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Research Papers: Evaporation, Boiling, and Condensation

Effects of Interface Velocity, Diffusion Rate, and Radial Velocity on Colloidal Deposition Patterns Left by Evaporating Droplets

[+] Author and Article Information
Collin T. Burkhart

Mem. ASME
Mechanical Engineering,
Rochester Institute of Technology,
76 Lomb Memorial Drive,
Rochester, NY 14623
e-mail: ctb6973@rit.edu

Kara L. Maki

School of Mathematical Sciences,
Rochester Institute of Technology,
85 Lomb Memorial Drive,
Rochester, NY 14623
e-mail: kmaki@rit.edu

Michael J. Schertzer

Mem. ASME
Mechanical Engineering,
Rochester Institute of Technology,
76 Lomb Memorial Drive,
Rochester, NY 14623
e-mail: mjseme@rit.edu

1Corresponding author.

Contributed by the Heat Transfer Division of ASME for publication in the JOURNAL OF HEAT TRANSFER. Manuscript received October 8, 2016; final manuscript received April 11, 2017; published online June 21, 2017. Assoc. Editor: C. A. Dorao.

J. Heat Transfer 139(11), 111505 (Jun 21, 2017) (9 pages) Paper No: HT-16-1645; doi: 10.1115/1.4036681 History: Received October 08, 2016; Revised April 11, 2017

This investigation experimentally examines the role of interface capture on the transport and deposition of colloidal material in evaporating droplets. It finds that deposition patterns cannot be characterized by the ratio of interface velocity to particle diffusion rate alone when the two effects are of the same order. Instead, the ratio of radial velocity to particle diffusion rate should also be considered. Ring depositions are formed when the ratio of radial velocity to the particle diffusion rate is greater than the ratio of interface velocity to diffusion. Conversely, uniform depositions occur when the ratio of radial velocity to diffusion is smaller than the ratio of interface velocity to diffusion. Transitional depositions with a ring structure and nonuniform central deposition are observed when these ratios are similar in magnitude. Since both ratios are scaled by diffusion rate, it is possible to characterize the depositions here using a ratio of interface velocity to radial velocity. Uniform patterns form when interface velocity is greater than radial velocity and ring patterns form when radial velocity is larger. However, Marangoni effects are small and Derjaguin, Landau, Verwey, and Overbeek (DLVO) forces repel particles from the surface in these cases. Further research is required to determine if these conclusions can be extended or modified to describe deposition patterns when particles are subjected to appreciable Marangoni recirculation and attractive DLVO forces.

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Figures

Grahic Jump Location
Fig. 1

Images depicting the methodology for image analysis of (a) interface shape and (b) fluorescence intensity data

Grahic Jump Location
Fig. 2

Representative fluorescent images and radial intensity distributions for (a) 25 nm, (b) 100 nm, and (c) 1.1 μm particles on SU8

Grahic Jump Location
Fig. 3

Representative fluorescent images and radial intensity distributions for DI water droplets containing 1.1 μm particles evaporated on (a) glass, (b) polyimide, (c) SU8, and (d) PTFE. Dashed lines represent initial contact diameters.

Grahic Jump Location
Fig. 4

Backlit side-view images of interface profiles of evaporating colloidal droplets at normalized times τ=0.0−0.9 on glass, polyimide, SU8, and PTFE. Vertical dashed lines indicate initial contact diameters of the droplets.

Grahic Jump Location
Fig. 5

Evolution of contact angle (θ) and normalized diameter (D/D0) for colloidal droplets containing 1.1 μm particles evaporated on (a) and (b) glass, (c) and (d) polyimide, (e) and (f) SU8, and (g) and (h) PTFE as a function of normalized time (τ). The center curves represent the mean values across all trials while the outer curves are two standard deviations above and below the mean values.

Grahic Jump Location
Fig. 6

Phase diagram for colloidal deposition as a function of the ratios of interface and radial velocities to diffusion rate. Images of the resulting depositions for each surface particle combination are shown inset: (i) SU8 25 nm, (ii) SU8 100 nm, (iii) SU8 1.1 μm, (iv) PTFE 1.1 μm, (v) polyimide 1.1 μm, and (vi) glass 1.1 μm. Open circles indicate ring depositions, hashed circles indicate platelike depositions, and closed circles indicate uniform depositions. Data points from Li et al. [38] are similarly plotted as squares. Results for (i) and (ii) as recalculated after repinning (dashed circles).

Grahic Jump Location
Fig. 7

Evolution of contact angle (θ) and normalized diameter (D/D0) for colloidal droplets containing (a) and (b) 25 nm, (c) and (d) 100 nm, and (e) and (f) 1.1 μm particles on SU8 as a function of normalized time (τ). The center curves represent the mean values across all trials while the outer curves are two standard deviations above and below the mean values.

Grahic Jump Location
Fig. 8

Ratios of interface velocity to radial velocity. Radial velocity after repinning is calculated is shown in cases where repinning was observed. Cases are enumerated as follows: (i) SU8 25 nm, (ii) SU8 100 nm, (iii) SU8 1.1 μm, (iv) PTFE 1.1 μm, (v) polyimide 1.1 μm, and (vi) glass 1.1 μm.

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