Thermal and Mechanical Stabilities of Core-shell Microparticles Containing a Liquid Core

11 Thorough understanding of the behaviour of core-shell microparticles with a liquid core is 12 essential for determining their performance in applications under different operation conditions. 13 This paper reports the behaviour of core-shell particles with a liquid core under thermal and 14 mechanical loads. First, we formulated an analytical model for the heating process of a core- 15 shell microparticle with a liquid core. Next, we utilised an axisymmetric model of an elastic 16 spherical shell upon compression to describe the deformation of a core-shell microparticle. 17 Finally, we conducted experiments to validate these models. Both thermal and mechanical 18 models agree well with the experimental data. The maximum temperature a core-shell 19 microparticle can withstand depends on the liquid, the geometry, and the material of the shell. 20 The critical compression force before rupture of a core-shell microparticle depends on the 21 Poisson’s ratio of the shell material and the shell thickness relative to the outer shell radius. The 22 rupture force and rupture temperature increase with increasing shell thickness. of the microcapsule. Early compression studies indicated


Introduction
26 fragrance molecules to stay with clothes during and after washing, and keep bleaches stable and 32 effective. Microcapsules packed with a small amount of liquid core are used as micro-reactors, 33 storage containers, and delivery carrier for food, drug, coating, and cosmetic products [2]. These degradation of microcapsule shell at the high temperature [19]. The release mechanism is an 57 essential factor for later applications, for which, e.g., the thermal stability of shell as a thermal 58 protection layer is important or controlled release of core components at a specific temperature 59 is desirable. The thermal stability is correlated with thermal tolerance of microcapsules in the 60 actual applications such as the temperature of storage and delivery of cargo, triggering 61 temperature in catalysis, and thermal tolerance of sensing systems [20]. Furthermore, size and 62 thickness of the shell affect the thermal behaviour of core-shell microparticles. The rupture 63 temperature has been found increase exponentially with decreasing size of the microcapsules. 64 A critical threshold size of approximately 10 μm was reported by Zhao et al. [20]. However, the 65 thermal behaviour of liquid core-shell microparticles is not yet fully understood due to the lack 66 of a theoretical model. 67 Intrinsic mechanical properties of microcapsules, particularly of the shell, affect their 68 performance. Under some circumstances, the release of encapsulated substances require stress, 69 e.g. massage for the release of perfume for personal skin care, and compression for encapsulated 70 ink in inkless paper and dyes in textile. The hydrodynamic shear stress in blood vessels is a 71 major problem for microcapsules in drug delivery [ 88 We previously prepared core-shell microparticles consisting of a HFE7500 fluorinated 89 oil core and a polymer shell of trimethylolpropane trimethacrylate (TMPTMA) using droplet-90 based microfluidics [33,34]. The core-shell microparticles can be tuned for different ratios of 91 shell thickness to outer radius. However, little information is available on thermal and 92 mechanical properties of these particles. The present study aims to experimentally investigate 93 the behaviour of core-shell microparticles with a liquid core during heating and compression 94 processes. The experimental results are compared with analytical models considering 95 parameters such as particle size and shell thickness. The thermal model predicts the stress 96 assumed to remain constant without any bend or inflation during the compression process, 106 and the internal volume of the shell remains constant as it deforms. The shell is compressed 107 between two rigid horizontal plates, Fig. 1(b). Because of the compression, the shell deforms 108 symmetrically with a distance x from top and bottom each and a total displacement of d=2x.  (2)

128
The force Fx is expected to depend slightly on the Poisson's ratio ν.

129
The elastic energy of buckling in the second regime is: The total displacement reaches a critical value max = 2 max , where the shell ruptures if 132 compression continues. The critical displacement can be determined by the continuity 133 condition 1 = 2 . In the following section, we conducted experiments on liquid core-shell 134 particles generated by our microfluidic approach to determine the constants A, B and C by fitting 135 the measured force versus displacement data. 136 2.2. Thermal behaviour of a core-shell particle with liquid core 137 In this section, the thermal characteristics of oil core-polymer shell particles are formulated 138 analytically. The core-shell particle is considered as a thick-walled particle because the wall 139 thickness is greater than a tenth of overall radius of the particle. Figure 2 shows the schematic 140 heating process of a core-shell particle and the forces acting on the spherical shell during the The liquid core is assumed to be in a single-phase before the rupture of the shell. For a single-phase oil core encapsulated in a solid shell, the volume V, the absolute temperature 160 T, and pressure P are interdependent quantities. The relationships between V, T, and P can be In (4), the partial derivatives are related to the two thermodynamic coefficients, volume 172 expansion coefficient α, and isothermal compressibility K with α= 1 ( ) P and K= − 1 ( ) T , 173 respectively. Substituting these coefficients into equation (7) results in: Integrating both sides of (5) results in the final internal pressure 2 :     The PDMS device has three cross junctions: HFE7500 oil was used as the inner phase at the 208 first junction and to disperse HFE7500 oil and produce oil core; TMPTMA was used as the 209 middle phase at this junction, Figure 3.

222
The formation of core-shell droplets in the PDMS device was monitored using an inverted 223 microscope (Nikon, Eclipse Ti) connected to a computer.

224
At the outlet, a long tubing was used to collect the generated core-shell droplets. Since 225 the core-shell droplets tended to aggregate while passing through the outlet tubing, the tubing 226 was exposed to an UV light to instantaneously cross-link the shell layer. However, the particles did not completely polymerise due to the limited residence time. Hence, the collected particles 228 were subsequently further exposed to UV light again for five minutes. The collected core-shell 229 particles at the outlet were then washed once with methanol to dissolve the bulk polymer waste 230 and to prevent particles from sticking during storage and drying. Finally, the generated core-   235 We characterised the mechanical properties of core-shell microparticles using compression 236 between two parallel plates. Figure 4 3.5.Thermal behaviour of core-shell microparticles 251 We carried out thermal stability experiments with a controlled hot plate, Figure 4

X-Ray Computed Microtomography (XCMT)
High resolution X-ray computed microtomography was also used to analyse 3D microstructure 264 of core-shell microparticles. Dried core-shell particles were placed on a cylindrical holder that  compression as well as the maximum load a core-shell particle can bear. The force 278 corresponding to a given displacement can be estimated using equation (2). Three sets of data 279 from three core-shell geometries were prepared and investigated. As the core and the shell may 280 not be centred (Fig. 6), the shell thickness is simply determined as the difference between the 281 outer radius and the inner radius h=b-a, Fig. 2(b). The ratios of the shell thickness to outer  314 We first investigated the thermal behaviour of the core-shell microparticles 315 experimentally by increasing the temperature and monitoring their response. Figure 8 shows

355
We studied liquid core-shell particles subjected to thermal and compression loads. We 356 established analytical models for the thermal and mechanical behaviour of thick-walled liquid 357 core-shell particle under heating and compression. We found that the maximum internal 358 pressure induced by the heated liquid core depends on the volume expansion coefficient and 359 isothermal compressibility of the liquid. We note that a relatively thick shell made of TMPTMA 360 and a thickness to ratio h/b=0.41 has a considerable thermal stability and can tolerate the 361 thermal expansion of HFE7500 oil core up to 115° C. In addition, we characterised the 362 mechanical behaviour of this thick wall core-shell particles. The compression of the core-shell 363 particle between two horizontal plates and the critical rupture force were modelled analytically. 364 We conducted validation experiments with three different particle geometries. The force-

373
There are no conflicts of interest to declare.