Subscriber access provided by UNIVERSITY OF THE SUNSHINE COAST
Technical Note
Shape Measurement of Ellipsoidal Particles in a CrossSlot Microchannel Utilizing Viscoelastic Particle Focusing Junghee Kim, Jun Young Kim, Younghun Kim, Seong Jae Lee, and Ju Min Kim Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.7b02559 • Publication Date (Web): 03 Aug 2017 Downloaded from http://pubs.acs.org on August 5, 2017
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Analytical Chemistry is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 18
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Analytical Chemistry
Shape Measurement of Ellipsoidal Particles in a Cross-Slot Microchannel Utilizing Viscoelastic Particle Focusing Junghee Kima, Jun Young Kimb, Younghun Kimc, Seong Jae Leeb, and Ju Min Kim*a, d
a
b
Department of Energy Systems Research, Ajou University, Suwon 16499, Korea Department of Polymer Engineering, The University of Suwon, Gyeonggi 18323, Korea c Department of Chemical Engineering, Kwangwoon University, Seoul 01897, Korea d Department of Chemical Engineering, Ajou University, Suwon 16499, Korea
*To whom correspondence should be addressed. *E-mail:
[email protected]; Fax: +82-31-219-1612; Tel.: +82-31-219-2475
ACS Paragon Plus Environment
Analytical Chemistry
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Abstract Shape measurement of nonspherical microparticles by conventional methods such as optical microscopy is challenging owing to particle aggregation or uncertainty regarding the out-of-plane arrangement of particles. In this work, we propose a facile microfluidic method to align particles inplane utilizing the extensional flow field generated in a cross-slot microchannel. Viscoelastic particle focusing is also harnessed to move particles toward the stagnation point of the cross-slot microchannel. We demonstrate that the shapes of ellipsoidal particles with various aspect ratios can be successfully measured using our novel microfluidic method. This method is expected to be useful in a wide range of applications such as shape measurement of nonspherical cells.
ACS Paragon Plus Environment
Page 2 of 18
Page 3 of 18
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Analytical Chemistry
Microfluidics-based particle manipulation has attracted much attention recently for chemical, biological, and environmental applications1-6 because particles can be precisely handled for accurate analysis, and a very small amount of sample is typically required. It has been demonstrated that physical properties of particles such as size and deformability can be measured5,6 and particle sorting based on these properties can be achieved for further analyses by microfluidic techniques7-9. However, most microfluidic approaches have been limited to spherical particles1,2,4,8,10 except for several shapebased separation techniques11-13. On the other hand, nonspherical particles have been extensively studied for engineering of water/air or water/oil interfaces.14-17 For instance, nonuniform particle distribution that occurs during the drying process, so-called “coffee ring effect”, can be significantly suppressed by using ellipsoidal particles,15 Capillary interaction among particles at the interface, which is significantly affected by the particle shape, plays a key role in suppressing the coffee ring effect.16,17 Therefore, identification of the particle shape is significant in designing and controlling the interfacial properties. However, particle shape measurement by conventional optical or scanning electron microscopy14 may have limited accuracy because not all of the particles are aligned on the same plane, and particle aggregation during sample preparation can be troublesome. Therefore, alignment of single nonspherical particle on the same plane is a prerequisite for accurate shape measurement. In this work, we employed particle alignment in the extensional flow field generated in a cross-slot microchannel [Figure 1a]. Rigid elongated particles are aligned when subjected to an extensional flow field.18,19 The flow kinematics near the stagnation point on the mid-plane between the upper and lower channel planes is assumed to be a purely planar extensional flow field [ = (−, , 0), where denotes the extensional rate]. The orientation of the aligned particles is stationary in the purely extensional flow because the extensional flow field has no rotational component.18,20,21 On the other hand, it is unlikely that particles emanating from the inlet approach the stagnation point along the mid-plane between the upper and lower channel planes when they are randomly suspended in a Newtonian fluid. In this work, the particles are focused along the channel centerline by viscoelastic particle focusing, and the three-dimensionally focused particle stream is
ACS Paragon Plus Environment
Analytical Chemistry
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
transported toward the stagnation point of the cross-slot channel.5,10,22,23 The particles in the viscoelastic channel flow migrate toward the channel centerline as a result of the imbalanced first or second normal stress difference (N1 or N2).24 N2 is typically assumed to be negligible because its magnitude is usually much smaller than that of N1.25 The elastic force ( ) exerted on a spherical particle is semi-empirically modeled as ~ ∇ ,23,26 where denotes the particle radius. When the suspending viscoelastic medium is modeled as an upper-convected Maxwell fluid, = 2λ in pressure-driven channel flow, where λ and are the relaxation time and viscosity of the viscoelastic fluid, respectively, and is the shear rate.25 Therefore, the particles migrate toward the low-shear-rate regions, which correspond to the channel centerline and the four corners of a rectangular channel.10 This viscoelastic particle focusing is widely used recently because it does not demand any complicated channel structure,10,23,27 and it can also be applied at a wide range of flow rates2,3,10. In addition, it has been demonstrated that various materials such as micro-/nano-sized particles, cells, and DNA molecules can be manipulated by viscoelastic focusing.9,12,28-30 In the current method, ellipsoidal particles suspended in a viscoelastic medium are focused along the channel centerline in a self-modulated manner and approach the stagnation point of the cross-slot channel on the mid-plane between the upper and lower channel planes. It was previously demonstrated that the cell deformability and damage can be successfully measured in a cross-slot microchannel using viscoelastic particle focusing.5,22 In the previous works,5,22 however, the extensional field generated in the cross-slot channel was harnessed only to stretch the deformable cells. Up to now, there was no study to demonstrate how the aspect ratio (AR) of the major axis to the minor axis of an ellipsoidal particle affects the particle focusing in viscoelastic fluid, which was systematically investigated in this work. Further, we observed that the ellipsoidal particles are aligned along the outlet direction at the stagnation point of the cross-slot channel, as shown in Figure 1b. Based on these studies, we propose a novel method to accurately measure the shapes of ellipsoidal particles using particle alignment in an extensional flow field combined with the viscoelastic particle focusing.
ACS Paragon Plus Environment
Page 4 of 18
Page 5 of 18
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Analytical Chemistry
Experimental Section Microfluidic Device and Operation. A schematic of the cross-slot channel is presented in Figure 1a, and more detailed information was provided in our previous work.5 In brief, the cross-slot channel has a single inlet and two outlets, and both the channel width (w) and height (h) are 50 µm throughout the straight channel regions. The side walls in the cross-slot region were designed to follow a hyperbolic function; that is, the side wall shape in the first quadrant has the relationship y = 1875/x µm (x ∈ [w/2, 3w/2]), and the walls in the remaining quadrants are axisymmetric or pointsymmetric to the wall in the first quadrant.5 A four-walled polydimethylsiloxane (PDMS) microfluidic channel was fabricated using conventional soft lithography techniques.31 The specific conditions for channel fabrication were those presented in our previous work,10 except that a slide glass coated with PDMS was used instead of a PDMS slab in this work. The flow rate was controlled using a syringe pump (PHD Ultra, Harvard Apparatus) equipped with 1 ml syringe (Hamilton). Materials. A 6.8 wt% polyvinylpyrrolidone (PVP) (Sigma-Aldrich, M.W. = 360,000 g/mol, molecular biology grade) solution in deionized water was used as a viscoelastic medium. The rheological properties of the PVP solutions are shown in Figure S1. The shear viscosity ( ) of the PVP solution was almost constant at 63 cP, and the relaxation time (λ) was measured to be 1.2 ms λ = lim →
! (")/"
" (")%,32 where " is the angular frequency, and
! (")
and
!
′(") are the
storage and loss moduli, respectively. Spherical polystyrene (PS) beads (diameter = 5.1 ± 0.08 µm) were initially synthesized by dispersion polymerization following the procedure presented in the previous work2 with slight modifications. Subsequently, ellipsoidal PS beads with different AR_opt’s (1, 1.5, 2, 3.2, and 4.8) were fabricated by a film stretching method using a prefabricated poly(vinyl alcohol) (PVA) film in which the spherical PS beads were embedded,33 where the AR_opt is defined as the aspect ratio of an ellipsoidal particle measured by optical method. In this work, the conventional optical and novel microfluidic methods were used to measure the aspect ratio (AR) of an ellipsoidal particle, which were denoted as subscripts ‘opt’ and ‘micro’, respectively. The PS beads were suspended in the viscoelastic medium at a concentration of 0.001 wt%, and a small amount of
ACS Paragon Plus Environment
Analytical Chemistry
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
surfactant (0.01 wt%, TWEEN 20; Sigma-Aldrich) was added to minimize adhesion among the PS particles and also suppress adhesion of the particles to the channel walls. Imaging. The particle dynamics in the cross-slot was observed with an inverted microscope (IX71, Olympus) equipped with an oil-immersed 60× objective with a long working distance (N.A. = 1.0), and images were acquired with a high-speed camera (MC2, Photron) (the shutter speed and frame rate per second were 1/8000 s and 3000, respectively). The particle shapes were measured by analyzing the acquired images using ImageJ software (NIH). The shape of a particle was measured as it passed the stagnation point, which corresponds to the red-circled region in Figure 1b and 1c. Images of stationary PS beads on a slide glass were obtained using a charge-coupled device camera (DMK23U445, The Imaging Source).
Results and Discussion Procedures to Measure the Shape of Ellipsoidal Particles. Images of stationary particles (Figure 2) observed with conventional optical method demonstrate aggregation of ellipsoidal particles (AR_opt > 1), which makes it difficult to measure their shape [Figure 2b–2e)]. In addition, it is unclear whether ellipsoidal particles exist on the same plane in their major axis direction. For this reason, the shapes of stationary particles that were separated from each other were measured when the conventional optical method was used. On the other hand, in our novel microfluidic method, the particle-bearing viscoelastic fluid leaving the inlet bifurcates into two flow streams, which meet at the cross-slot [Figure 1a]. In this work, the viscoelasticity of the polymer λ
solution is characterized by the Deborah number (De = )), which represents the ratio of the material's +
relaxation time (λ) to the flow time scale (* = ,); - is the characteristic length scale (= h), and U is the characteristic velocity (= Reynolds number (Re =
.1 , /23
. 25 ), /0
where Q is the total flow rate from the inlet [Figure 1a]. The
where 4 is the fluid density), which is used to characterize the ratio of
the inertial force to the viscous force, is small (
