The present invention relates to flow diffusion methods for determining the viscosity of a fluid sample.

Background

The measurement of the viscosity of complex solutions is a ubiquitous problem in biological, biophysical and biotechnological sciences. In addition, viscosity plays an important part in a wide range of technological applications that are based on fluid flow.

It is desirable to measure the viscosity of a fluid sample using high-throughput techniques using low volumes of sample. Several micro-rheological approaches have been described as alternatives to conventional viscometers and rheometers.

A common strategy for determining fluid viscosity involves monitoring the diffusion motion of tracer particles of known size. The movement of a single tracer particle may be monitored by video-microscopy (see, for example, Valentine et ai. and Tseng et a/.). Alternatively, the motion of tracer particles may be monitored by recording fluctuations in the average light scattering or fluorescence signal of the tracer particles (see, for example, Mason et ai.; He et a/.; Palmer et a/.; and Goins et al.). In these approaches, the viscosity of the fluid is readily quantified from the measured apparent diffusion coefficient of the tracer particle, as understood from the Stokes-Einstein relationship between viscosity and diffusion.

In diffusion wave spectroscopy (DWS) and single particle tracking, a generalized Langevin equation of motion can be applied to correlate the time evolution of the measured mean square displacement with the storage and loss moduli of the fluid (see, for example, Tseng et ai. and Mason et a/.).

The present invention provides alternative methods for the determination of viscosity, such as relative viscosity.

Summary of the Invention

The present invention generally provides a method for determining the viscosity of a fluid sample. The method comprises the step of monitoring the diffusion of an added component of known size through the fluid sample over time. From the measured diffusion profiles it is possible to determine the viscosity of the fluid sample.

Accordingly, in a first aspect of the invention there is provided a method for determining the viscosity of a fluid sample, the method comprising the steps of:

(ii) providing a flow of the fluid sample;

(iii) providing a component flow, wherein the component flow is a flow of the fluid sample further comprising a tracer component;

(iv) generating a laminar flow of the flow (ii) with the flow (iii) in a channel, such as a microfluidic channel;

(iv) measuring the lateral diffusion of the tracer component across the flows; and

(v) determining the viscosity of the fluid from the measured diffusion profile, wherein the size of the tracer component is known or is determined.

In an embodiment of the invention, the method further comprise the preliminary step of (i) adding a tracer component to a part of the sample fluid.

The tracer component is a component that is added to the fluid sample for the purpose of determining the viscosity of the sample.

The methods of the invention are performed in a flow device, such as a microfluidic device. The use of fluidic devices in the method of the invention provides for low cost and relatively simple instrumentation, as compared with, for example, those methods that make use of a relatively expensive auto-correlator. Furthermore, fluidic techniques can allow the use of small volume samples, and permit rapid analysis times.

The methods of the invention are insensitive to the composition of the fluid sample, and the methods are also insensitive to the size of the tracer component added to a part of the fluid sample. As shown herein, this is in contrast to viscosity methods based on dynamic light scattering, where viscosity measurements are complicated where an added tracer component is of a comparable size to other components within the fluid sample.

It follows that the methods for the invention are accordingly not restricted to the use of specific components or specific fluid samples.

Furthermore, the fluid methods of the present invention may be combined with other fluidic techniques. The fluidic devices used in the methods of the invention may be provided in fluid commutation with other fluid devices, and these devices may be integrated within a single flow chip.

In other aspects of the invention there are provided fluidic devices for use in the methods of the first aspect of the invention.

Further aspects and embodiments of the invention are discussed in further detail below.

Summary of the Figures

Figures 1 a and 1 b are schematics of a fluidic device for use in a diffusion method according to an embodiment of the invention.

Figure 2 shows the diffusion profiles for 47 nm particles in 10 wt % (blue) and 50 wt % (red) in aqueous glycerol solutions as measured in the fluidic device of figures 1a and 1b. The diffusion profiles were measured at twelve different diffusion times, at 12 positions along the diffusion channel. The diffusion distance (mm) is the distance from the junctions of the large cross section channel with the diffusion channel. The low and high viscosity samples were 10 wt % and 50 wt % glycerol in water solutions respectively.

Figure 3 is a series of images (left) showing the diffusion of particles across the fluid flows in a diffusion channel in the fluidic device of figures 1a and 1 b. Also provided are the diffusion profiles (right) for each of the images, where the profiles include the measured (dotted lines) and simulated (continuous lines) diffusion profiles. The images make use of a 10 wt % glycerol in water solution. Positions I, II, III and IV are located at 5.3 mm, 18.6 mm, 30.4 and 88.7 mm respectively along the diffusion channel.

Figure 4 shows the change in relative viscosity (ηΓ) with change in glycerol concentration (mg/mL) in an aqueous sample solution, as measured in the fluidic device of figures 1a and 1 b (circles) at 25°C, and by conventional dynamic light scattering technique (squares). The reported literature values are also shown (triangles). The dashed line represents an empirical function reported in the literature.

Figure 5 shows the change in relative viscosity (ηΓ) with the change in bovine serum albumin (BSA) concentration (mg/mL) in an aqueous sample solution, as measured in the fluidic device of figures 1a and 1b (circles) using standard nanoparticles of 100 nm diameter at 25°C, and by conventional dynamic light scattering techniques (squares).

Figure 6 shows the size distributions (nm) measured by dynamic light scattering for a solution of 100 g/L BSA with 0.05 % standard nanoparticles of 47 nm diameter (a) or 100 nm diameter (b).

Figures 7a and 7b are schematics of fluidic devices for use in a diffusion method according to embodiments of the invention, where (a) is a series device having a 2 channel configuration and (b) is a series device having a 3 channel configuration.

Figures 8a and 8b are schematics of fluidic devices for use in a diffusion method according to embodiments of the invention, where (a) is a parallel device having a 2 channel configuration and (b) is a parallel device having a 3 channel configuration.

Detailed Description of the Invention

The present invention provides a method for determining the viscosity of a fluid sample. The fluid sample for analysis is used in parts, with one part having a tracer component added to it, whilst another part is not so modified. A laminar flow of the fluid parts is generated in a channel, and the tracer component in the one part is permitted to diffuse into the other part. The movement of the tracer component over time is monitored. From the measured diffusion profiles the viscosity of the fluid sample may be determined.

The fluid flows are provided in a channel of a flow device, such as a microfluidic device.

Flow devices for measuring diffusion are known in the art, and are described in further detail below.

The present inventors have previously described in WO 2014/064438 the use of diffusion techniques to analyse multicomponent mixtures. In a validation of that approach, the inventors monitored the diffusion of components having a known size. However, it is not apparent from this earlier work that the diffusion techniques could and should be used to determine the viscosity of a test sample using a tracer component.

To validate the sizing measurements, WO 2014/064438 describes the diffusion of fluorescently-labelled 25 and 100 nm polystyrene particles, separately and together, into a blank aqueous flow. The polystyrene particles are used at a concentration of 0.2 % by volume. It is not apparent from this work that particles of known size and relatively low concentration could and should be added to a test sample, in order to determine the viscosity of that test sample.

The methods of WO 2014/064438 look at the movement of components out of the sample fluid flow into a blank (of buffer) fluid flow. In contrast, the methods of the present case look at the movement of a tracer component into a sample fluid flow.

Fluid Sample

The methods of the invention allow the viscosity of a fluid sample to be determined. The methods of the invention allow fluids of unknown composition to be analysed, and there is no limitation as to the nature and concentration of the components within the fluid sample.

In one embodiment, the fluid sample is a fluid sample whose viscosity is unknown.

The fluid sample may be a liquid sample.

In the methods of the invention, the fluid sample is provided in at least two parts. The first part of the fluid sample is a part having a tracer component. This tracer component may be

added to the fluid sample prior to the analysis. The other part is fluid sample that does not contain the tracer component. In the methods of the invention flows of these parts of the fluid sample are brought into contact to form a laminar flow. The component is then permitted to diffuse from the flow of the first part to the flow of the other part.

A reference to a sample flow is a reference to a fluid flow of a part of the fluid sample that does not contain the tracer component.

A reference to a tracer component flow is a reference to a fluid flow of a part of the fluid sample that does contain the tracer component.

Thus, the composition of the sample flow and the tracer component flow may be identical save for the presence of the tracer component in the tracer component flow. Thus, a reference to the properties of the sample flow may be taken as a reference to the tracer component flow, as context dictates.

The sample fluid may have within it, either dissolved or dispersed, one or more components. Such components include biological molecules, and the components may contain polypeptide, polysaccharide or polynucleotide groups.

For example, a component in the sample fluid may be a protein, including an antibody.

In one embodiment, the sample fluid contains a plurality of components. It follows that the diffusion measurements follow the movement of the tracer component across

multicomponent flows. In one embodiment, the fluid sample is a multicomponent mixture. In one embodiment, the composition of the sample fluid or the concentration of components within the fluid may be unknown.

When the tracer component is provided in a part of the fluid sample this tracer component is in addition to the components within the fluid sample.

The fluid sample may contain a component having a molecular weight, such as a weight average molecular weight, of at least 300 Da, 500 Da, at least 1 ,000 Da (1 kDa), or at least 2 kDa.

The fluid may contain a component having a molecular weight, such as a weight average molecular weight, of at most 5 kDa, at most 10 kDa, at most 20 kDa, at most 50 kDa or at most 100 kDa.

For example, as noted above, the sample fluid may contain one or more proteins.

The fluid sample may contain (or may be suspected to contain) components that are of a similar size to the tracer component. The inventors have found that the fluid techniques of the invention allow viscosity to be determined regardless of the size of the tracer component, and regardless of the size of components that are present within the fluid sample. Thus, the methods of the invention may be used to determine the viscosity of a fluid sample having a complex and/or unknown composition.

The present inventors have found that previously described dynamic light scattering methods for determining viscosity are problematic where an added tracer component has a similar size to one or more components within the fluid sample. In particular, the convolution of the light scattering signals from the tracer component and the similarly-sized components in the fluid sample do not permit accurate sizing of the tracer particles, therefore resulting in an inaccurate viscosity determination. This is shown in the worked examples of the present case.

Given that the results from the dynamic light scattering experiments may not always be reliable, it may be necessary to repeat the dynamic light scattering experiments using tracer components of varying size in order to validate the calculated viscosity values. Alternatively it may be necessary to increase the intensity of the light scattering signal in some other way, such as increasing the concentration of the tracer components within the fluid sample.

However, this latter approach is not often desirable, as the addition of a significant amount of the tracer component is likely to cause a change in the viscosity of the sample.

A reference to the size of a component in the fluid sample may be a reference to the radius, such as the hydrodynamic radius, of the component. The size of the components may be determined using standard analytical techniques. For example, diffusion measurements may be used to determine the size of components within the fluid sample.

A component present in a fluid sample may have a radius of at least 0.05 nm, at least 0.1 nm, at least 0.5 nm, at least 1 nm, or at least 5 nm.

A component present in a fluid sample may have a radius of at most 10 nm, at most 15 nm, at most 25 nm, at most 50 nm, at most 100 nm, or at most 200 nm, or at most 500 nm.

A component may have a radius in a range with upper and lower limits selected from those given above. For example, the present invention is particularly suitable for determining the viscosity of fluid samples holding components having radii in the range 0.5 to 500 nm, such as 0.5 to 200 nm, such as 0.5 to 100 nm, such as 0.5 to 15 nm.

Typically, the fluid sample is an aqueous fluid. An aqueous fluid may additionally comprise a miscible organic solvent. This may be provided to retain components in solution or suspension. For example, DMSO may be present together with water.

In the methods of the invention a tracer component-containing fluid is prepared from the fluid sample, by the addition of the tracer component into a part of the fluid sample. A further part of the fluid sample is provided, and this part does not contain the tracer component. This further part is used to generate the sample flow.

A part of the fluid sample containing the tracer component may be prepared by simple admixture of the tracer component with a part of the fluid sample.

The tracer component may be used at relatively low concentrations. The detection methods for use in the present invention, such as fluorescent detection, allow for the detection of the tracer component at very low concentrations.

The tracer component is added to the fluid sample at a level sufficient to substantially maintain the viscosity of the fluid. Thus, the viscosity of the sample flow and the tracer component flow are substantially the same, if not the same.

The determined viscosity values for the sample fluid will not be sufficiently accurate if there is a large difference in the viscosity of the sample fluid and the tracer component fluid.

The tracer component is provided in an amount sufficient to allow for its detection in the fluid flows. The minimum amount of tracer component that may be used will depend upon the analytical properties of the tracer component, such as the fluorescence efficiency tracer component, and the detection efficiency of the detector that is used.

The tracer component may be provided in a part of the fluid sample at a concentration of at least 0.001 , at least 0.005, or at least 0.01 wt %.

The tracer component may be provided in a part of the fluid sample at a concentration of at most 0.05, at most 0.1 , at most 0.2, at most 0.5, or at most 1.0 wt %.

The tracer component may be used at a concentration that is selected from a range having upper and lower limits selected from the values given above. For example, the tracer component may be provided in a part of the fluid sample at a concentration in the range 0.01 to 0.05 wt %.

The fluid sample may contain one or more components, and these components will also be present in the tracer component flow. The concentration of these components in the tracer component flow is substantially the same as those components within the sample flow. Thus, the addition of the tracer component to a part of the fluid sample is not associated with a dilution or concentration of the components in the fluid. Thus, when the tracer component flow and the sample flow are contacted in a laminar flow, there is substantially no concentration gradient of the components across the fluid flows. In contrast, there is a concentration gradient of the tracer component across the fluid flows, and it is the change in this concentration gradient that is monitored over time.

In certain embodiments of the invention, the method provides the use of two flows of the fluid sample that do not contain the tracer component. These fluid flows may be generated from a single part of the fluid sample or these fluid flows may be generated from two parts of the fluid sample, as described in further detail below. These two flows are provided either side of the tracer component flow.

Tracer Component

The tracer component for use in the methods of the invention is a component whose size is known. The size of the component may be known in the art or it may be determined prior to the use of the component in the methods of the invention. The size of the tracer component may be known from the literature and/or this information may be provided from a commercial supplier of the component.

Alternatively, the size of the tracer component may be determined after the component has been used in the methods of the invention.

The size of the tracer component may be determined (or confirmed) using standard analytical techniques.

A reference to the size of the tracer component may be a reference to the radius, such as the hydrodynamic radius, of the component.

The tracer component used in the method of the invention may have a radius of at least 0.05 nm, at least 0.1 nm, at least 0.5 nm, at least 1 nm, or at least 5 nm.

The tracer component used in the method of the invention may have a radius of at most

10 nm, at most 15 nm, at most 25 nm, at most 50 nm, at most 100 nm, or at most 200 nm, or at most 500 nm.

The tracer component may have a radius in a range with upper and lower limits selected from those given above. For example, the present invention is particularly suitable for use with tracer components having radii in the range 0.5 to 500 nm, such as 0.5 to 200 nm, such as 0.5 to 100 nm, such as 0.5 to 15 nm.

The size of the tracer component may be of a substantially similar size to a component held within the fluid sample.

In one embodiment, the tracer component has a radius that is at least 10%, at least 20%, at least 40% at least 50%, at least 60%, at least 70%, at least 80%, or at least 90% that of the radius of a component in the fluid sample.

In one embodiment, the tracer component has a radius that is at most 100 % (i.e. the same as that of the component), at most 150%, at most 200%, at most 500% or at most 1 ,000% that of the radius of a component in the fluid sample.

The tracer component may have a radius that is in a range selected from the upper and lower limits given above. For example, the tracer component has a radius that is from 10% to 200% that of the radius of a component in the fluid sample.

Where diffusion measurements are to be made for multiple fluid samples having a range of viscosities (or a predicted range of viscosities), it is preferred that a tracer component having a small size is used for all experiments.

As explained above, the methods of the invention allow viscosity to be determined even where the tracer component has a comparable size to a component within the fluid sample.

The tracer components typically have a uniform size amongst their population. Using a substantially homogenous population of tracer particles ensures that the determined viscosity values are accurate.

Thus, in one embodiment, the trace components for use in the invention are substantially monodisperse.

In one embodiment, the tracer component has a size distribution where the standard deviation is less than 15%, less than 10%, less than 5% or less than 1% from the mean.

The size distribution of the tracer component may be known from information supplied by the commercial supplier of the tracer component. The size distribution may also be determined by spectroscopic measurements, including microscopy measurements of a sample population of tracer components.

It is possible to use tracer components having different sizes, where the relative numbers of each of the differently sized tracer components are known. The diffusion profiles of multicomponent mixtures may be resolved, for example through the use of the techniques developed by the present inventors as described in WO 2014/064438. For simplicity, it is preferred that tracer components are used having substantially the same size.

The tracer component may be a dissolved in the fluid sample. However, the present invention may also be used to follow the movement of tracer components that are dispersed within a fluid. Thus, the fluids used in the method may be colloidal, and may be a sol or an emulsion, where the tracer component is the dispersed phase.

The amount of tracer component required to perform an analysis according to the method of the invention is not large, and very small quantities of material may be passed through the microfluidic device. It is also possible to collect the fluid exiting the diffusion channel, and this may be reanalysed.

The method of the invention includes the step of measuring the diffusion of the tracer component across fluid flows. The tracer component may be detectable using standard analytical techniques such as fluorescent spectroscopy, luminescent spectroscopy, UV-vis spectroscopy amongst others.

The tracer component is typically a component that does not or is not expected to react with components within the fluid sample. For this reason the use of proteins as tracer components is to be avoided when measuring the viscosity of biological fluids. There is a risk that a protein tracer would interact with components of the biological fluid (e.g. such as other proteins).

The tracer component may be a molecule, such a polymeric molecule.

The tracer component may be a particle.

In one embodiment, the tracer component is a polymer, such as a polymer particle. The tracer component may be polystyrene, such as a polystyrene particle.

In one embodiment, the tracer component is a dye molecule.

A component may have be labelled, to assist detection. For example, in the exemplary methods described herein fluorescently-labelled polystyrene particles are used as a tracer component.

The tracer component may be chosen for its analytical properties. For example, the tracer may possess detectable functionality that is not present (or not thought to be present) within the sample fluid.

For example, the tracer component may be fluorescently active, and the fluorescent excitation and emission wavelengths may be such as are not typical or expected for the components within the sample fluid. In the present case, tracer components that have fluorescent activity are used to determine the viscosity of bovine serum albumin-containing solutions. The fluorescent activity of the tracer particles is not shared with the bovine serum albumin. Thus, the fluorescent signals detected are associated solely with the tracer component, and not any other component of the sample fluid.

Methods

The present invention may be used to determine the viscosity, such as the relative viscosity, of a fluid sample. The methods of the invention look to measure the diffusion of a tracer component of known size through the fluid sample. The diffusion of the tracer component over time allows the viscosity of the fluid to be determined.

In the methods of the invention a flow of the fluid sample is established. This is the sample flow. A flow of a fluid sample containing a tracer component is also established. This is the component flow. These sample and component flows are brought into contact, and a laminar flow is generated in a diffusion channel. Once the laminar flow is established the diffusion of the component from the component flow into the sample flow is monitored. Thus, the location of the component across one or all of the fluid flows may be determined at different diffusion times. From the measured diffusion profiles of the component, the viscosity of the fluid sample may be determined.

The channel is referred to as a diffusion channel, as it is a fluid channel where the diffusion of the component is permitted and monitored.

The flow rate of each flow is maintained at a substantially constant level during the analysis steps. The analysis may be undertaken only when a stable flow is established in the diffusion channel.

The flow rates used are not particularly limited, and it is not necessary for the flow rates of the sample flow and the tracer component flow to be the same.

The flow rate of the tracer component flow may be altered independently of the flow rate of the sample flow.

In practice, the flow rates are selected to accommodate the diffusion of the component across the fluid flows. The flow rate is selected such that there is sufficient residency time within the flow channel to allow for diffusion of the tracer component to be monitored.

The channel is a part of a fluidic device. The fluidic device is adapted for use with a detector at a plurality of locations in the channel. The channel is in fluid communication with supply channels for the sample flow and the component flow.

In some embodiments, two sample flows are provided on either side of the tracer component flow. The method of the invention may therefore look at the diffusion of the tracer component in the tracer component flow into either or both of the flanking sample fluid flows. The use of two sample fluid flows is advantageous as these may be used to provide a stable balancing pressure across the tracer component flow.

The composition of the two sample flows is identical. Typically the flow rate of the two sample flows is identical.

The methods of the invention are typically performed in flows having a low Reynolds number. For example, the Reynolds number of a flow may be 1 or less, 0.5 or less, 0.1 or less, or 0.05 or less.

The methods of the invention may be performed at or around room temperature, for example 15, 20 or 25°C. Alternatively, the methods of the invention may be conducted at lower temperatures, such as 5 or 10°C, or higher temperatures, such as 35, 40 or 50°C. The method of the invention may also include the step of measuring the temperature of the fluids for use in the invention.

In one embodiment the fluid flow rate of the laminar flow is at least 1 , at least 5, at least 10, at least 50, or at least 100 μΙ_η"1.

In one embodiment the fluid flow rate of the laminar flow is at most 200, at most 400, at most 500, at most 1 ,000, at most 2,000 or at most 5,000 μΙ_η"1.

In one embodiment, the flow rate of the laminar flow is a value selected from a range having upper and lower values selected from the values above. For example, the flow rate may be in range 5 to 400 pLh"1.

The fluid flow rate is the flow rate at steady state.

The laminar fluid flow rate refers to the combined flow rate of the sample flow (or sample flows) and the tracer component flow. The flow rates of the tracer component flow and the sample flow may be adjusted to achieve the desired flow rate in the diffusion channel.

The use of microfluidic devices with flow rates in the range indicated above means that relatively small quantities of component fluid may be used in an analytical run. For example, volumes in the range are sufficient to establish a steady state flow in the diffusion channel for the purposes of obtaining at least one diffusion profile reading.

In one embodiment, the total volume of fluid used in the method is at most 50, at most 100, at most 200, at most 500, or at most 1 ,000 μΙ_.

In one embodiment, the total volume of fluid used in the method is at least 0.1 , is at least 0.5, is at least 1 , is at least 5, or is at least 10 μΙ_.

In one embodiment, the total volume of fluid used is a value selected from a range having upper and lower values selected from the values above. For example, the total volume may be in range 1 to 50 μΙ_.

The total volume of fluid refers to the combined volumes of the tracer component fluid and the sample fluid used in the method.

In one embodiment, the lateral diffusion of the tracer component from the tracer component flow into the sample fluid flow is measured at a plurality of diffusion times.

The lateral diffusion may therefore be measures at a plurality of locations along the diffusion channel. The separation between measurement points is not particularly limited, but may be of sufficient distance that the recorded diffusion profiles have noticeably changed between measurement points.

In an alternative embodiment, the lateral diffusion of the tracer component is measured at a single location in the channel. Once the lateral diffusion has been recorded, the flow rates of the tracer component and sample fluid flows are altered, and once a stable flow is established, the lateral diffusion of the tracer component is measured at the single location. Thus, diffusion profiles at different diffusion times may be obtained through changes to the flow rates. The flow rates may be changed one, two, three or more times.

Fluidic Device

The method of the first aspect of the invention makes use of a diffusion channel which is a part of a fluidic device, for example a microfluidic device.

Thus, the fluidic device comprises a diffusion channel. The diffusion channel holds the laminar flow of the tracer component flow with a sample fluid flow, or the diffusion channel holds the laminar flow of the tracer component flow with two sample fluid flows, which are provided either side of the tracer component flow.

The diffusion channel is adapted for use with an analytical device, which is suitable for determining the lateral distribution of a tracer component at one or more locations along the diffusion channel.

The diffusion channel may be in fluid communication with an upstream channel having a larger cross section (a large cross section channel). The diffusion channel may accordingly be referred to as a small cross section channel. The use of large and small cross section channels in a diffusion device is described by the present inventors in WO 2014/064438, the contents of which are hereby incorporated by reference in their entirety.

The use of microfluidic channels to hold the tracer component and sample fluid flows ensures that the flows take place at low Reynolds numbers, and consequently convection and diffusion are the only relevant mechanism of mass transport within the system.

Accordingly, this allows accurate numerical calculations to be performed.

The general dimensions of the channels in the device are selected to provide reasonable mobilisation rates and analysis times. The dimensions of the device may also be selected to reduce the amount of fluid required for a sufficient analysis run.

The diffusion channel (and the large cross section channel, where present) is a channel having suitable dimensions allowing for the generation and maintenance of a laminar flow of two (or three) streams within. The laminar flow of two streams means that the flows are side by side and are stable. Thus, there are typically no regions where the fluids recirculate, and the turbulence is minimal. Typically such conditions are provided by small channels, such as microchannels.

Devices for use in dispersive measurements are well known in the art, and are described, for example, by Kamholz et al. (Biophysical Journal, 80(4): 1967-1972, 2001) and

WO 2014/064438.

A reference to a channel herein, such as a diffusion channel, is a reference to a channel having a substantially rectangular cross section. Thus, the channel may be formed of a substantially flat base with walls which extend substantially vertically therefrom, and optionally a top cover. Typically, the base and the walls are formed into a silicone substrate. The cover may be a glass cover, for example a standard glass slide or a borosilicate wafer. The detection apparatus may be provided above the diffusion channel.

A reference to width is a reference to the lateral diffusion dimension in the channel (which is referred to as d in some prior art references).

The diffusion channel has a substantially constant width throughout its length.

The width of the diffusion channel may be at most 500 pm, at most 700 pm, at most 1 ,000 m, or at most 2,000 pm.

The width of the diffusion channel may be at least 5 pm, at least 10 pm, at least 50 pm, at least 100 pm or at least 200 pm.

In one embodiment, the width of the diffusion channel may be in a range selected from the upper and lower values given above. For example, the width may be in the range 10 to 500 pm.

The length of the diffusion channel may be of a length suitable to allow the diffusion of the component in the component flow to the channel edge forming the boundary for the sample fluid flow. Thus, by the time the fluid flows have reached the end of the diffusion channel, all the components present in the component flow have reached the maximal entropic configuration.

The length of the diffusion channel is sufficient to allow a tracer component to diffuse from the tracer component fluid flow into the sample fluid flow. For tracer components, such as polymers, having the molecular weights described herein, diffusion channel lengths of 1 mm length or more are generally sufficient.

In one embodiment, the diffusion channel is at least 0.5 mm, at least 1 mm, at least 2 mm, or at least 5 mm long.

In one embodiment, the diffusion channel is at most 10 mm, at most 20 mm, or at most 50 mm long.

In one embodiment, the diffusion channel length may be in a range selected from the upper and lower values given above. For example, the diffusion channel length may be in the range 0.5 to 50 mm, such as 1 to 20 mm.

The flow of the fluids is along the longitudinal axis of the diffusion channel. The diffusion of the tracer component in the tracer component flow into the sample flow is transverse to the longitudinal axis of flow, across the width of the channel.

In some embodiments at least a part of the diffusion channel is convoluted. Thus, the diffusion channel may include a turn or series of turns, for example. The use of a convoluted geometry allows the size of the device to be minimised. The use of a convoluted path may also provide multiple flow channels within a single detection zone. In a single detection zone multiple channels (corresponding to different flow distances and therefore different diffusion times) may pass across a detector allowing multiple and simultaneous measurements to be made.

The diffusion channel may receive the sample and tracer component fluid flows directly from supply channels for each of the sample and tracer component fluids. Thus, the fluid flows contact at a junction of the upstream region of the diffusion channel.

In another embodiment, the diffusion channel may receive the sample and tracer component fluid flow from an upstream large cross section channel.

Fluid exiting from the diffusion channel may be collected for further analysis. Thus, the diffusion channel is in fluid communication with a sample collection reservoir. Alternatively, the diffusion channel may be in fluid communication with a further fluidic device, for example a device for measuring a second physical property of the fluid. In one embodiment, the diffusion channel is upstream of a second analytical device for determining the

hydrodynamic radius of a component within the sample fluid (which component may be referred to as an analyte).

The fluidic device may be provided with supply channels providing fluid communication between the reservoir and the diffusion channel, either directly or via a large cross section channel. Where two sample flows are to be provided into the diffusion channel (and either side of the tracer component flow), each of the sample fluid flows may be delivered independently from different reservoirs. However, each of the sample fluid flows may be provided form a single reservoir that is linked to the large cross section channel via two supply channels.

The dimensions of each supply channel are not particularly limited and may be similar to or the same as the diffusion channel. In one embodiment, each supply channel has a width that is greater than the width of the diffusion channel. In one embodiment, each supply channel has a width that is less than the width of the diffusion channel.

Claims:

1. A method for measuring the viscosity of a fluid sample, the method comprising the steps of:

(ii) providing a flow of the fluid sample;

(iii) providing a component flow, wherein the component flow is a flow of the fluid sample further comprising a tracer component;

(iv) generating a laminar flow of the flow (ii) with the flow (iii) in a diffusion channel, such as a microfluidic diffusion channel;

(iv) measuring the lateral diffusion of the tracer component across the flows; and

(v) determining the viscosity of the fluid from the measured diffusion profile, wherein the size of the tracer component is known or is determined.

2. The method of claim 1 , comprising the preliminary step (i) adding a tracer component to a part of the fluid sample.

3. The method of claim 1 or claim 2, wherein step (iv) is measuring the lateral diffusion of the tracer component across the flows at a plurality of diffusion times.

4. The method of any one of the preceding claims, wherein the tracer component is fluorescent, and the lateral diffusion of the component is measured by fluorescence.

5. The method of any one of the preceding claims, wherein the channel is a microfluidic channel.

6. The method of any one of the preceding claims, wherein the flow (ii) and the flow (iii) are brought into contact in a large cross section channel, and the contacting flows are permitted to flow from the large cross section channel into the diffusion channel.

7. The method of any one of the preceding claims, wherein step (ii) provides two flows of the fluid sample, and the sample fluid flows are brought into contact with and provided either side of the fluid flow comprising the tracer component.

8. The method of any one of the preceding claims, wherein the component has a radius, such as a hydrodynamic radius, in the range 0.5 to 200 nm, such as 0.5 to 100 nm.

9. The method of any one of the preceding claims, wherein the component flow is a flow of the fluid sample further comprising a tracer component, and the tracer component is present at 0.2 wt % or less.

10. The method of any one of the preceding claims, wherein the component is substantially monodisperse.

11. The method of any one of the preceding claims, wherein the component is a polymeric molecule.

12. The method of any one of the preceding claims, wherein the fluid sample is an aqueous sample.

13. The method of any one of the preceding claims, wherein the fluid sample comprises one or more components.

14. The method of claim 13, wherein the fluid sample comprises a component having a polypeptide, polynucleotide or polysaccharide group.

15. The method of claim 13 or claim 14, wherein the fluid sample comprises a component having a radius, such as a hydrodynamic radius, in the range 0.5 to 200 nm, such as 0.5 to 100 nm.

16. The method of any one of claims 13 to 15, wherein the tracer component has a radius, such as a hydrodynamic radius, that is from 10% to 200% that of the radius of a component in the fluid sample.