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Dingsheng Liu Bejan Hakimi Michael Volny Joelle Rolfs Robbyn K. Anand Frantisek Turecek Daniel T. Chiu 《Biomicrofluidics》2014,8(4)
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 = μaUa/σ, and electric Bond number, Boe = E2(εD/σ). Ca and Boe describe the ratio of viscous force to interfacial tension force and the ratio of electric-field force to interfacial tension force, respectively. Here, μa (1 mPa s), σ (8.5 mN/m), ε (7.1 × 10−10 F/m), E, Ua, 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 Boe, 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 Boe 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 Boe 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. 相似文献
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Internet Protocol (IP) technology has played an important role in the management of access to scholarly work within institutions since the advent of electronic publishing in the late 1990s. Despite recent efforts to explore alternative modes of authentication, the ubiquitous use of IP address authentication within the systems of libraries, publishers, and third party vendors around the world suggests it will continue to play a significant role for the foreseeable future. However, recent studies show that a large amount of the IP address data held within publisher systems is incorrect, improperly assigned, or requires clarification. Poor source data carries significant risks—not the least of which translates into unreliable usage reporting for libraries and publishers. The IP Registry is designed to help mitigate these challenges by creating a Registry of cleaned data that libraries and publishers can interact with in a centralized fashion thereby helping to ensure accuracy and reducing the level of resources necessary to manage and communicate changes to existing data. The database of more than 60,000 academic, corporate, and government libraries was created through the amalgamation of raw source data from more than 170 scholarly publishers. In addition to cleaned IP address data, the Registry includes standardized English naming, aliases, local language naming (and characters), hierarchies, and organizational IDs for each record. The importance of accurately understanding current usage and how libraries utilize published content is clear. Regardless of what the future holds, the need for a clean and stable source of IP address data is real for libraries, publishers, and vendors alike. 相似文献
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Zheng Zhang Joelle Nagle Bethany McKishnie Zhen Lin Wanjing Li 《Pedagogies: An International Journal》2019,14(1):33-61
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. 相似文献
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Jerónimo García-Fernández Pablo Gálvez-Ruíz Jesús Fernández-Gavira Luisa Vélez-Colón Brenda Pitts Ainara Bernal-García 《Sport Management Review》2018,21(3):250-262
Due to its importance in fitness centers, a number of authors have explored and analyzed loyalty. However, two characteristics not yet examined are service convenience in fitness center chains and low-cost fitness centers (an emerging business model). In the present study, the authors sought to understand the relationship among perceived quality and service convenience on perceived value, satisfaction, and client loyalty in low-cost fitness centers. Clients (N = 763; 381 women and 382 men) from three low-cost Spanish fitness centers were surveyed, revealing a positive relationship among the variables studied. Findings demonstrate the importance of proper management of non-monetary sacrifices and perceived quality by the managers of these sport organizations, since client loyalty could depend on factors of these emerging sport models. 相似文献
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The purpose of the current study was to estimate reliability, internal consistency and construct validity of the Measure of Instruction for Creative Engagement (MICE) instrument. The MICE uses an iterative process of evidence collection and scoring through teacher observations to determine instructional domain ratings and overall scores. The results demonstrated the sound inter-observer reliability, teacher stability and score validity of the MICE. We found (a) a low proportion of rater variance (0.14–5.99%), (b) moderate to highly correlated within-teacher ratings ranging from r(17) = 0.663, p < 0.01 to r(17) = 1.000, p < 0.01 and (c) a statistically-significant difference between classroom teachers and teaching artists, t(56) = 7.37, p = 0.000. These results relate to the development of classroom environment instruments and the substantive development of pedagogy that supports creative thinking and behaviours, both of which are a priority for enhancing teacher accountability and student learning. 相似文献
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William T. Pilkington Don Graham William R. Meyer Michael Pitts Gerald Mast 《Communication Booknotes Quarterly》2013,44(4):82-84
William T. Pilkington and Don Graham, eds., Western Movies (Albuquerque: University of New Mexico Press, 1979–$13.95/$6.95) William R. Meyer, The Making of the Great Westerns (New Rochelle, New York: Arlington House, 1979 – $20.00) Michael Pitts, Famous Movie Detectives (Metuchen, New Jerseys Scarecrow Press, 1979 – $11.95) Gerald Mast's The Comic Mind (Chicago: The University of Chicago Press, 1979– $7.95 paper) 相似文献
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James H. Pitts 《Clearing house (Menasha, Wis.)》2013,86(5):197-199
Dealing with student behavior is often cited as one of the most frustrating aspects of teaching, yet many classroom teachers receive no pre-service training in the basics of behavior management. This article describes the process of implementing a quick behavioral analysis for the purpose of designing a basic intervention. It will provide general education teachers with the means to choose the appropriate interventions when behavioral problems occur in their classrooms. 相似文献
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ABSTRACTCultural 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. 相似文献
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Scott A. Myers James P. Baker Heather Barone Stephen M. Kromka Sara Pitts 《Communication Research Reports》2018,35(2):131-140
Guided by rhetorical/relational goal theory, the purpose of this study was to examine whether students’ impressions of their instructors’ credibility, attraction, and homophily are influenced by four specific rhetorical and relational communication behaviors that instructors use when communicating with their students in the classroom. Participants were 193 undergraduate students enrolled in a variety of introductory communication courses at a large mid-Atlantic university. Using a series of multiple regression analyses, it was found that (a) clarity emerged as the sole indicator of instructor competence, trustworthiness, and task attraction; (b) clarity and confirmation emerged as indicators of goodwill; and (c) humor and confirmation emerged as indicators of social attraction and attitude homophily. 相似文献