EML4930/6930 Introduction to Microfluidics & bioMEMS Spring 2020
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Midterm Exam
Problem XXXXXXXXXXpt)
Select all co
ect answers that describe the image below (black patterns – electrodes):
A. It shows a dielectrophoresis phenomenon.
B. White particles and blue particles show same dielectrophoresis behavior.
C. White particles and blue particles show different dielectrophoresis behavior.
D. White particles show positive dielectrophoresis.
E. The non-separated white and blue particles further away from electrodes are due to
weak dielectrophoresis force.
Problem XXXXXXXXXXpt)
Consider flow of water ( = 1000 ; = 1 ) through a circular microchannel of length 5
cm. What is the largest magnitude pressure drop that can be applied while maintain strictly
laminar flow if the microchannel diameter is (a) 5 and (b) 50 m.
EML4930/6930 Introduction to Microfluidics & bioMEMS Spring 2020
2 | 3 P a g e
Problem XXXXXXXXXXpt)
Consider the following microfluidic channel with embedded microelectrodes on a side wall, a
suspension of a mixture of two particle populations (blue and red) is focused in the center of the
channel under a pressure-driven sheath flow. Under the influence of electric field, the trajectory
of the focused particles will be biased due to dielectrophoresis (DEP) force. Depending on the
sign of the real part of the CM factor, the particles can exhibit positive DEP or negative DEP so
that they will be attracted towards to the electrodes or pushed away from the electrodes. Using
this method, we can separate the two different types of particles.
Given the dielectric properties in the following table, calculate the real part of the CM factor and
identify an electrical frequency from a range of 1 kHz to 10 MHz to separate blue particles from
ed particles. Assume laminar flow and spherical particles with a same size. Using your selected
electrical frequency, predict the collection sites (A, B, C) for each particle type. Show details of
your calculation to support your prediction.
parameter r, relative permittivity σ (S/m), electrical conductivity
medium XXXXXXXXXX
Blue particle 40 0.2
Red particle XXXXXXXXXX
EML4930/6930 Introduction to Microfluidics & bioMEMS Spring 2020
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Problem 4 (15 pt).
The electrical permittivity o the electrolyte affects the thermodynamic efficiency of an
electrokinetic pump owing to the dependence of electroosmotic flow on the permittivity.
Read the attached reference and summarize:
(a) The dependence of the flow rate on permittivity,
(b) How can the permittivity of a solution be changed?
(c) What other properties of a solution will also be changed that can affect the pumping
efficiency?
Problem XXXXXXXXXXpt)
Access to the FAU li
ary or Google Scholar, search for a journal publication showing
microfa
ication process. Understand the procedure of microfa
ication described in the paper.
Show explicitly in your answer regarding:
(a) the cross-sectional view of the step-by-step fa
ication process;
(b) point out whether the photoresist material used in each step is positive or negative. Attach
the full text of the paper together with your answer.
doi:10.1016/S XXXXXXXXXXX
Increasing the performance of high-pressure, high-efficiency
electrokinetic micropumps using zwitterionic solute additives
David S. Reichmuth, Ga
iela S. Chirica, Brian J. Ki
y*
Microfluidics Department, Sandia National Laboratories, P.O. Box 969, MS 9951, Livermore, CA 94551, USA
Accepted 14 January 2003
Abstract
A zwitterionic additive is used to improve the performance of electrokinetic micropumps (EK pumps), which use voltage applied across a
porous matrix to generate electroosmotic pressure and flow in microfluidic systems. Modeling of EK pump systems predicts that the additive,
trimethylammoniopropane sulfonate (TMAPS), will result in up to a 3.3-fold increase in pumping efficiency and up to a 2.5-fold increase in the
generated pressure. These predictive relations comparewell with experimental results for flow, pressure and efficiency. With these improvements,
pressures up to 156 kPa/V (22 psi/V) and efficiency up to 5.6% are demonstrated. Similar improvements can be expected from a wide range of
zwitterionic species that exhibit large dipole moments and positive linear dielectric increments. These improvements lead to a reduction involtage
and power requirements and will facilitate miniaturization of micro-total-analysis systems (mTAS) and microfluidically driven actuators.
Published by Elsevier Science B.V.
Keywords: Micropump; Electroosmosis; Zwitterion; Dielectric increment
1. Introduction
Micro-total-analysis systems (mTAS) have received a great
deal of recent attention owing to their ability to improve the
performance of chemical analysis systems by reducing foot-
print, reagent volumes, and electrical power needs. As a
crucial component of mTAS research, micropumps have been
investigated as a means to move fluids and actuate microscale
mechanical components. Previous investigators have pre-
sented micropumps in a variety of formats, as reviewed in
ecent papers [1,2]. Electrokinetic micropumps (EK pumps)
have been shown to generate pressures above 57 MPa
(8000 psi) [3] or flow rates above 1 ml/min [2], making them
attractive for miniaturization of HPLC systems [4], cooling of
microelectronics, and actuation of microscale mechanical
components [5].
EK pumps use electroosmosis in charged porous media to
generate a pumping function. Electroosmotic flow (EOF) in
porous matrices has been used in a variety of applications,
including capillary electrochromatography [6,7], and micro-
fluidic pumping [2,3,8]. EK pumps are ideally suited fo
mTAS, since they can straightforwardly meter the very low
flow rates (nl/min or ml/min) that are typically used, and can
generate high pressure (>10 MPa) required for chromato-
graphic separations.
An EK pump is realized experimentally by applying
voltage across a porous bed possessing a charged solid–
liquid interface (Fig. 1). Electroosmosis due to the applied
electrical field causes fluid flow and generates a pressure
whose magnitude depends in part on the fluidic resistance of
the channels downstream of the pump. Pump performance is
dictated by substrate material and geometry as well as fluid
properties. This paper presents the use of fluid additives to
improve the pressure and flow rate performance of EK
pumps. Important achievements include the demonstration
of 156 kPa/V (22 psi/V) and 5.6% efficiency, both (to ou
knowledge) the highest performance reported for EK pumps.
2. Theory
In certain limits, EK pumps can often be modeled by
straightforward equations. This section derives performance
elations for EK pumps, with special attention to the effect of
solutes. These relations will be used throughout the paper to
illustrate the effects of uncharged, zwitterionic solute addi-
tives on pump performance.
EK pump performance parameters were first derived fo
capillaries using a Helmholtz double layer model [9], and
later expanded to incorporate Gouy–Chapman double layers
Sensors and Actuators B XXXXXXXXXX–43
* Co
esponding author. Tel.: þ XXXXXXXXXX; fax: þ XXXXXXXXXX.
E-mail address: bjki
XXXXXXXXXX (B.J. Ki
y).
XXXXXXXXXX/03/$ – see front matter. Published by Elsevier Science B.V.
doi:10.1016/S XXXXXXXXXXX
of finite size [10]. Building on early work, which considered
simple geometries and often linearized the Poisson–Boltz-
mann equation [11], recent work has expanded this analysis
to include detailed accounts of pore sizes and shapes [12]
and fully explore the input parameter space [13] to evince
nonlinear and limiting effects.
Here, we are concerned primarily with the relative per-
formance change caused by adding solute to the pumped
uffer, and anticipate working with high enough buffe
concentrations (�10 mM) that the Debye length [14] can
e assumed small compared to the effective pore radius
(�100–200 nm). For this simple case, the electroosmotic
flow (EOF) may be treated as uniformly proportional to the
electric field throughout the pump medium.
Assuming a cylindrical capillary geometry with radius a
and phenomenological zeta potential z (V), as well as a
liquid with viscosity m (Pa s), Stokes’ flow equations can be
combined with an electroosmotic forcing term to give the
EOF profile as
uðrÞ ¼ Px
4m
ða2 � r2Þ � ee0zE
m
(1)
where Px (Pa/m) is the pressure gradient along the axis, r the
adial position, and E (V/m) is the uniform electrical field. In
this paper we use e to denote the nondimensional dielectric
constant such that the fluid permittivity is given by the product
of the dielectric constant e and the permittivity of free space e0.
Eq. (1) can be used to derive a number of performance
elations for EK pumps that consist of linear capillaries
and operate in the thin double layer limit. Practical EK pumps
consist not of linear capillaries but rather a porous bed; thus
Eq. (1) can quantitatively treat porous media only if additional
parameters (e.g., formation factors, porosity, tortuosity) are
used to adapt the microchannel geometry to that of the porous
ed. These additional parameters add multiplicative factors to
Eq. (1) and the derived results to follow. However, the theory
in this section is concerned primarily with the relative per-
formance increase observed upon addition of specific fluid
additives; thus the treatment for an idealized linear capillary
system will be retained; it is simple and sufficient for this
purpose. This derivation has been presented in [9], but is
epeated here for clarity.
From Eq. (1) we can derive that the maximum pressure
per volt generated in such a capillary (i.e., the pressure
performance at zero net flow rate) is
DPmax
V
¼ 8ee0z
a2
(2)
Fig. 1. EK pump operation and characterization. (a) Schematic of experimental setup. Voltage applied across a capillary packed with silica microspheres
leads to flow and pressure generation. The fluidic resistance of the output channel controls the pressure and flow rate. Pressure is measured with a transduce
and flow is measured by observation of meniscus motion through the output channel. (b) Expanded view of EK pump. Voltage gradient induces EOF from left
to right; pressure gradient induces Pouiseille flow from right to left. (c) Expanded view of pores in between microspheres. Flow pattern is a linea
superposition of solenoidal EOF from left to right and pressure-driven Poiseuille flow from right to left.
38 D.S. Reichmuth et al. / Sensors and Actuators B XXXXXXXXXX–43
where V is the applied voltage. As a practical example, we
can use Eq. (2) to estimate that for a packed bed of 0.5 mm
silica beads (effective pore radius a � 100 nm) and a fluid
consisting of a 10 mM aqueous Tris (tris(hydroxymethyl)a-
minomethane hydrochloride) buffer (z � 60 mV), the max-
imum pressure achieved will be 35 kPa/V (4.9 psi/V).
Expanding the microchannel model to consider an a
ay
of identical