Against The Machine: adolescents' interests and meaning‐making in the creation of multimodal identity texts

This paper draws on a Canadian qualitative case study grounded in multiliteracies theory to describe the meaning‐making processes of four students aged 13‐14 years as they created history projects. Students were invited to explore curriculum content in self‐chosen ways and to produce presentations in a range of formats. The data we present and discuss were collected through participant observation and in‐situ interviews with four students who selected digital formats. We examine these data using multiliteracies concepts: specifically multimodality and identity texts. We argue that multimodal literacy practices have potential to bridge gaps between students' in‐school and out‐of‐school lives and underscore the importance of allowing students to draw on their out‐of‐school identities and interests to guide explorations of curriculum content.     

A review of key parameters for effective electrophoretic deposition in the fabrication of solid oxide fuel cells

This paper reviews recent progress in electrophoretic deposition (EPD), particularly in solid oxide fuel cells (SOFCs). EPD is a simple, cost-effective, and geometrical flexible colloidal process. With its excellent control of thickness and other morphological characteristics, it is favored for the fabrication of SOFCs because each component layer of an SOFC has different requirements. However, the effectiveness of EPD is closely related to the suspension stability and EPD processing parameters. Maintaining a stable suspension and optimizing the EPD processing parameters are essential to achieve a dense and uniform deposition layer. Key parameters in maintaining the suspension stability are generally categorized into colloidal related parameters, including particle size and solid loading, and suspension media related parameters, including dielectric constant and conductivity. The effects of these parameters are often reflected by the zeta potential of the suspension, which can be manipulated by using charging agents to maintain a stable state. The deposition time and applied voltage are key parameters in optimizing the EPD process through their effects on the deposition rate. The effects of these parameters on particle surface charges and on the EPD mechanism are discussed.     

Leveraging competency framework to improve teaching and learning: A methodological approach

A number of engineering education programs have defined learning outcomes and course-level competencies, and conducted assessments at the program level to determine areas for continuous improvement. However, many of these programs have not implemented a comprehensive competency framework to support the actual delivery and assessment of an individual course. This paper highlights how a competency framework can be used across the life cycle of a course to effectively deliver and assess course content, and give valuable, timely feedback to students thus, improving teaching, student motivation and learning. A framework for leveraging course competencies during course design and delivery is presented, and addresses the following five phases of a course, namely, content design, assessment design, content delivery and assessment, assessment results analysis and feedback, and content review. Using a large first-year core course of the BSc (Information Systems Management) program, at School of Information Systems, Singapore Management University, Singapore—called Information Systems Software Foundation (ISSF)—as an example, this paper shows how course competencies support the framework’s five phases. Data from a student survey indicates that the framework has contributed to enhancing their motivation to learn, provides enhanced learning experiences in terms of helping students prepare for each assessment, providing better feedback by raising awareness of what they know and do not know, and revisiting topics that relate to competencies that have not been fully acquired. Results from interviewing instructors revealed that the competency framework provides valuable and timely feedback on how students are performing, and additionally what changes are required to both the content and method of delivery in order to improve teaching. This contributes towards more effectively closing the “teaching and learning loop”.     

How Do We Market to Distance Populations,and Does It Work?: Results From a Longitudinal Study and a Survey of the Profession

This article discusses the results of a longitudinal survey conducted by Kansas State University Libraries to assess the marketing of library services to the university's distance education population. The latest implementation of the survey revealed overall similar or increased awareness of and satisfaction with library services compared to the previous survey. Also included are results of a survey of library professionals on best practices for distance education librarians. Findings include the importance of developing relationships with campus distance education offices and faculty and the need to be persistent with marketing efforts to a changing population.     

Approach-avoidance conflict in preweanling rats: A developmental study

Ten- and 15-day-old rat pups were trained with two procedures to approach an anesthetized mother, and then were punished for approaching. Both ages of subjects exhibited increased latencies to reapproach the mother, indicating passive inhibitions, but only the older pups retreated. All but one of the younger pups eventually reached the mother within 3 min after the punishment, while only half of the older pups did so. In a second experiment examining the development of locomotor avoidance reactions, 5- to 20-day-old rats were shocked without the mother present. Fifteen- and 20-day-old rats significantly decreased their activity patterns in reaction to shock and spent significantly less time in the shock area than either of the younger aged pups. These results suggest that flight reactions are components of a rat’s defensive repertoire that appear very rapidly between 10 and 15 days of age.     

TO BECOME ONE'S OWN MEDIATOR

Promoted by the Free University of Brussels, supported by the EEC, the transnational action‐research “To become one's own mediator” is applying and evaluating a violence prevention programme in elementary schools. We are creating and introducing the programme to the children (ages 8 to 10) and training the educational personnel. Inquiries are being made: questionnaire on transgressions, semi‐directive conversation about definitions of “violence”, test of attitudes in front of situations including social interaction, etc… Through that action research, our goal is to encourage our decision makers to include socio‐emotional programme in the school basic cognitive training. (It should be noted by U.K. readers that ‘violence’ is used in European research to describe acts of incivility. The role of a primary school teacher is also less global For example children will usually be supervised outside the classroom by people employed for that task).     

Key Elements of a Successful Multi-System Collaboration for School-Based Mental Health: In-Depth Interviews with District and Agency Administrators

The alarming number of youth with unmet mental health needs in the US is a significant social problem. The pilot school-based mental health project described here established an innovative multi-system partnership between an urban school district, a public mental health agency, and a local university to better meet the mental health needs of youth in one community. This qualitative study employed in depth interviews with six key administrators who were instrumental in developing and executing the project to explore the most important factors that promoted the successful collaboration. Results of the interviews identified five major themes: (a) perceptions of the project, (b) barriers to collaboration, (c) motivating factors, (d) sustainability, and (e) lessons learned. Findings may be especially helpful to other communities interested in establishing a multi-system intervention to support at risk youth. Implications for practice and research are discussed.