Deterministic ratchets for suspension fractionation

T. Kulrattanarak

Research output: Thesisinternal PhD, WU


Driven by the current insights in sustainability and technological development in
biorefining natural renewable resources, the food industry has taken an interest in
fractionation of agrofood materials, like milk and cereal crops. The purpose of fractionation
is to split the raw material in several functional ingredients. For example,
milk can be split in fractions containing milk fat, casein micelles, and whey proteins.
Traditionally, separation processes in food industry are mainly aimed at separating
fluid from a suspension stream. Frequently membrane technology is used this type of
separation; membranes seem an obvious choice because they are able to sieve components
during mild fractionation of many foods, which are suspensions by nature,
like milk, or are suspended in liquid during processing (such as starch granule suspensions).
However, membrane separation is hindered by fouling of the pores by the
food ingredients and accumulation of these components in front of the pore, which
makes fractionation with membranes more challenging than plain separation of fluid
and solids. That is why we have investigated the possibilities of alternative technologies
such as microfluidic devices, and evaluated them under conditions required for
food applications.
Microfluidic devices are currently investigated for fractionation in biological applications,
like sorting of DNA or cells. Due to the large degree of freedom in design,
these devices are very suited for innovative fractionation technologies. First, we have
evaluated various designs available in literature in chapter 2, which concludes that
so-called deterministic ratchets are the most promising technology for fractionation of
food suspensions. This conclusion is based on the high yield, compactness of equipment,
and high selectivity that can be reached with such devices. In chapters 3 6,
we report on detailed investigations on deterministic ratchets through 2D simulation
(chapter 3), image analysis in comparison with simulation results (chapter 4), and full
3D simulations in combination with the previously mentioned methods (chapter 5).
In the last chapter, our findings are summarized in classification and design rules, and
an outlook for future developments is given.
Deterministic ratchets are microchannels, containing a regularly spaced array of
obstacles, through which the particle suspension flows. The essential property of
these ratchets is that each obstacle row is displaced slightly laterally with respect
to the previous row. Small particles follow the streamlines of the fluid, and zigzag
around the obstacles, while particles larger than a certain critical size bump into the
obstacles, and are consequently displaced from their streamline. The larger particles
will continuously be displaced in a direction in which the obstacles are placed, and
have a certain angle with the flow direction. The small particles are moving in the
direction of the liquid flow, which implies under an angle of zero degrees. Via the
difference in migration angle of the zigzag and displacement motion, particles can be
fractionated, and collected from different outlets.
An important property of deterministic ratchets is the size of the particles relative
to the width of the so-called flow lane, which determines whether it will show zigzag
motion or not. This we have investigated intensively in chapter 3 by means of 2-D
flow field simulation. The critical particle size is related to the width of the flow lanes,
within which the zigzagging particles will move, and we have determined the flow lane
widths for various designs. The distribution of the flow lane width is found to depend
strongly on the design of the ratchets. For a limited number of designs the original
hypothesis of the inventors of the deterministic ratchets holds, and the flow lanes are
symmetrically distributed over the space in between obstacles in one single row. In
general, ratchets have an asymmetric flow lane distribution, and typically, ratchet
designs suitable for food applications show a strong asymmetric flow lane distribution.
An asymmetric flow lane distribution implies that there is not one critical flow lane
width but two that determine the type of motion of particles inside the ratchets. As a
first approach we have taken these as the first and last (and largest) flow lane width,
df,1 and df,N. Consequently, particles are expected to show alternative motions that
are in between zigzag and displacement motion. Its existence has become evident in
the experiments described in chapter 4, and we have named it mixed motion. The
mixed motion is irregular, in contrast to the zigzag and displacement motion, and has
a migration angle which is intermediate between the angles corresponding to zigzag
and displacement motion, 0 < _ < _max. The particles moving in the ratchets we have
tracked by high speed recording, and the migration angle were quantified through tailor-made image analysis. As expected, the transitions between the different types
of particle motion seem to occur on the basis of the critical length scales, df,1 and df,N.
However, this conclusion can not be stated with high certainty because of the large
experimental error due to the wide particle size distribution of the used suspensions.
Because the ratchets used in chapter 4 has not been specifically designed to investigate
various particle behaviors, we have designed new ratchets based on the critical
length scales, df,1 and df,N, via 2D flow simulations, in order to allow detailed investigation.
Although these critical length scales do not take all aspects that play a
role during particle movement in a ratchet into account, we have stated that they can
be used as an initial guideline for ratchet designs. Next, we have performed detailed
and computationally intensive, 3D simulations, that include the particles. These 3D
simulations are performed to check the validity of the classification rules, derived from
the 2D simulations, that only include fluid flow. The simulation results show that the
transition between zigzag and mixed motion occurs indeed at the critical length scale,
df,1, being the width of the first flow lane. However, the length scale determining the
occurrence of displacement motion is larger than the last lane width, df,N, and might
even be uncorrelated with it. We have concluded that this second critical length scale,
df,c, can only be determined via 3D simulations. The thus obtained classification rules
are investigated experimentally and we have been able to correlate the migration angle
of many observed particles exhibiting mixed motion, to the critical length scales. This
makes us confident, that we now have identified the relevant critical length scales in
deterministic ratchets.
In the concluding chapter, we discuss the approach that we chose to ultimately derive
the classification rules, and discuss the implications of the corrected length scales
on the key performance indicators of ratchets, that are relevant to food applications.
We find that obtaining the correct critical length scales requires computationally intensive
3D simulations. Specifically for compact ratchet designs, which are relevant for
food application, the critical lane width df,c is not much different from df,N, obtained
via 2D flow simulations - and 2D simulation may thus offer a more time-efficient way
of estimating df,c. Further, we have discussed the existence of mixed motion in terms
of selectivity during fractionation for polydisperse suspensions, and have found that
the yield, compactness, and selectivity, all decrease, but at the same time it also opens
possibilities for fractionation in multiple streams in one step.
Original languageEnglish
QualificationDoctor of Philosophy
Awarding Institution
  • Wageningen University
  • Boom, Remko, Promotor
  • van der Sman, Ruud, Co-promotor
  • Schroen, Karin, Co-promotor
Award date28 Apr 2010
Place of Publication[S.l.
Print ISBNs9789085856146
Publication statusPublished - 2010


  • suspensions
  • fractionation
  • fluid mechanics
  • simulation models
  • two dimensional flow
  • particles
  • separation technology

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