Journal of Microencapsulation, November 2005; 22(7): 745–759
Monodisperse structured multi-vesicle microencapsulation using flow-focusing and controlled disturbance RODRIGO BOCANEGRA1, JOSE´ LUIS SAMPEDRO1, ´ N-CALVO1, & MANUEL MARQUEZ2,3 ˜A ALFONSO GAN 1
Escuela Superior de Ingenieros, Universidad de Sevilla, Sevilla, Spain, 2Kraft Foods R&D, The Nanotechnology Laboratory, Glenview, IL, USA, and 3Los Alamos Nacional Laboratory, Chemistry Division, Los Alamos, NM, USA (Received 31 January 2005; accepted 20 March 2005)
Abstract A method to produce monodisperse structured microcapsules in the diameter range from 10–100 mm is here presented. Flow-focusing is a well known technique whereby a steady capillary micro-jet is generated by the action of a highly accelerated co-flowing stream forced through a small orifice. The micro-jet breaks up owing to capillary instability, giving rise to droplets with a narrow size distribution. In the present study, flow-focusing gives rise not to simple but to compound capillary jets. At break-up, under suitable control parameters, such jets give rise to microcapsules where an outer liquid (shell liquid) surrounds a core liquid integrated by one or more vesicles. Furthermore, under adequate stimulation combining a sinusoidal signal with intermitent pulses, the jet break-up can be controlled. Highly monodisperse microcapsules are produced; fundamental geometric parameters (main diameter, shell thickness or number of cores) are reliably controlled. Rather than using a gas flow to focus the concentric stream of two immiscible liquids, this study has investigated in some detail the evolution of a concentric stream of three immiscible liquids forced through a small orifice. The selection of the surface tension coefficients between the three phases ensures the robust production of a microcapsule structure involving a plurality of vesicles homogeneously distributed in the capsule bulk, the number of cores being a freely chosen parameter. Such composite microcapsules find a broad field of technological applications in the pharmaceutical, food or biotechnology industries. Keywords: Microencapsulation, flow-focusing, jet break-up, controlled release
Introduction The production of single and multiple-core microcapsules is becoming of special interest in a broad spectrum of applications and technological fields. Microcapsules contain an active agent or core material surrounded by a coating or shell made up of materials such as
Correspondence: Rodrigo Bocanegra, Universidad de Sevilla, Escuela Superior de Ingenieros, Dpto. Meca´nica de Fluidos, Camino de los Descubrimientos s/n, 41092 Sevilla, Spain. Tel: þ34 954 593640. E-mail:
[email protected] ISSN 0265-2048 print/ISSN 1464-5246 online # 2005 Taylor & Francis DOI: 10.1080/02652040500273639
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polymers, carbohydrates, fats and waxes. Microencapsulation aims at the protection of the integrity of food ingredients such as vitamins, flavours or salts from external agents (oxygen, water or light) (Hardas et al. 2000; Yoshii et al. 2001). Target substances are, thus, protected from an aggressive environment, e.g. in the controlled delivery of drugs or food additives. However, the internal geometry of microcapsules (size, shape and distribution of core vesicles) determines its behaviour and the release pattern of the encapsulated substance as the microcapsule proceeds through the environment. Indeed, it is not only the accurate definition of the external microcapsule geometry that matters. Whenever the release mechanism of the encapsulated liquid is a key consideration, the internal structure and component distribution of microcapsules becomes a critical aspect of their application. Methods based on the production of an emulsion prior to the encapsulation process are common in the industry (Lee et al. 2000). In these methods, an immiscible sheath liquid is introduced and encapsulates an inner core liquid, giving rise to an emulsion: a non-continuous phase, the core liquid, and a continuous phase, the shell liquid. Once the emulsion is produced, several routes may be followed to produce droplets and, subsequently, capsules, consisting of core fluid encased by a layer of shell fluid: coacervation, phase separation or spray drying. Spray drying is the simplest method to disperse an emulsion into an aerosol of compound droplets which are subsequently dried up. Many other methods based on different physical principles are also available: rotary disc atomization, fluid bed atomization, centrifugal head co-extrusion or stationary nozzle co-extrusion, among the most usual. However, the methods above give rise to serious quality shortcomings: high polydispersity, low controllability of the microcapsule structure, low efficiency and high core losses. Recent advances in methods based on the formation, control and break-up of jets have attracted the interest of many researchers. Two main areas can be discerned: electrospray (Loscertales et al. 2002; Bocanegra et al. 2004) or flow-focusing (Gan˜a´n-Calvo 1998). In the electrospray technique, a coaxial jet of immiscible liquids is generated by the action of electro-hydrodynamic forces; the jet diameters are in the nano/micrometric range. Two concentric electrified needles are used to feed the liquids: the core liquid is fed through the inner needle while the shell liquid flows through the outer one. Under some specific range of the applied voltage and liquid flow rates injected, the compound meniscus at the tip of the coaxial needle adopts a conical-like shape from whose vertex a slender compound jet is emitted. The jet diameter does not depend on the applied voltage or geometric parameters such as the diameter of the needles; it depends on just two parameters: liquid flow rates and electrical conductivity (Lopez-Herrera et al. 1999, 2003). Unlike electrospray, the flow-focusing technique relies on mechanical forces to produce a capillary micro-jet. A liquid jet issues from a capillary tube whose end faces an orifice; both the liquid jet and an additional focusing fluid are forced to flow to the outer ambient through the orifice. Under a specific range of operational parameters and geometrical configuration, owing to mechanical stress, the liquid meniscus at the tip of the needle adopts a cusp-like shape (Figure 1); in analogy to electrospray, the cusp emits a slender steady jet of nano/micrometric dimensions. At break-up, this jet gives rise to an aerosol of nearly monodisperse droplets (Gan˜a´n-Calvo 1998; Gan˜a´n-Calvo et al. 1999). In microencapsulation flow-focusing applications, two concentric needles are also used. Subsequent break-up of the compound jet yields monodisperse micro-droplets, the outer liquid shell coating the core liquid. Thus, both electrospray and flow-focusing are suitable for the production of coaxial jets. However, it is important to remark that generating a steady coaxial jet does not necessarily guarantee small microcapsules. In fact, the characteristics of the microcapsules
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Figure 1. Liquid meniscus with a cusp-like shape emitting a slender jet in a typical flow-focusing pattern.
depend not only on the steadiness of the coaxial jet, but also on the dynamics of natural perturbations growth and the eventual break-up process (Leib et al. 1986; Lasheras and Hopfinger 2000). Break-up depends on a wide number of parameters, mainly the geometric dimensions of the jet and the physical properties both of the core and shell liquids and of the surrounding (focusing) liquid: viscosity, inter-facial tension, density. . .. Theoretical and numerical studies published in the literature foresee two possible break-up modes in a coaxial jet (Meseguer and Sanz 1985; Chauhan et al. 2000): stretching and squeezing. In the first mode, the disturbance grows in phase in both the inner and the outer jets; in the second mode, the disturbance is out of phase in both jets (Figure 2). However, the break-up process may be enhanced and controlled (its monodispersity may be improved) by artificial excitation (Freitas et al. 2004; Yeo et al. 2003). Forcing a sinusoidal disturbance with a wavelength set at the maximal growth rate of the coaxial jet is known to improve the break-up uniformity and consequently the monodispersity: to that end, the forcing amplitude must be above the amplitude of all other natural disturbances in the environment. This research aimed at the production of monodisperse microcapsules with controlled inner structure. It used a core liquid whose inter-facial tension with the focusing liquid was larger than that of the shell-to-focusing liquid. This implies that the free energy associated with the interfaces is minimal and leads to a stable configuration when the shell liquid encases the core liquid. A further property concerns the core liquid viscosity: it has been chosen to be of the order of the viscosity of the focusing liquid and almost two orders of magnitude below the viscosity of the shell liquid. In situations where the jet broke up naturally, it was observed that the inner jet split up into rather monodisperse droplets while the outer shell-liquid jet remained significantly undisturbed; this is in agreement with what can be expected from the liquid viscosity ratio. In addition, the
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dINNER-JET
dINNER-JET
dOUTER-JET
dOUTER-JET
(a)
(b)
Figure 2. Break-up of coaxial jets: (a) stretching mode; (b) squeezing mode.
break-up wavelength of the inner jet was significantly shorter than the wavelength of the outer jet. Consequently, long chains of monodisperse vesicles formed in the core of the shell-liquid jet after it broke up into micro-capsules: the resulting pods comprised a number of cores ranging from 5–50 depending on the shell-to-core liquid flow rates ratio and the viscosity ratio. This study, therefore, aimed at controlling the internal structure and size of the pods (multi-vesicle microcapsules). The goal was not only to improve the monodispersity of the microcapsules but also to control the number of inner cores. A sinusoidal signal was applied to the compound jet so that the break-up of the core jet took place while the shell jet remained undisturbed (Figure 3). Every certain number of cycles, a strong pulse was introduced in the signal, producing the sudden break-up of the outer jet. The number of inner cores in each pod equalled the number of cycles between two pulses (Figure 4). In order to obtain solid microcapsules, once the compound micro-droplets were produced, solidification of the liquid shell took place. In the work here presented, the liquid forming the outer shell of the microcapsules was a specific photopolymer solidifying under ultraviolet radiation. The experimental set-up and physical process, therefore, are described in detail in the following sections of the paper. Once the compound jet breaks up and for a certain period of time, while the shell of the resulting microcapsules remains liquid, a double emulsion configuration appears until the solidification process occurs. A double emulsion is usually defined as a multiplephase dispersion wherein droplets entrapping smaller droplets are suspended in a continuous liquid phase (Garti 1997; Benichou et al. 2004). A familiar classification includes either water-in-oil-in-water (W/O/W) or oil-in-water-in-oil (O/W/O) emulsions. Double emulsions are thermodynamically unstable with a strong tendency for coalescence,
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200µm 200µm
(a)
(b)
Figure 3. Coaxial jet under a sinusoidal excitation of 50 Hz: (a) The inner jet is fully developed into droplets while the outer liquid remains undisturbed; (b) resulting single-core microcapsules.
200 µm
Figure 4. Compound filament just after break-up. The number of cores is controlled by imparting artificial excitation. Seven cores in twin microcapsules are observed.
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flocculation and creaming. Due to the potential applications of this kind of emulsions in the pharmaceutical or food industry, in the last decades many studies have been carried out with the main aim of improving the stability. The main pathway followed in these studies has been to find proper and suitable combinations of emulsifiers; both hydrophilic and hydrophobic emulsifiers, properly added to the double emulsion, strengthen either water/ oil or oil/water inter-phases and, consequently, improve their stability (Sugiura et al. 2004). Stabilizers are also frequently used to increase the viscosity of the phases, slowing down the instability process. In the present research, neither emulsifiers nor stabilizers were used, since the process of solidification of the microcapsules was triggered as soon as the coaxial jet broke up, so that instability problems prior to the solidification were avoided. However, in future works using new combinations of liquids with different properties to those used in this research, use of surfactants may become mandatory to attain satisfactory results. As will be described next, the case under consideration here is unusual (water/polymer/ oil), but it avoids instability problems such as coalescence of the internal aqueous cores into larger cores, coalescence of the oil droplets suspended in continuous phase or expulsion of the inner cores; such problems are frequently observed in W/O/W emulsions. Theoretical background Flow-focusing consists on an atomization technique which relies on aerodynamic forces to produce capillary micro-jets (Gan˜a´n-Calvo 1998). In this method, a flow rate Q is injected through a capillary needle. This needle is placed inside a pressure chamber as sketched in Figure 5(a). Liquid is injected through the needle, giving rise to a meniscus at the extremity of the needle. An orifice in the chamber wall facing the tip of the needle opens the camber to the outer ambient. A fluid, either a gas or a liquid, is forced to escape the chamber through the orifice by means of an extra pressure in the chamber, Pg. When Pg reaches a certain value, the surface tension stresses at the meniscus are overcome, so that it is pulled into a cusp-like shape from whose vertex a very slender micro-jet issues with diameter dj. In general, the system will be steady when the pressure drop, Pg, is sufficiently higher than
FOCUSED LIQUID
FOCUSING LIQUID
FOCUSED LIQUIDS: ENCAPSULATED AND ENCAPSULATING
FOCUSING LIQUID
COMPOUND MENISCUS
dj
MENISCUS (FIGURE 1)
PRESSURE CHAMBER
(a)
(b)
Figure 5. Flow-focusing atomizer; (a) simple jet; (b) compound atomizer with two concentric needles.
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the maximum surface tension stresses, of the order of /dj, where is the inter-facial tension between the focused liquid and the fluid in the chamber. Once the jet exits the orifice, the pressure gradient (the main axially accelerating force) vanishes and the jet evolves under the influence of the viscous shear stresses produced by the fluid stream and the capillary stresses. Perturbations of the capillary jet grow downstream until the jet eventually breaks up. A theoretical and experimental study of this process was carried out by Gan˜a´n-Calvo (1998) with outstanding conclusions. Under an adequate geometric configuration, the diameter of the jet, dj, is independent of the geometric properties of the chamber such as the diameter of the orifice or the thickness of the wall, and it depends only on two parameters: flow rate of the focused liquid Q and pressure drop Pg. The scaling law for dj is as follows, being the density of the focused liquid: 1=4 8 dj ffi Q1=2 ð1Þ 2 Pg The system is steady and reliable just for a specific range of the Weber number We, defined as the aerodynamic to capillary force ratio: We ¼
Pg dj
ð2Þ
The limits of the steady range depend on the geometry of the atomizer. Nevertheless, it can generally be said that for We values typically below 4, the pressure drop Pg is too small to trigger the process. On the other hand, when We ranges from 4–20, an adequate Pg is able to atomize the jet issuing from the needle; and the break-up mode is axisymmetric and monodisperse. When We is higher than 20, the focusing fluid stream becomes increasingly turbulent and the quality and steadiness of the jet deteriorate. Non-axisymmetric disturbances appear coupled to the axisymmetric ones generating a non-axisymmetric and polydisperse break-up (Gordillo et al. 2001). Break-up of a simple jet into droplets has been analysed for more than a century following the initial investigations of Rayleigh (1879). It is well known that axisymmetric circumferential capillary forces destabilize a liquid jet and the fastest-growing disturbance leading to break-up has a wavelength Kmax ¼ 4.51dj which determines the droplet diameter Dd at break-up as Dd ¼ 1.89dj. However, under realistic operation, a jet is disturbed by a heterogeneous spectrum of environmental disturbances. As a consequence, the jet will experience simultaneous growth of a variety of wavenumbers centred around Kmax, resulting in imperfect droplet monodispersity. In order to improve monodispersity, experimental efforts have stimulated the jet with a sinusoidal artificial excitation having a wavelength close to Kmax, so as to enhance maximum growth disturbance (Rutland and Jameson 1970, 1971). Several studies have also looked at the dependence of the jet break-up on factors such as the viscosity of the jet fluid (Chandrasekhar 1961) or the effect of the characteristics of the surrounding fluid (Tomotika 1935). These factors are important, since they influence the occurrence of satellite droplets at break-up, disrupting the aerosol quality when high monodispersity is required. In this process, once the coaxial jet is produced by flow-focusing, it leaves the atomizer through the pressure chamber orifice and break-up occurs downstream producing compound spherical droplets. The characteristics and quality of the resulting microcapsules depend not only on the steadiness of the coaxial jet but also on the break-up process. From the point of view of microencapsulation, a satisfying break-up mode would
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result in compound droplets with the outer liquid forming a shell and the inner liquid being aggregated as one or more cores trapped by the shell. Spilling of the inner liquid or production of satellite droplets would affect the general monodispersity of the sample as well as the efficiency of encapsulation. From simple free energy considerations, a general sufficient condition to achieve encapsulation is to have a shell-forming liquid with a surface tension with the ambient liquid smaller than the encapsulated liquid with that ambient. The break-up mode of a coaxial jet depends on a large number of parameters: geometric dimensions of the coaxial jet and physical parameters of the shell and core liquids, as well as of the focusing and atmospheric fluids (Shkadow et al. 1996). As a consequence, the theoretical and analytical solution to this problem turns out to be complicated. Many studies have been carried out to characterize the break-up of a coaxial jet. Meseguer and Sanz (1985) completed the one-dimensional temporal analysis of an inviscid coaxial jet under axisymmetric disturbances. They found two growing modes in the downstream motion of the disturbances leading to break-up. The first is a stretching mode, where the core/shell interface grows in phase with the shell/environment interface; this mode is driven by capillary forces at the shell/core interface (Figure 2(a)). A second mode was also found (squeezing mode) in which the two interfaces grow out of phase; this mode is driven by the capillary forces at the shell/environment interface (Figure 2(b)). In an analytical and numerical work, Chauhan et al. (2000) studied the temporal stability of viscous axisymmetric coaxial jets. The dependence of break-up modes on the characteristics of the coaxial jets was studied and Kmax and its associated growth rate max were determined both in the stretching and the squeezing modes (see Figure 2). However, as said above, in most environments not only the maximal growth disturbance is found, but a full spectrum, generally heterogeneous and random. Break-up is not determined by a single wavelength and the resulting compound droplets are not monodisperse. In analogy to the simple jet case, a practical way of improving the monodispersity of compound jets break-up consists of imparting a controlled disturbance with a wavelength Kmax, whose amplitude should be sufficiently larger than all other ambient modes. As a result, the break-up characteristics and the droplet monodispersity will improve. During research, the approach from Chauhan et al. (2000) was followed in order to predict the maximal growth wavelength leading to coaxial jet break-up. Numerical and experimental results were obtained and compared. In order to improve the quality of the microcapsules, artificial excitation was applied as described above. However, designing the inner structure of the microcapsules is no easy task. The main goal of the work was to select, with enough reliability, the number of inner cores in the microcapsules. To that end, when using artificial excitation, the applied signal was not exactly sinusoidal. The wavelength required to trigger break-up of the core jet was determined experimentally prior to destabilizing the shell jet: this wavelength turned out to be quite close to the numerical value expected for the maximal growth wavelength, Kmax, in the stretching mode. Once the inner jet was broken, a strong pulse was introduced in the signal every certain number of cycles, nc. When this pulse was applied (Figure 6), the outer jet broke up suddenly, giving rise to a spherical drop, which resulted in a microcapsule after shell polymerization with a number of inner cores equal to nc. This method turned out to be highly reliable, giving the possibility of controlling not only the diameter and the monodispersity of the microcapsules, but also their inner structure. Additionally, either
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W
W
n· W = n · microcapsules
(a)
(b)
Figure 6. Break-up-inducing input: (a) Sinusoidal signal, wavelength W; (b) sinusoidal signal, wavelength W, with a pulse every n periods.
the thickness of the shell or the diameter of the inner cores in the microcapsules was adjusted by varying adequately the different flow rates in the atomizer. Experimental set-up During this research, several experiments were carried out. Flow-focusing atomizers with dual-concentric-needles were used (Figure 5(b)). Most of the experimental data were obtained with an atomizer with the following dimensions: the outer diameter of the inner needle was 200 mm; the inner and outer diameters of the outer needle were 350 and 500 mm, respectively. The diameter of the orifice located in the wall chamber was 500 mm. The thickness of the wall chamber was 150 mm. Water-based ink was the liquid selected for encapsulation. Small amounts of glycerine were added to control the viscosity of the ink according to the requirements of each experiment. The shell liquid, i.e. the liquid forming the outer jet, was a photopolymer (a modified methacrylate, type SK9, Summers Optical Corp, USA), whose physical properties are given in Table I (before polymerization). In the experimental set-up designed for this research, the microcapsules generated at break-up were introduced in an ultraviolet reactor to achieve the shell polymerization and produce solid microcapsules. The focusing fluid filling the pressure chamber, whose acceleration as it leaves the chamber pulls the compound liquid jet, was a silicon-based oil (FS5, ESQUIM Corp., Spain). Its physical properties are given in Table I. The basic experimental set-up is sketched in Figure 7. Three different syringe-pumps were used to feed the atomizer. Each one determined suitable flow rates for the core liquid, the shell liquid and the focusing liquid. The atomizer was submerged in the upper part of a cylindrical tank also filled with focusing silicon oil, FS5. At the same time, the cylindrical tank was located inside an ultraviolet reactor to solidify the microcapsules. The atomizer was physically connected to a loudspeaker imposing controlled disturbances to the concentric jets. The loudspeaker was in turn connected to an amplifier and a signal was generated by a waveform generator. Break-up of the jet was monitored by a CCD camera connected to a monitor as well as a computer. The polymerization and solidification process of SK9 under an ultraviolet radiation of intensity I starts after a critic time tc, following which a layer of polymerized solid SK9 starts to grow. The threshold level Ec is defined as the minimum exposition triggering
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SK9, shell liquid
FS5, focusing liquid
1000 1.0 — —
1080 100.0 4.5 13.2
950 5.0 15.6 —
AMPLIFIER
WAVEFORM GENERATOR
3
Density (kg m ) Viscosity at 25 C (cps) Surface tension (mN m1)—INK1 Surface tension (mN m1)—FS5
LOUD SPEAKER INK SYRINGE PUMPS
SK9 FS5
ATOMIZER
LIGHT SOUCE
CCD CAMERA
COMPUTER
ULTRAVIOLET REACTOR FS5 TANK
Figure 7. Basic sketch of the experimental set-up.
the solidification process in the polymer: Ec ¼ Itc
ð3Þ
Once Ec is reached, the thickness of the layer starts to grow. This growth may be simulated by the following equation: E t dc ¼ Dp ln ¼ Dp ln , ð4Þ Ec tc dc being the thickness of the solidified layer and E the exposition applied to the surface of the polymer. Dp and tc are dimensional constants to be determined experimentally depending on the wavelength spectrum of the ultraviolet radiation. These constants were obtained for SK9 under the radiation in the reactor: Dp ¼ 0:565 mm Tc ¼ 4:61 s
ð5Þ
The design of the ultraviolet reactor and the height of the cylindrical tank were made taking into account the requirements issuing from equations 8 and 9. Given the physical
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properties of the fluids under consideration, the residence time or ‘flight time’ of the microcapsules through the reactor was above 20 s, long enough to ensure the correct solidification of the whole microcapsule. Results and discussion Many experiments were carried out along this research aimed to find the best experimental conditions for microencapsulation. However, only the most representative are shown here. First, the combination of liquids (ambient-shell-core ¼ FS5-SK9-ink) complied the freeenergy considerations to achieve encapsulation (see Table I), i.e., SK9-FS5 ¼13.2 mN m1 < SK9-ink ¼ 15.6 mN m1. The experimental conditions were also numerically simulated following the method by Chauhan et al. (2000), in order to predict the maximal growth wavelength Kmax, either in the stretching or the squeezing mode.
Spontaneous break-up Initially, several experiments were performed without any artificial excitation, to monitor spontaneous break-up of coaxial jets. The experimental conditions were as follows: QINK1 ¼ 7 mL h1 ,
QSK9 ¼ 14 mL h1 ,
QFS5 ¼ 350 mL h1
ð6Þ
Introducing equation 10 in equations 1 and 2 gives: We ¼ 10
douterjet ¼ 400 mm
ð7Þ
Given the parameters described above, the maximal growth wavelengths in both break-up modes were the following: Kmaxsqueezing ¼ 785 mm,
Kmaxstretching ¼ 411 mm
ð8Þ
From the images generated by the CCD camera, the break-up of the coaxial jet produced during the experiment was analyzed. It was observed that the jet broke up according to the stretching mode pattern (Figure 8), at a wavelength of 437 mm. It is worth noting
437µm
Figure 8. Natural break-up evolution of a coaxial jet without artificial excitation.
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500mm 500 Figure 9. Microcapsules produced under natural break-up conditions; heterogeneous microcapsules, with varied number of cores.
that the experimental wavelength was quite close to the numerical result: 411 mm. However, though the core jet breaks up quite cleanly and with acceptable monodispersity, the outer jet does not break up following a fixed pattern. Hence, the resulting microcapsules contained a different number of cores (Figure 9).
Forced break-up: Sinusoidal signal In order to improve control on the geometry and the number of cores per microcapsule, a new set of experiments was carried out where controlled disturbances were imparted to the jet, in particular a simple sinusoidal excitation. In this case, the experimental conditions were identical as in the first set of experiments, equations 8 and 9. Thus, from equations 8–11, the break-up frequency f ¼ Vl1 for both modes is obtained (V being the average axial speed of the coaxial jet and l the theoretical wavelength for this experiment) as: fmax squeezing ¼ 118 Hz,
fmax stretching ¼ 61 Hz
ð9Þ
When applying excitation to the atomizer, it was observed that an adequate break-up of the jets took place only for a narrow frequency range at 50 Hz (Figure 3). This frequency was closer to the theoretical break-up frequency in the stretching mode: 61 Hz. Figure 3(a) shows the inner jet as it gives rise to perfect monodisperse droplets while the outer jet remains undisturbed. Downstream from the position where the pictures in Figure 3(a) were taken, the outer jet is observed to break-up under the same frequency as the inner jet, i.e. in obedience to the stretching mode pattern. Individual microcapsules were obtained, with a single core surrounded by a shell of outer liquid (Figure 3(b)).
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In this second set of experiments, the production of single-core monodisperse microcapsules was successfully attained. The final diameter of the microcapsules as well as the thickness of the shell was controlled by varying the flow rates in the experiment. The diameter of the resulting microcapsules ranged from 30–700 mm, while the thickness of the shell could be modified from 5–200 mm. However, sinusoidal forcing by itself is apparently not amenable to a controlled production of multiple-core capsules.
Forced break-up: Sinusoidal þ pulse signal In a third set of experiments, tyis study operated with working parameters as in the first or the second set. A composite signal was used, combining a sinusoidal excitation with pulses. A simple sinusoidal forcing around the stretching mode (50 Hz), as described above, led to a successful break-up of the core jet. However, one did not have any control over the outer jet, so that the outer jet broke up following the pattern imposed by the inner one and giving rise to single core microcapsules. The proposed answer to this limitation was to introduce a strong pulse in the sign every certain number of cycles. When the inner jet is fully developed into droplets and the outer jet is still undisturbed (Figure 3(a)), a strong pulse induces immediate break-up of the outer jet. Hence, the microcapsules produced at break-up contain an exact number of cores equal to the number of cycles between two pulses. In the experiment, a signal was introduced with seven cycles every two pulses. In Figure 4, two identical microcapsules were observed, just after been produced, containing exactly seven cores. This technique turned out to be highly reliable, allowing a rigorous control of the number of cores per capsule, in direct response to the distribution of pulses in the signal. Full control was achieved on the microcapsule geometry: diameter, shell thickness and number of inner cores. It is worth noting that the loss of inner liquid, not encapsulated, was almost negligible. Since the inner liquid chosen for the experiments was black ink, any spill would be readily noticeable due to the darkening of the FS5 in the environment caused by the solution of some of the pigments contained in the ink. Several problems could mar the complete success of the experiments: the most prominent risk is microcapsule coalescence. However, this may be easily solved by means of different expedients: the use of surfactants reduces the inter-facial tension between the shell liquid and the environmental fluid. Hence, coalescence is inhibited. The use of a faster photopolymer could also provide a solution, so that the time required for the outer surface of the shell to solidify is shortened, avoiding coalescence. Conclusions The main goal of the research presented in this paper was the definition of a feasible and reliable technique to produce structured microcapsules with diameters ranging from a few tens to hundreds of microns. The geometry of the microcapsules was largely controlled by varying the parameters of the experiments. The final diameter of the microcapsules and the thickness of the shell were controlled by adjusting the flow rates of the liquids involved. Additionally, the desirable monodispersity of the microcapsules as well as the number of inner cores per shell was finely controlled by applying an adequate artificial excitation. This type of microcapsule finds application in a broad range of fields. In the pharmaceutical industry, drugs to be protected from an aggressive environment are easily encapsulated.
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However, when an accurately controlled release of these drugs is required, the problem becomes difficult to solve. In this case, an accurate design involving parameters such as monodispersity, shell thickness or the exact amount of drug to be delivered and protected within the microcapsule turns out to be essential. Structured microcapsules as those described in this paper are ideal for this kind of application. In this research, the choice of the core and shell liquids, as well as of the focusing liquid, is to be considered a simple illustration of the technique. Many other liquids may be chosen depending on the technological goals. Gelatin, sodium alginate or any fat such as cocoa butter may be used as encapsulating liquid.
Acknowledgements The authors are in debt to J. M. Gordillo for his valuable assistance in numerical problems. The manuscript was perused by P. Riesco-Chueca, who provided insightful comments.
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