Article pubs.acs.org/Langmuir
Influence of Surface Wettability on Microbubble Formation Daniel J. Wesley, Rachel M. Smith, William B. Zimmerman, and Jonathan R. Howse* Department of Chemical and Biological Engineering, University of Sheffield, Mappin Street, Sheffield S1 3JD, United Kingdom S Supporting Information *
ABSTRACT: The production and utilization of microbubbles are rapidly becoming of major importance in a number of global applications, from biofuel production to medical imaging contrast agents. Many aspects of bubble formation have been studied, with diffuser characteristics (such as pore size, pore orientation) and gas flow rate all being shown to influence the bubble formation process. However, very little attention has been paid to the influence of surface wettability of the diffuser and the detailed role it plays at the triple interface of gas− liquid−diffuser. Here, we investigate how the wettability of the diffuser surface impacts upon the dynamics of the bubble formation process and examine the effect both at the orifice and upon the bubble cloud produced as a result of the engineered wetting variations. Experimental data shown here indicate the presence of a switching point at a contact angle of θ = 90°, where bubble size vastly changes. When a surface exhibits a contact angle below 90°, bubbles emitted from it are considerably smaller than those emitted from a surface with an angle in excess of 90°. This effect is observable over flow rates ranging from 2.5 to 60 mL min−1 from a single pore, an array of controlled pores, and the industrially relevant and commercially available sintered metals and sintered ceramic diffusers. It is also observed for both thiol and silane modified surfaces, encompassing a range of contact angles from 10° to 110°. In addition, the importance of the diffuser plate’s surface topography is discussed, with elevated roughness acting to reduce the effect of surface chemistry in some instances.
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INTRODUCTION AND BACKGROUND Since the mid 1990s, bubbles have become an emerging theme within engineering, with the number of applications growing and diversifying into a wide range of areas such as biofuel production,1−6 microflotation,6−11 and medical imaging and drug delivery.12−18 More recently, the use of very small bubbles has become of great interest, with microbubbles, defined as being in the size range of 1−999 μm now being extensively studied and utilized.6,7,9,14,16,19−25 From an engineering viewpoint, the appeal of microbubbles is relatively straightforward. They are desirable due to their high surface area (for a given gas volume) and possess an inherent ability to facilitate high levels of mass transfer, which is directly related to the interfacial area between two phases, as characterized by eq 1. J = KlS(cg − c l)
that algae is of current interest to the scientific community, as it has the ability to efficiently convert solar energy into chemical energy sources such as cellulose, oils, and starch in high yields.1,27,28 Furthermore, algae have been shown to be able to produce other forms of fuel such as glycogen and hydrogen.1−5 As a result, the high mass transfer capability of microbubbles has been employed in algal growth and processing, and it has been shown that higher yields of both algae and biofuel can be attained through the application of microbubbles.7,8,11,29 To maximize the biofuel production from algae, the photosynthesis process must be maximized. The algae must be subjected to elevated levels of CO2, while maintaining the exposure to sunlight. Zimmerman et al.26 have commented upon the design of an airlift loop bioreactor (ALB), which has been reviewed more thoroughly by Jones.30 These ALBs are perceived to be more efficient than the other variants used for algal growth, and they hold great promise for further study.26 To create microbubbles, and improve mass transfer, the reactor utilizes the oscillating flow created by the fluidic oscillator of Zimmerman and Tesar31−34 to generate bubbles with a mean diameter of 300 μm, raising CO2 levels to above the 380 ppm of atmospheric air. Work by Hanotu7 and Ying8 has shown a
(1) −1
where J is the interphase mass transfer rate (mol s ), Kl is the mass transfer coefficient (m s−1), S is the interfacial area (m2), and cg and cl are the molar concentrations of gas and liquid, respectively.26 Many current renewable energy sources, and in particular those centered upon biofuel production, still focus upon higher plant life, and as a result high levels of inefficiency are an inherent problem due to the poor conversion of solar energy to a pure form of chemical energy.1 It is due to these inadequacies © XXXX American Chemical Society
Received: October 7, 2015 Revised: January 11, 2016
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DOI: 10.1021/acs.langmuir.5b03743 Langmuir XXXX, XXX, XXX−XXX
Article
Langmuir 30−40% enhancement in algal growth with the application of microbubbles. Pilot scale studies by Zimmerman, utilizing CO2 from the combustion of waste exhaust gases emitted during steel production, showed how the microbubbles efficiently stripped deleterious oxygen from the reactor during the growth regime, increasing algal proliferation. The separation and harvesting of algae from the growing medium is also one of the key challenges to the commercial viability of biofuel production. Gudin35 estimates that the harvesting step could account for 20−30% of the total energy usage in biofuel production, although Molina stipulates this figure could be much higher, and as large as 60%.29 Historically separation has been carried out by flocculation and bioflocculation followed by sedimentation. Development routes for algal separation include using the Jameson cell in an induced air flotation (IAF) step, dissolved air flotation (DAF), and dispersed air flotation, which offer the possibility of >95% separation. However, all of these techniques have drawbacks when compared to microflotation. Microbubble generation by a robust technique, such as the fluidic oscillator systems presented by Zimmerman and Tesar,31−33,36 have proven to be a breakthrough in energy efficient separation. The “no moving part” nature and robustness of the oscillator leads to energy usage 2 or 3 times lower than that used in DAF and dispersed air flotation, for example.11 In the work by Hanotu,11 microflotation was presented as a viable alternative for algal separation, finding separation efficiencies of more than 96% could be achieved with the fluidic oscillator, in addition to the significant energy saving. Despite the increasing interest in microbubbles, the ability to control bubble formation to create monodisperse bubble clouds with a chosen bubble size is still lacking. Many factors such as pore size, pore orientation, surface tension, and flow rate influence bubble size and formation. Many groups have alluded to the importance of wettability on bubble formation, but very few fundamental studies exist on the effect of surface wettability on bubble size, especially in aqueous systems.37−45 It is important, and prudent, to recognize the substrate from which the bubbles are emitted plays a vital role in their formation. Bubbles created by a nozzle, capillary, or needle have a small surface around the orifice meaning the effect of surface wettability is greatly reduced, and in some cases negated altogether. This is exemplified by Zhu,46 who used both a glass capillary and a polytetrafluoroethylene (PTFE) capillary. They found the departure time from the glass capillary was longer than that of the PTFE, which is in contrast to the observations of Corchero.42 Bubbles emitted from a hydrophobic surface are significantly larger than those emitted from a hydrophilic surface, implying an increased adhesive component of the force balance during bubble growth from a hydrophobic surface, and thus a prolonged growth phase and departure time. Gerlach41 modeled the process of bubble formation at a submerged orifice, finding that if the bubbles’ instantaneous contact angle during growth becomes equal to the Young’s contact angle (as defined in Figure 1, where the contact is shown as the angle made by the fluid touching the solid surface), found under equilibrium conditions, the bubble can grow outward from the pore onto the surface of the diffuser plate. Yasuda44,45 corroborates this hypothesis, and suggests maintaining the surface contact angle at θ < 45° prevents the bubble from growing past the orifice boundary. This view is further supported by Kukizaki,37,43 who modified Shirasu porous glass (SPG) membranes with silanes. Unlike other
Figure 1. Contact angles (θ) of a sessile liquid drop (water) on a solid horizontal surface in air, (a) θ < 90°, (b) θ = 90°, and (c) θ > 90°; and an air bubble sitting on a solid surface in water, (d) θ < 90°, (e) θ = 90°, and (f) θ > 90°.
studies that used single, controlled pores, SPG is a sintered glass powder with a random distribution of numerous small pores. Using organosilanes, Kukizaki modified the surface of the SPG to yield a variety of surface wettabilities before bubbling and agrees with the importance of a 45° contact angle.37 Many groups agree that the bubble volume at detachment from a surface with θ > 90° is significantly larger than those from surfaces with θ < 90°.38−41 Corchero42 carried out wettability experiments using a combination of acrylic and Teflon plates, drilled to contain a single pore of between 0.5 and 1 mm in diameter. They reported static contact angles of 68° and 123°, respectively. To create an intermediate surface with θ = 90°, domestic Vaseline was smeared onto the surface, and, as a result, it was observed that surface wettability played an important role in bubble formation. The volume of the bubbles produced from the acrylic sheet (θ = 68°) is lower than that of the Vaseline (θ = 90°) and the Teflon (θ = 123°). However, the flow rates for the three systems under investigation were not equivalent, which is likely to greatly influence the result, nor is the quality of the intermediate contact angle surface of sufficient quality or robustness to allow for full insight into the underlying phenomena. These studies provide a brief insight into the role of surface wettability, with two dominant theories being postulated: that the contact angle of the surface must be maintained at 99.8% was obtained from Sigma-Aldrich and used as received unless stated. Twenty-five mm diameter, rolled stainless steel disks (70 μm thick) were obtained from Photofabrication services Ltd. (UK) with 250 μm circular holes etched through the disk. A single pore was etched through the center to create the “single pore” disks. A pattern of 7 holes was etched through the disks to give a central hole with a hexagonal group of holes surrounding it, with a center to center distance of 2.25 mm between all adjacent holes. Sintered steel disks (25 mm Diameter, 3 mm thick) were obtained from Hengko Technology Co. Ltd. (Shenzhen City, China) with a random array of pores with a mean diameter of 5 μm. C
DOI: 10.1021/acs.langmuir.5b03743 Langmuir XXXX, XXX, XXX−XXX
Article
Langmuir Table 2. Summary of Diffuser Type and Modification Techniques Utilized in This Study diffuser
material
photofabrication steel disk photofabrication steel disk Hengko steel diffuser point-four diffuser point-four diffuser point-four diffuser
rolled stainless steel rolled stainless steel sintered steel sintered ceramic sintered ceramic sintered ceramic
pore structure single 7 pores multiple, multiple, multiple, multiple,
pore size (μm) 250 250 5
