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Ind. Eng. Chem. Res. 2009, 48, 8872–8880
Enhancing the Throughput of Membrane Emulsification Techniques To Manufacture Functional Particles Qingchun Yuan,† Nita Aryanti,† Gemma Gutie´rrez,†,‡ and Richard A. Williams*,† Institute of Particle Science and Engineering, School of Process, EnVironmental and Materials Engineering, UniVersity of Leeds, Leeds, LS2 9JT U.K., and Department of Chemical and EnVironmental Engineering, UniVersity of OViedo, C/Julia´n ClaVerı´a 8, 33006 OViedo, Spain
Formulation technologies increasingly demand the manufacture of droplets with user-controlled size and size distributions for applications in emulsions, capsules, and semisolid particulates. These are used in various functional consumer products. This paper introduces methods to enhance throughput for two types of membrane emulsification system, using cross-flow and rotating membrane technology. Modification of the interaction between the dispersed phase and the surface of the membrane, the inner wall of the pores (through control of hydrophobicity), pore orientation, and pore shape is demonstrated to increase emulsion droplet productivity. Such methods can also be deployed in the production of capsule materials using emulsion precursors. The possible mechanisms underlying the enhancements are discussed. It is concluded that noncircular pores can offer significant process benefits for the production of uniform droplets and semisolid particulates. It is demonstrated that the droplet formation rate can be doubled through optimization of the orientation of noncircular pores in a rotating membrane process. 1. Introduction A range of membrane emulsification technologies have been developed to make particles with precisely controlled size and size distributions.1 This has included complex nano-/microparticles for value-added functional products targeted in the food, drink, pharmaceutical, personal care, household care, and agriculture chemical sectors. It has been suggested that membrane emulsification provides a competitive approach in size control and energy savings compared to existing commercial high shear processes, such as rotor-stator, high pressure, and ultrasonication.2 Broadly, the principal membrane emulsification technologies developed include several drop-by-drop methods such as the following: (i) cross-flow and stirred membrane emulsification, which use static and fixed membranes (plates, slotted plates, or tubes) and a moving continuous phase (ii) rotating and vibrating membrane emulsification, which use moving membranes to generate relative motion between the membrane and the continuous phase (iii) microchannel reactors that consist of bundles of microcapillaries with coaxial junctions that enable injection of one fluid phase into or around another We shall be concerned primarily with processes of types i and ii, for which factors such as membrane specification (pore and surface properties), formulation of emulsion systems (dynamic interfacial tension, compatibility to membrane surface, viscosity), and operational parameters are important. Only some of these factors have been studied previously, in terms of precise size control of the process for a variety of complex systems.3-6 To date some membrane technologies have been developed to the stage of demonstration of larger scale manufacture. Throughput (productivity) is an important issue in commercial production, since for applications beyond the area of specialist * To whom correspondence should be addressed. Tel.: +44 (0) 113 343 2801. Fax: +44 (0) 113 343 2781. E-mail: r.a.williams@ leeds.ac.uk. † University of Leeds. ‡ University of Oviedo.
high value particles (e.g., for medical diagnosis application) materials may be required in quantities on the order of several metric tons per day. Some limitations have been observed when the disperse phase has a high viscosity or where it proves necessary to use membranes with very small nano- or submicrometer pores. Throughput is defined as the volumetric flux of the disperse phase normalized by the actual membrane area and time. The flux (Jd) is a function of membrane pore properties, disperse phase viscosity (η), and operational parameters (∆P), as described by Vladisavljevic´ et al. using a modified Poiseuille equation (eq 1):4 Jd )
∆Pεdp2 32ηδmξ2
(1)
where ε is the porosity of the membrane surface, ξ is the mean tortuosity factor of the pore, δm is the thickness of the membrane, and dp is the volume-based mean pore diameter. Evidently, the equation shows that flux density can be enhanced through use of membranes with larger pore sizes, higher pore density, reduced tortuosity, and use of thinner membranes. The maximum pore density is limited by a minimum pore spacing (the distance to the adjacent pores) required to avoid coalescence of droplets from adjacent pores. The throughput is proportional to the transmembrane pressure and inversely proportional to the viscosity of the disperse phase. This equation has been well validated through permeation studies using pure liquids on glass and ceramic membranes.4,7 However, the equations based on Poiseuille flow in uniform capillaries do not take into account the interaction of the disperse phase with the pore surface and pore shape. Little research has been published that considers flux enhancement through modifying the interaction of the disperse phase to the pore wall. There has also been little examination of the effect of the pore shape and orientation on the disperse phase flux and the rate of droplet detachment. The work presented in this paper discusses the factors that determine the throughput, which include membrane properties,
10.1021/ie801929s CCC: $40.75 2009 American Chemical Society Published on Web 05/12/2009
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disperse phase properties, and operational parameters in order to design a new generation of membranes to achieve high throughput without sacrificing droplet uniformity and increasing the cost of production. It is demonstrated that the control over wettability of the disperse phase on the pore wall can improve the throughput of the disperse phase. This was studied using ceramic membranes and a cross-flow technique (section 2.2). Pore shape is another factor that could be optimized to favor the production of uniform particles at a high throughput. This is explored by the use of laser-drilled tubular membranes that enables creation of membrane materials containing pores of chosen size, shape, and positioning. These membranes can be rotated in the continuous phase while dispersing another liquid or mixture through the pores, which will be referred to as rotating membrane emulsification (section 2.3). 2. Experimental Section 2.1. Materials. In the results reported here a number of oils, including monomer solutions, were emulsified as disperse phases. The oils were Newtonian fluids and had viscosities ranging from 11 to 2199 mPa · s. The continuous aqueous phase contained different surfactants (2 wt % concentration) and stabilizer in order to obtain stable droplets of initial makeup. Sodium dodecyl sulfate (>90%, Fluka Ltd., U.K.) was applied in the preparation of finer emulsions (up to ∼10 µm) using crossflow membrane emulsification (section 2.2). Larger droplets (a few tens of micrometers in size) were also made using the crossflow technique and stabilized using Cremophor RH40. Coarse droplets larger than 100 µm were prepared using the rotating membrane method (section 2.3) in an aqueous solution of 2 wt % Tween 20 (Fisher Chemicals Ltd., U.K.) under stagnant conditions and 2 wt % Tween 20 and 0.1 wt % carbomer (Carbopol ETD 2050, Surfachem Ltd., U.K.) under rotation conditions. Carbomer is a cross-linked poly(acrylic acid) polymer and was applied as a bifunctional additive to thicken the aqueous phase and stabilize oil droplets. The pH value of the aqueous solution was adjusted to be ∼5 to have a viscosity of around 100 mPa · s at shear rate of 1 s-1. 2-Propanol (99.9%, Fluka Ltd., U.K.) was added to a viscous vegetable-based oil (with a viscosity of 530 mPa · s at 20 °C) to lower its viscosity and increase the wettability of the disperse phase to the pore wall of the ceramic membranes. Droplets of monomer solutions (polyisocyanates) were encapsulated by adding an amine based water-soluble cross-linking agent after emulsification. 2.2. Cross-Flow Membrane Emulsification. A cross-flow membrane emulsification technique was used to manufacturing finer and larger oil-in-water emulsions; see Figure 1. The technique uses a tubular ceramic membrane, which is statically fixed in a cylindrical stainless steel shell module. The disperse phase is in the annulus under pressure, and the aqueous (product) phase is recirculated via a pump. The oil phase is forced to permeate the membrane pores under a compressed air head and forms oil droplets on the inner surface of the membrane in the path of the flowing aqueous phase. The flowing aqueous phase results in a shear force at the inner surface of the membrane channel, which assists in the detachment of the oil droplets formed in the continuous phase. The droplets are then recirculated in the system until the desired oil concentration is reached, whereafter the product can be bled off. During production of emulsions the pressure used to force the oil through the membrane and the pump speed to generate the cross-flow are closely controlled. The pressure difference between both phases in the membrane module (0.01-0.8 MPa)
Figure 1. Concept diagram of cross-flow membrane emulsification.
Figure 2. X-ray microtomographic image of a cross section of the ceramic membrane used. The channel diameter is 4 mm.
is known as the transmembrane pressure (∆P), and the circulation velocity of the continuous phase in the membrane channel (up to 1.75 m/s) is referred as the cross-flow velocity (Vcf). These two parameters are important and practical operational controls. Specially fabricated tubular ceramic membranes (Fairey Industrial Ceramics Limited, U.K.), with average pore sizes of 0.2, 1, and 5 µm on the active inner surface of the tubes, were used in this study. The ceramic tubes used were 600 mm in length and 20 mm in outer diameter with a multichannel configuration, each having seven separate channels with an inner diameter of 4 mm. Figure 2 shows an X-ray microtomographic image of a cross section of the membrane wall. The bulk structure of the membranes has random-shaped tortuous pores, configured by sintered alumina powder and formed a wall porosity of approximately 35%. The inner channel surface was slip-coated with finer alumina, using a proprietary method, to form size-controlled pores.8 The pores in the coating layer had a span (for the definition, see section 2.4) of approximately 0.3-0.8 and a surface porosity of ∼10%. All experiments were carried out at room temperature of ∼19 °C. For each experimental run, a continuous phase of 1 × 10-3 m3 was used, and emulsion samples were taken when the continuous phase contained about 30% v/v of the oil. The samples were analyzed by laser scattering for the droplet size and size distribution (see section 2.4). 2.3. Rotating Membrane Emulsification. Tubular laserdrilled stainless steel membranes were used to prepare coarse droplets by using the rotating membrane emulsification method. This technique used a quasi-static continuous phase and a rotating tubular membrane that is vertically arranged, as shown in Figure 3.
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b. The average dimensions of the pores (a, b) are 80 × 81 µm (membrane A), 105 × 46 µm (membrane B), 128 × 37 µm (membrane C), and 38 × 122 µm (membrane D). Circular equivalent diameters of the slots were calculated based on pressure drop of circular pores using the following equation.11 de ) 1.3(ab)0.625 /(a + b)0.25
Figure 3. Concept diagram of rotating membrane emulsification.
Figure 4. Stainless steel membranes with laser-drilled pores. The pore arrays on the membranes have dimensions of 80 × 81 µm (membrane A), 105 × 46 µm (membrane B), 128 × 37 µm (membrane C), and 38 × 122 µm (membrane D). Table 1. Slotted Pore Geometry Data Measured
The membrane tube was mounted in a stirrer motor (IKA Eurostar digital agitator) and carefully positioned in the middle of a stationary cylindrical container with an inner diameter of 30 mm. The amount of continuous aqueous phase used was 30 mL, and the membrane rotation rate in each experiment was kept at a constant value in the range 0-1000 rpm. Under stagnant conditions, the droplet detachment rate was counted by visual observation and a stopwatch. The disperse oil phase was fed into the membrane tube by a syringe pump in the range 0.075 × 10-6-75 × 10-6 m3 h-1 (Razel A99FMZ, Fisher Scientific Ltd., U.K.). The syringe pump was connected with the membrane tube through a rotary transfer connection. The droplets formed were sampled on glass slides and analyzed using optical microscopy (see section 2.4). 2.4. Analysis and Characterizations. Laser scattering was carried out (Mastersizer 2000, Malvern Instruments Ltd., U.K.) in a “Hydro S” dispersion cell. The emulsion sample was dispersed in deionized water within the dispersion cell and stirred and pumped at 1000-1750 rpm (depending on droplet stability) to be circulated through the measuring cell. The scattering pattern was deconvoluted (to produce the particle size distribution) using Mie theory. The concentration of droplets in the sample cell was controlled by ensuring light obscuration in the range 10-15%. Each sample was examined in three runs. Each run consisted of 10 measurements, and each measurement took 10 s. The average results are reported as d0.1, d0.5, and d0.9, which are the droplet diameters of volume percentage at 10%, 50%, and 90%, respectively. The size distribution is expressed by the parameters span and coefficient of Variation (CV), calculated by span )
membrane
length a, µm
length b, µm
perimeter, µm
area, µm2
De, µm
CV, %
A B C D
80 105 128 38
81 46 37 122
322 302 330 320
6480 4830 4732 4637
88 72 72 71
4.7 5.8 8.5 7.8
The disperse phase is introduced into the membrane tube and penetrates through the membrane pores, forming droplets at the outer surface of the membrane tube. The rotation of the tube provides operational shear and centrifugal forces to detach the droplets together with other prevailing forces. Under extreme conditions of high rotation other surface-scouring effects are feasible arising from Taylor vortices.9 The stainless steel membranes used have a diameter of 8 mm and a wall thickness of 0.5 mm. One square and three rectangular pores were manufactured in a bespoke array of 6 × 6 through laser drilling. Pores were arranged along the tube axis, as shown in the SEM images of the membrane pores in Figure 4. The spacing between a pore to adjacent ones is 1 mm. Three such pore arrays were located on a circumference band on each membrane tube. Further details are given elsewhere.10 The pore properties are given in Table 1. The length of the side of the pore that is parallel to the axis of the membrane tube is denoted as a, and the length perpendicular to the axis is
(2)
σ CV (%) ) × 100; dav
d0.9 - d0.1 d0.5 σ)
(3)
∑ (d
i
- dav)2
N-1
(4)
where di and dav are the ith droplet diameter and number-average diameter, σ is the standard deviation of the droplet diameters, and N is the total number of droplets analyzed. A stereomicroscope (Nikon Ltd., Model SMZ800) was used to observe the emulsion droplets and microcapsules. Its objective lens could be adjusted at a magnification of 1-6.3 times. The image was recorded by a digital camera (Spot Insight QE Model 4.2) and controlled by SPOT Advanced software. A high-speed camera (Redlake MotionXtra HG-100K) and a digital camera (Cannon 100D) were used to catch the images of droplet formation. The electronic scanning micrograph of microcapsules was taken using a Philips XL30 ESEM. The rheological property of both the oils and the continuous phase were measured using a Bolin rheometer (C-VOR). The measurement was carried out at controlled shear stress ranging from 0.01 to 75 Pa at 20 °C. X-ray microtomography (Phoenix Nanotom, Germany) was applied to examine the threedimensional characteristics of the membrane porous structure.12
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Figure 5. Effect of membrane pore size on the size of oil droplets using ceramic membrane and cross-flow technique. The emulsions were prepared from different oil phases.
membrane with an average pore size of 1 µm. The cross-flow velocity is a major factor governing the droplet size: higher velocity generates smaller droplets. Compared to the effect of velocity, the influence of the transmembrane pressure upon the droplet size is subtle, especially as the result of other effects: principally at higher transmembrane pressures more smaller pores become active as their critical breakthrough pressure is exceeded. Hence control over the extent of polydispersity of the membrane pore is critical. At high droplet formation rates there is also a potential for a diffusion-limited supply of surfactant to occur at the newly created droplet surfaces and at extreme pressure for jetting to occur. It has been reported that the droplet uniformity is closely related to the uniformity of pores in size. The emulsions shown in Figure 6 have CV values of approximately 30 ( 5%, which is largely limited by the CV of the membrane pores (>10%). The pores in ceramic membranes have random shapes, but these have been manufactured to achieve a relatively narrow equivalent pore diameter distribution. However, as in any polydisperse pore size membrane, the larger pores become active at a lower transmembrane pressure. The cross-flow of the continuous phase generates a shear force at the membrane surface to detach droplets formed at the membrane surface with other forces together. The higher the velocity, the larger shear force is produced, which tends to shear-down smaller droplets as calculated from the friction factor correction:14 f)
Figure 6. Average droplet sizes of emulsions of the oil of 49 mPa · s as a function of transmembrane pressure and cross-flow velocity, prepared using a pilot cross-flow membrane emulsification rig and a ceramic membrane with an average pore size of 1 µm.
3. Results and Discussion 3.1. Size and Uniformity Control. Droplet size and uniformity of fine and larger emulsions were prepared using oils with a wide range of viscosity (11-2199 mPa · s) as the disperse phase. The emulsification was carried out using ceramic membranes and the cross-flow technique. Figure 5 shows the dependence of average droplet size on membrane pore size. Experimental results have shown that droplet production differs with pore sizes.3,13 When the membrane pore is ∼0.2 µm, the droplet size and size distribution are well controlled by the pore size and the droplet size is largely independent of cross-flow velocity since the droplet sits on the membrane surface well within the boundary layer created by the movement. The droplet size has a diameter 3-4 times that of the pore size. When the pore size is larger, say approaching 1 µm, the droplet size starts to vary with the operational parameters as shown in Figure 6 and ref 3. As the pore size further increases, the droplet size varies more significantly with operational parameters. In this work, the emulsion prepared from 5 µm membranes could be arranged to have average droplet sizes in the range 30-50 µm, and each emulsion produced has similar uniformity. Figure 6 demonstrates the effect of transmembrane pressure and cross-flow velocity on the average droplet sizes of vegetable oil of 49 mPa · s. The droplets were produced from a ceramic
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2τw Fυw
2
)
{
16/Re 0.0792Re
-1/4
(Re < 500) (500 e Rw e 20 000)
(5)
where f is the friction factor of porous surface, τw is the wall shear force, υw is the linear velocity of the emulsion in the membrane channel, F is the emulsion density, and Re is the Reynolds number of the flowing in the channel. The emulsions prepared from oil phases containing monomers and active components were further encapsulated as microcapsules. Figure 7 shows the microcapsules manufactured from membranes of 0.2 and 5 µm. The micrographs show that the particles prepared have good uniformity and their sizes are in consistent with the data shown in Figure 5. 3.2. Effects on Oil Flux. For a given membrane, the oil flux is determined by oil viscosity and operational conditions, as described by eq 1. Figure 8 shows the variation of oil flux with oil viscosity at different transmembrane pressures using the cross-flow technique and a ceramic membrane with an average pore size of 1 µm. As expected, the oil flux decreases with the increase in the oil viscosity and increases with the increase in the transmembrane pressure. The logarithmic value of the oil flux (log J) has a good linear relationship with the logarithmic value of oil viscosity (log η) at a given transmembrane pressure, as shown in Figure 8. These relations follow the form of eq 1. However, when 2-propanol was added to the oil of 530 mPa · s to lower its viscosity, the oil flux was much higher than expected based on its viscosity. The addition of 10 wt % 2-propanol decreased the viscosity of the oil from 530 to 230 mPa · s. Surprisingly, the oil flux was at the level of the oil of 49 mPa · s; see Figure 9. The low- and high-viscosity oils have very similar hydrophobicities, both exhibiting a contact angle of 31° on a glass slide, and interfacial tensions to the aqueous phase used of 2.7 and 3.2 mN/m, respectively. Comparing the oils, 2-propanol is water soluble and can readily wet the ceramic membrane. It is believed that the addition of 2-propanol increased the wettability of the oil phase on the pore wall, and consequently the oil flux was enhanced.
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Figure 7. Spherical submicrometer particles and microparticles prepared via cross-flow membrane emulsification and ceramic membranes with average pore size of (a) 0.2 and (b) 5 µm.
Figure 8. Effect of oil viscosity on oil throughput at different transmembrane pressures, using ceramic membrane and cross-flow technique.
Figure 9. Comparison of oil fluxes of oils of 49, 230, and 530 mPa · s using a ceramic membrane and cross-flow technique. The oil of 230 mPa · s was a mixture of the oil of 530 mPa · s with 10 wt % 2-propanol. The mixture had a viscosity of 230 mPa · s, while its oil flux was much more improved and similar to that of the oil of 49 mPa · s. The cross-flow velocity used was 1.75 m/s.
Figure 10 shows the size distribution of the three emulsions prepared at the same conditions. It can be seen that the emulsions from the oils of 49 and 530 mPa · s had virtually identical size and size distribution, while the emulsion of the 230 mPa · s (the mixture of the oil of 530 mPa · s with 10 wt % 2-propanol) showed larger size and similar size distribution. The larger size
Figure 10. Size distribution of the three emulsions from the oils of 49, 230, and 530 mPa · s using ceramic membrane of 1 µm at ∆P ) 0.3 MPa, Vcf ) 1.75 m/s. The 230 mPa · s was the oil of 530 mPa · s mixed with 10 wt % 2-propanol.
is likely to be caused by the increased wettability of the 10 wt % 2-propanol in the oil phase on the detachment surface of the membrane. The results indicate that the interaction between the disperse phase and the pore is a significant factor in the throughput enhancement. The membrane pore wall having good wettability to the disperse phase allows the oil phase to permeate more quickly in the pores, and hence results in significantly higher oil fluxes at the same transmembrane pressure and cross-flow velocity. Therefore, we can modify the pore wall to be
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Figure 11. High-speed camera image showing the formation and detachment of oil droplets on the tubular stainless steel membrane with square pores under stagnant conditions. Shutter speed 1/2000 s.
hydrophobic for oily disperse phases. The hydrophobic pore makes the oil permeate the pores more readily and avoids the aqueous phase diffusing into the pore, thus increasing the pore activity and giving higher oil fluxes.15 The hydrophilic detachment part is retained to avoid the spread of oil phase over the membrane for emulsion quality control. The consequence will be a dramatically improved oil flux, without sacrificing the control of droplet size and uniformity. The knowledge can be used to design high-throughput membranes, especially for viscous disperse phase emulsification. 3.3. Effect of Pore Shape and Orientation. Pore shape and orientation are other factors that could be optimized for desired interfacial interaction/instability to favor production of uniform particles at a high throughput. Some earlier work using porous barriers created using slotted plates and silicon microarrays has highlighted that different phenomena can occur associated with these geometric factors.16,17 Microchannels, slotted symmetric pores, and slotted asymmetric pores may produce monodisperse emulsions by spontaneous detachment dominated by interfacial instability. It is noticed that rectangular pores in the rotating technology formed more uniform droplets than round pores, especially at a moderate rotation speed.9,10 In the new work reported in this paper we consider the response of a rotating membrane, examining the effect of pore shape and orientation with respect to the rotational flow. Aspect ratio, pore length divided by pore width, is used to assess the pore shape. Outcomes are represented in terms of the observed droplet formation rates. Laser-drilled tubular membranes with square and rectangular pores were investigated for the effect of pore shape and orientation on droplet detachment rate under stagnant and rotating conditions. Paraffin oil with a low viscosity of 24 mPa · s and a specific density of 0.79 was used in this study. Figure 11 shows an image of oil droplets forming and detaching on a stagnant membrane. It was observed that almost all the square pores (membrane A) were active in generating droplets at a low hydraulic pressure of the oil phase. Some of the rectangular pores of membranes B, C and D were not active at the low hydraulic pressure, because the square pores have a relatively larger cross-section area (6480 µm2, Table 1), corresponding to a lower capillary pressure drop, and more uniform pores than the rectangular pores (4830, 4732, and 4637 µm2 for membranes B, C and D, respectively; Figure 4). An increase in oil flux (or transmembrane pressure) activated the pores. The droplets developed and detached as individual entities from the membranes as demonstrated in Figure 11. The phenomena differed from the behavior observed from slotted pores on flat membranes.18 In Kobayashi et al.’s work on flat membranes, polydisperse emulsions with some very large
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droplets formed when the pore aspect ratio was lower than 3. The difference might result from the membrane disposition and operational conditions used. The flat membrane was horizontally arranged and the disperse oil phase was introduced from the underneath of the flat membrane at a very low flux (corresponding to a very low transmembrane pressure). In our work, the tubular membranes had vertical disposition (generating static hydraulic pressure) under a sufficient oil flux. All the slotted pores with aspect ratios ranging from 1 to 3.5 produced individual droplets without any coalescence in a controlled manner of drop by drop. Round pores exhibit the same performance.19 The droplet detachment rate under stagnant conditions was estimated by counting production from the pores. The counting was carried out at given oil fluxes. As expected, the drops produced under stagnant conditions are much larger than those produced under rotating conditions (Figures 11 and 13). The large drops were observed to be very close to each other before their detachment. Hence the drops at the top of the membrane areas tend to be dragged up by the lower ones due to the vertical disposition of the membranes. For this reason only the pores located at the bottom of the pore array were used to examine the droplet formation rate. The results are given in Figure 12. It shows that droplet detachment rates increased for each membrane as the oil flux increased. At fixed oil flux, the droplet detachment rate is the fastest from the square pores (membrane A). It has been shown that the pore fluid velocity (linear flow velocities of oil in the pore) dominates the droplet sizes:10 the slower the pore fluid velocity, the lower the diameter ratio of the droplet to the corresponding pore. The three membranes with rectangular pores have similar pore cross-section areas (Table 1). Their difference in droplet detachment rates could be related to the pore shape and orientation. The droplet detached from vertically arranged pores (membranes B and C) at a higher rate than from horizontally arranged pores (membrane D). The droplet production rate of membrane C is approximately twice of that of membrane D at the oil flux range studied. Membrane C has a higher droplet detachment rate than membrane B. The reason for this will be discussed later. The droplet generation rate was also studied under rotation conditions.10 The variation of average droplet generation rates and their CVs as a function of rotation speed are shown in Figure 13a,b. The generation rates were calculated assuming that 108 pores displayed the same droplet production activity. The production rate is sensitive to the speed of rotation. When the rotation speed was lower than 400 rpm, the droplet detachment rates from the membranes were virtually constant at
