Confocal Imaging of Confined Quiescent and Flowing Colloid-polymer Mixtures

Summary

Confocal microscopy is used to image quiescent and flowing colloid-polymer mixtures, which are studied as model systems for attractive suspensions. Image analysis algorithms are used to calculate structural and dynamic metrics for the colloidal particles that measure changes due to geometric confinement.

Abstract

The behavior of confined colloidal suspensions with attractive interparticle interactions is critical to the rational design of materials for directed assembly^1-3, drug delivery⁴, improved hydrocarbon recovery^5-7, and flowable electrodes for energy storage⁸. Suspensions containing fluorescent colloids and non-adsorbing polymers are appealing model systems, as the ratio of the polymer radius of gyration to the particle radius and concentration of polymer control the range and strength of the interparticle attraction, respectively. By tuning the polymer properties and the volume fraction of the colloids, colloid fluids, fluids of clusters, gels, crystals, and glasses can be obtained⁹. Confocal microscopy, a variant of fluorescence microscopy, allows an optically transparent and fluorescent sample to be imaged with high spatial and temporal resolution in three dimensions. In this technique, a small pinhole or slit blocks the emitted fluorescent light from regions of the sample that are outside the focal volume of the microscope optical system. As a result, only a thin section of the sample in the focal plane is imaged. This technique is particularly well suited to probe the structure and dynamics in dense colloidal suspensions at the single-particle scale: the particles are large enough to be resolved using visible light and diffuse slowly enough to be captured at typical scan speeds of commercial confocal systems¹⁰. Improvements in scan speeds and analysis algorithms have also enabled quantitative confocal imaging of flowing suspensions^11-16,37. In this paper, we demonstrate confocal microscopy experiments to probe the confined phase behavior and flow properties of colloid-polymer mixtures. We first prepare colloid-polymer mixtures that are density- and refractive-index matched. Next, we report a standard protocol for imaging quiescent dense colloid-polymer mixtures under varying confinement in thin wedge-shaped cells. Finally, we demonstrate a protocol for imaging colloid-polymer mixtures during microchannel flow.

Introduction

This paper demonstrates (a) confocal imaging of quiescent and flowing confined colloid-polymer mixtures in two and three dimensions and (b) particle-tracking and correlation analyses of the resultant images to obtain quantitative information on the phase behavior and flow properties.

Colloidal suspensions with attractive interparticle interactions appear ubiquitously in technological applications as materials for directed assembly^1-3, drug delivery⁴, improved hydrocarbon recovery^5-7, and energy storage⁸. A common feature of these applications is that the particles must be flowed through fine geome....

Protocol

1. Preparation of Colloid-polymer Mixtures

Note: This protocol uses poly(methyl methacrylate) (PMMA) particles, sterically stabilized using poly (12-hydroxystearic acid) and labeled with a fluorescent dye (such as Nile Red, rhodamine B, or fluorescein), that were synthesized following a standard recipe⁵².

1. Prepare a 3:1 w/w mixture of cyclohexyl bromide (CXB) and decahydronaphthalene (DHN) as a stock solvent. This mixture nearly matches the density and index of refraction of the particles. Add an organic salt, tetrabutylammonium chloride (TBAC)⁴⁶, to the solvent at a concentration of 1.5 mM to partially....

Results

To demonstrate confocal imaging and particle-tracking, we investigated the effect of confinement on the phase behavior of colloid-polymer mixtures^63-65. For these experiments the colloid diameter was 2a = 0.865 μm. The colloid volume fraction was fixed at Φ = 0.15 and the concentration of polymer c_p was varied from 0 to 23.6 mg/ml. Representative confocal images are shown in Figure 2⁶³, left column. From the particle positions obtained using tracking alg.......

Discussion

Colloidal suspensions are widely studied as models for confined phase behavior, because micron-sized colloidal particles exhibit significantly slower dynamics than atoms and molecules and thus can be readily imaged and tracked over time¹⁰. For these fundamental studies, understanding the effect of interparticle attractions on confined phase behavior offers the opportunity to explore phenomena such as capillary condensation and evaporation^21,22,67. In addition, confined attractive suspensions appear .......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Cyclohexyl bromide	Sigma Aldrich	135194	CAS Number  108-85-0, Molecular wt. = 163.06, Used in stock solvent
Decahydronapthalene	Sigma Aldrich	D251	CAS Number 91-17-8, Molecular wt. = 138.25, Used in stock solvent
Nile Red	Sigma Aldrich	72485	Fluorescent dye
Fluorescein 5(6)-isothiocyanate	Sigma Aldrich	F3651	Fluorescent dye
Rhodamine B	Sigma Aldrich	83689	Fluorescent dye
Dynamic Light Scattering	Brookhaven Instruments	BI-APD	DLS equipment used for particle size measurement
Polystyrene	Varian/Agilent	PL20138-23	Polystyrene (polymer) for inducing depletion attraction
Tetrabutyl(ammonium chloride) (TBAC)	Sigma Aldrich	86870	monovalent salt
UV Adhesive	Norland Adhesive	NOA 68T	Part Number 68T01 (UV cured adhesive)
VT Eye	Visitech	VT Eye	confocal scanner
VT Infinity	Visitech	VT Infinity	confocal scanner
Microscope	Leica	DMI3000B	Inverted Microscope
Centrifuge	Thermo Scientific	Sorvall ST 16	1-5,000 rpm
Teflon tubing	smallparts	SLTT 26-72	Zeus PTFE Sublite Wall Tubing 26 AWG 0.016" ID x 0.003" Wall
Epoxy	Devcon	DA051	5 min epoxy
Syringe	Micromate/Cadence	5004	glass syringe with metal luer lock tip
Syringe tips	Nordson	7018462	32 GA precision tips
Syringe pump	New Era Pump system Inc.	NE1002X	Programmable microfluidic pump (syringepump.com)
Weigh balance	Mettler Toledo	AB204-S	0.0001-220 g
PMMA particles	synthesized		poly(methylmethacrylate) colloidal particles