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Model of laser energy distribution: (a) Variation in laser beam diameter in the propagation direction, (b) Distribution of light intensity on the vertical plane
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Using a laser to heat microfluid has the advantages of non-contact local operation, high accuracy, and good adjustability. In this study, a focused infrared laser with a 1550-nm wavelength was applied to heat an oil–water-oil double-emulsion droplet in a microchannel. The Finite Volume Method was used to numerically study the thermocapillary flow a...
Citations
... 28,29 Nevertheless, the thermocapillary flow of droplets in DEs has been studied to a limited extent. For example, Wang et al. 30 examined the heat transfer characteristics of a DE droplet whose center was heated by a photothermal process. However, the central heating process induced neither the inner droplet nor the outer droplet to move away. ...
The use of double emulsions (DEs), which represent colloidal structures composed of droplets nested within droplets, can provide for unparallel droplet manipulation in droplet‐based microfluidic technology due to their unique core–shell structures. The controlled release of cores in DEs is of particular interest. However, this process remains poorly explored. In this work, the thermocapillary flow induced by a temperature gradient is used as a driving force to control the core release and the impacts of different linear temperature gradients, core diameters, shell diameter, and core/shell diameter ratios on the thermocapillary flow and core release characteristics of DE droplets consisting of a water‐in‐n‐hexadecane‐in‐water system within a cylindrical microchannel are investigated. Most of the core and shell diameter conditions considered result in a double‐core release process, where the inner droplet volume is partially ejected before the remaining core is rewrapped by the outer droplet, and the remaining inner droplet volume is ejected later during a second core release event. However, relatively small core diameters of 50 and 75 μm produce conditions where the full inner droplet volume is ejected during a single‐core release process. In addition, we provide empirical relationships for accurately determining the time at which core release initially occurs under given DE parameters as well as for precisely determining whether the applied conditions will lead to single‐ or double‐core release processes. Therefore, the results of this study provide insights enabling the development of accurate inner droplet release technologies under thermocapillary migration.
... The authors found that the steady migration velocity of the droplet decreased as increasing the value of the Marangoni number. Wang et al. (2021) invested the heat transfer and thermocapillary flow of a double emulsion droplet. There were two or four thermocapillary vortices formed around the middle phase depending on the sign of the temperature coefficient of the interfacial tension on both interfaces. ...
The present paper focuses on the motion due to the thermocapillary force of a droplet in a circular tube through the front-tracking-based simulation. The tube profile in the axial direction is generated with a sinusoidal function that induces a constriction with depth d at the middle. The droplet is slowed down as it migrates from the cold region (ahead of the constriction) to the hot region in the downstream. Various parameters including the Marangoni number Ma, the capillary number Ca and the depth of the constriction d are varied to better understand the thermocapillary motion of the droplet under the influence of the constriction. The simulation results show that when the Ma number increases, the influence factor of the constriction increases and the migration velocity of the droplet decreases. Increasing the depth of the constriction decreases the migration velocity of the droplet.
... Another report states that the PTC effect is a thermocapillary (TC) flow, since fluid flow can take place at a very low velocity in the absence of deformation [1]. In general, a temperature change, ∆T = T − T 0 , on the surface of a liquid can be induced using various heat sources, including focused electromagnetic radiation of any wavelength (ultraviolet, visible, or infrared) [12,[17][18][19][20][21][22][23]. In two limiting cases, PTC flow takes place in a liquid, which strongly absorbs the electromagnetic radiation at its surface, and/or in a liquid, which is either transparent or weakly absorbs the radiation of a pumped laser or a halogen lamp [13]. ...
The photothermocapillary (PTC) effect is a deformation of the free surface of a thin liquid layer on a solid material that is caused by the dependence of the coefficient of surface tension on temperature. The PTC effect is highly sensitive to variations in the thermal conductivity of solids, and this is the basis for PTC techniques in the non-destructive testing of solid non-porous materials. These techniques analyze thermal conductivity and detect subsurface defects, evaluate the thickness of thin varnish-and-paint coatings (VPC), and detect air-filled voids between coatings and metal substrates. In this study, the PTC effect was excited by a "pumped" Helium-Neon laser, which provided the monochromatic light source that is required to produce optical interference patterns. The light of a small-diameter laser beam was reflected from a liquid surface, which was contoured by liquid capillary action and variations in the surface tension. A typical contour produces an interference pattern of concentric rings with a bright and wide outer ring. The minimal or maximal diameter of this pattern was designated as the PTC response. The PTC technique was evaluated to monitor the thickness of VPCs on thermally conductive solid materials. The same PTC technique has been used to measure the thickness of air-filled delaminations between a metal substrate and a coating.
The thermocapillary-driven core release of double-emulsion droplets offers advantages 1 such as simplicity in implementation and a broad range of applications, providing unique 2 strengths in the field of core component release. However, the characteristics and mech-3 anisms related to thermocapillary-driven core release were still unknown. This article 4 employed the Volume of Fluid (VOF) method to investigate the core release character-5 istics of double-emulsion droplets driven by thermocapillary. The range of Marangoni 6 (Ma) number is from 10 to 500. The results indicate that effective control of the migra-7 tion and release dynamics of double-emulsion droplets can be achieved through viscosity 8 regulation, which induces the obvious difference between the two types of double-emul-9 sion droplets. The regulation of both viscosity and surface tension is efficient in control-10 ling the release type of the core. A phase diagram distinguishing between one-off and 11 This is the author's peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset. PLEASE CITE THIS ARTICLE AS 2 two-off release types is presented. Additionally, by characterizing dimensionless num-1 bers, a formula for the characteristic release time of double-emulsion droplets is derived. 2 The current study contributes to achieving precise control of double-emulsion droplets, 3 expanding the scope of applications for double-emulsion droplets, and establishing a 4 fundamental groundwork for subsequent investigations into the release methods of dou-5 ble-emulsion droplets driven by thermocapillary. 6
