DESC:FIELD OF THE INVENTION
[0001] The present disclosure pertains to system and method for measurement of rheology. In particular, the present disclosure pertains to microfabricated probes and methods for obtaining rheological properties of fluids using the same.

BACKGROUND OF THE INVENTION
[0002] Background description includes information that may be useful in understanding the present invention. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed invention, or that any publication specifically or implicitly referenced is prior art.
[0003] Rheology refers to measurement of the mechanical properties of a fluid. The subject of rheology is of great interest to the physicists, engineers and is significant from the industrial point of view as well. Paints, gels, emulsions, oil recovery, plastics manufacturing are some of the many places where it finds a great relevance. Conventional bulk rheology suffers from a lot of limitations, like the low frequency range that can be probed, large amount of material required (mililitres), failure to probe microscale heterogeneities, etc. Microrheology is the method of using micron sized probes to measure the fluidic properties. It removes almost all of the above mentioned problems associated with bulk rheology. Microrheology requires a very small amount of fluid (microlitres), and hence is particularly suitable for analyzing expensive and rare samples like biomaterials.
[0004] Conventional microrheological techniques can be broadly divided into the passive and active methods. In passive rheology, no external energy or force is applied to the probe(s) and their response to thermal noise is observed. These thermal fluctuations observed over a considerable amount of time can provide valuable information about the local rheological properties of the surrounding media. The common methods employed to do passive rheology include light scattering techniques like Dynamic Light Scattering (DLS), Diffusing Wave Spectroscopy (DWS), Particle Tracking by video microscopy, Laser deflection particle tracking (LDPT) or Quadrant photodetector (QPD) measurements.
[0005] In light scattering techniques like DLS and DWS a monochromatic light source and detection optics are used, such that the intensity fluctuations of the light scattered from a particle of known size is detected which gives information about the viscoelastic properties of the surrounding media. These techniques have got the advantage of probing the frequency response over a wide range and information up to about 107 Hz can be extracted. Though the data produced are reproducible and reliable, they fail to give any information about the heterogeneity of the media and also require a measurement time of about 10s or more to extract any information.
[0006] In particle tracking measurements (PTM), fluorescence or bright field microscopy is used to measure the displacements resulting from the thermal fluctuations of the probe particles. The fluid properties can be measured by the well known Stokes' Einstein relation which relates the diffusion coefficient D of the particle of radius a, to the local viscosity ?; k being the Boltzmann's constant and T, the absolute temperature. However a very few fluids, known as the Newtonian fluids, can be completely described by only the viscosity. Most of the fluids encountered in daily life have elastic properties as well. Their complete rheological description requires the measurement of the complex stress modulus , where is called the storage modulus and measures the elastic properties and is called the loss modulus and measures the viscous properties of the surroundings. Here ? refers to the frequency at which the sample is being probed. For such Non- Newtonian or viscoelastic fluids one has to look into the Generalized Stokes Einstein relation (GSER) [10] for performing passive rheology. Here s is the variable after Laplace transform, is thus the transformed mean square displacement. Measurement of the displacements of a single particle may be affected by the interaction with the surrounding heterogeneity of the media. This problem can be removed by tracking the position fluctuations of two particles (Two point Microrheology) and using correlation functions to extract the rheological properties. The technique is suitable for measurements in a spatially heterogeneous media, but the frequency range of probing is limited by the frame rate of the video capture, which is typically around 100 fps or less.
[0007] Another way of doing the particle tracking would be to use quadrant photodetectors (QPD). A probe laser is used to scatter light from a single probe particle, which is arranged to fall on a photodiode, divided into four quadrants. This basically gives position information of the particle, but it does not have the limitation of the frame rate of video microscopy and can be used to probe over a wider frequency range (up to 100 KHz).
[0008] In passive methods, in order to produce statistically significant data so that the various important time scales involved can be probed, measurement should be carried out over a long time. The spatial resolution is limited to the area of coverage of the fluctuating particle.
[0009] In active methods, on the other hand, an external force is applied to the probe and its response is studied. Active micro rheology thus has the capability of probing materials which are very viscous and in which passive rheology fails, because of very minimal fluctuations produced. This can be accomplished by using AFM or most commonly by manipulating the motion of probe particles by optical or magnetic forces/torques.
[00010] Optical Tweezers have been widely used to rotate microparticles by transferring the angular momentum using light. If the light beam is rotationally symmetric, then the particles will rotate at a constant angular velocity and in this case the torque generated by the beam is balanced by the rotational fluidic drag. Thus, by knowing the applied torque and the rotation speed one can measure the viscosity of the surrounding media.
[00011] Magnetic particles are used to do active rheology under oscillating as well as rotating fields. Spherical magnetic particles have been used to do measurements in mucus or inside the cellular cytoplasm. In previous studies, researchers used ferromagnetic nanowires and rotated them using a rotating magnetic field to probe the nonlinear rheological properties of wormlike micelle solutions. Rotating magnetic nanorods have also been used to probe the viscoelastic properties of aging protein films with very high sensitivity. The fast measurement time of these techniques has been exploited to measure the temporal evolution of the viscosity during photo polymerization reactions. In known techniques, magnetic microbuttons have been oscillated using magnetic fields to do rheology of sheared phospholipid films.
[00012] For any micro rheology method described above, one important parameter of importance is the measurement time. Active micro rheology, though faster than its passive counterpart, requires measurement times of the order of at least a second for small enough probe particles, because of the averaging required to remove the inherent thermal fluctuations in the system. Further, the known techniques are not suitable for obtaining complete rheological map of a heterogeneous fluidic environment. A further disadvantage of the aforementioned techniques is that the analysis is inherently complicated and makes interpretation of data highly challenging. Also very fast particle manipulation at the nanoscale still being a technical challenge, studying the rheological evolution of a complex system with high temporal and spatial resolutions is something that has to be overcome.
[00013] The present invention satisfies the existing needs, as well as others, and generally overcomes the deficiencies found in the prior art.

OBJECTS OF THE INVENTION
[00014] It is an object of the present disclosure to provide a system and method for measurement of rheological properties of fluids.
[00015] It is a further object of the present disclosure to provide a system and method for measurement of local rheological properties of heterogeneous liquids.
[00016] It is another object of the present disclosure to provide a system and method for obtaining complete rheological map of a heterogeneous fluidic environment.
[00017] It is another object of the present disclosure to provide a system for measurement of linear and non-linear rheology of fluids, while requiring microliter or smaller sample volumes.
[00018] It is another object of the present disclosure to provide a system that can facilitate quick measurement of local rheological properties of fluids.
[00019] It is another object of the present disclosure to provide a system that can facilitate real time monitoring of fluid flow rates in various processes.
[00020] It is another object of the present disclosure to provide a system that can facilitate rheological measurements in systems where very quick changes in mechanical properties are encountered.
[00021] It is another object of the present disclosure to provide a rheological measurement system which is capable of being used for in vivo characterization of bodily fluids/tissues or disease diagnosis.

SUMMARY OF THE INVENTION
[00022] Embodiments of the present disclosure relate to microrheological probe capable of being used for obtaining rheological properties of fluids and fluid interfaces, wherein the probe can include micro or nanometer scale particles with a defined shape and contain a magnetic material. The microrheological probes of the present disclosure can have elongate configuration with defined shape such as helical, rod shaped, disc or ellipsoid. The elongated microrheological probe can be rotated about its short or long axis very quickly, and its angular trajectory can be followed as a function of time.
[00023] According to embodiments, the microrheological probes of the present disclosure can be trackable. Further, the probes can exhibit a variety of dynamical configurations when dispersed in a fluid and subjected to a rotating or oscillating magnetic field.
[00024] In an embodiment, the microrheological probes of the present disclosure can be torqued or translated externally, and it can be controllably translated when dispersed in a fluid and subjected to a rotating or oscillating magnetic field.
[00025] In another aspect, the present disclosure provides a method for fabricating a microrheological probe, wherein the method can include the steps of a) providing micro or nanometer scale particles using physical vapor deposition technique, wherein each of the particles have elongated structure and have a shape selected from the group consisting of helical, rod-like, disc and ellipsoid; b) evaporatively depositing an adhesive layer on the surface of micro or nanometer scale particles; and c) evaporatively depositing a ferromagnetic material on the adhesive layer.
[00026] In another aspect, the present disclosure provides a rheometer which can include a microfluidic cell comprising one or more microfluidic channels and configured to contain a microrheological probe and further configured to receive at least one fluid from a fluid chamber; at least one magnetic or optical tweezer configured to hold the probe in a position within the microfluidic cell; an electromagnetic array comprising at least one pair of electromagnets configured to magnetically actuate the probe; an imaging system connected to the microfluidic cell to capture the dynamics/speed of the actuated probe; and a system configured to receive and analyze the captured images from the imaging system.
[00027] Various objects, features, aspects and advantages of the inventive subject matter will become more apparent from the following detailed description of preferred embodiments, along with the accompanying drawing figures in which like numerals represent like components.

BRIEF DESCRIPTION OF THE DRAWINGS
[00028] The accompanying drawings are included to provide a further understanding of the present disclosure, and are incorporated in and constitute a part of this specification. The drawings illustrate exemplary embodiments of the present disclosure and, together with the description, serve to explain the principles of the present disclosure.
[00029] FIG. 1 is SEM image of an exemplary nanofabricated helical probe in accordance with embodiments of the present disclosure.
[00030] FIG. 2 shows a schematic of propeller motion of a nanofabricated helical probe in accordance with embodiments of the present disclosure.
[00031] FIG. 3 illustrates orientation of a processing nanofabricated helical probe as a function of time for 1s under a rotating magnetic field in accordance with embodiments of the present disclosure.
[00032] FIG. 4 illustrates exemplary configuration of a microfluidic cell to carry out rheological measurement in accordance with embodiments of the present disclosure.
[00033] FIGs. 5A-C illustrate measurement of viscosity across a boundary of two fluids in accordance with embodiments of the present disclosure.
[00034] FIGs. 6A and 6B are graph showing relative viscosity measured across the two fluid boundaries in accordance with embodiments of the present disclosure.
[00035] FIGs. 7A and 7B are graph showing time evolution of mixing of two fluids in accordance with embodiments the present disclosure.
[00036] FIG. 8 is SEM image of an exemplary nanofabricated helical probe used for elasticity measurements in accordance with embodiments of the present disclosure.
[00037] FIG. 9 is a graph showing ratio of effective pitch of a nanofabricated helical probe and the measured viscosity as the probe is moved across the boundary from water to methyl cellulose, in accordance with embodiments of the present disclosure.
[00038] FIG. 10 is a graph showing ratio of effective pitch as a function of concentration of methyl cellulose in accordance with embodiments of the present disclosure.
[00039] FIGs. 11A and 11B illustrate two different levels of actuation in accordance with embodiments of the present disclosure.
[00040] FIG. 12 illustrates an exemplary schematic block diagram of a rheometer apparatus in accordance with embodiments of the present disclosure.

DETAILED DESCRIPTION OF THE INVENTION
[00041] The following is a detailed description of embodiments of the disclosure depicted in the accompanying drawings. The embodiments are in such detail as to clearly communicate the disclosure. However, the amount of detail offered is not intended to limit the anticipated variations of embodiments; on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present disclosure as defined by the appended claims.
[00042] Each of the appended claims defines a separate invention, which for infringement purposes is recognized as including equivalents to the various elements or limitations specified in the claims. Depending on the context, all references below to the "invention" may in some cases refer to certain specific embodiments only. In other cases it will be recognized that references to the "invention" will refer to subject matter recited in one or more, but not necessarily all, of the claims.
[00043] Unless the context requires otherwise, throughout the specification which follow, the word “comprise” and variations thereof, such as, “comprises” and “comprising” are to be construed in an open, inclusive sense that is as “including, but not limited to.”
[00044] Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.
[00045] As used in this specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the content clearly dictates otherwise. It should also be noted that the term “or” is generally employed in its sense including “and/or” unless the content clearly dictates otherwise.
[00046] In some embodiments, the numbers expressing quantities of ingredients, properties such as concentration, reaction conditions, and so forth, used to describe and claim certain embodiments of the invention are to be understood as being modified in some instances by the term “about.” Accordingly, in some embodiments, the numerical parameters set forth in the written description are approximations that can vary depending upon the desired properties sought to be obtained by a particular embodiment. In some embodiments, the numerical parameters should be construed in light of the number of reported significant digits and by applying ordinary rounding techniques. Notwithstanding that the numerical ranges and parameters setting forth the broad scope of some embodiments of the invention are approximations, the numerical values set forth in the specific examples are reported as precisely as practicable.
[00047] The recitation of ranges of values herein is merely intended to serve as a shorthand method of referring individually to each separate value falling within the range. Unless otherwise indicated herein, each individual value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g. “such as”) provided with respect to certain embodiments herein is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention otherwise claimed. No language in the specification should be construed as indicating any non-claimed element essential to the practice of the invention.
[00048] The headings and abstract of the invention provided herein are for convenience only and do not interpret the scope or meaning of the embodiments.
[00049] The following discussion provides many example embodiments of the inventive subject matter. Although each embodiment represents a single combination of inventive elements, the inventive subject matter is considered to include all possible combinations of the disclosed elements. Thus if one embodiment comprises elements A, B, and C, and a second embodiment comprises elements B and D, then the inventive subject matter is also considered to include other remaining combinations of A, B, C, or D, even if not explicitly disclosed.
[00050] Various terms as used herein are shown below. To the extent a term used in a claim is not defined below, it should be given the broadest definition persons in the pertinent art have given that term as reflected in printed publications and issued patents at the time of filing.
[00051] Embodiments of the present disclosure relate to microrheological probes capable of being used for obtaining rheological properties of fluids and fluid interfaces. Specifically, the present disclosure relate to micro and nanofabricated probes and a method for obtaining complete rheological map of a heterogeneous fluidic environment using the same. According to embodiments, the microrheological probes can include micro or nanometer scale particles with a defined shape and contain a magnetic material. The microrheological probes of the present disclosure can have elongate configuration with a defined shape such as helical, rod shaped, disc or ellipsoid. The elongated microrheological probe can be rotated about its short or long axis very quickly, and its angular trajectory can be followed as a function of time.
[00052] According to embodiments, the microrheological probe of the present disclosure can be trackable. Further, the probes can exhibit a variety of dynamical configurations when dispersed in a fluid and subjected to a rotating or oscillating magnetic field.
[00053] In an embodiment, the microrheological probes of the present disclosure can be torqued or translated externally, and it can be controllably translated when dispersed in a fluid and subjected to a rotating or oscillating magnetic field.
[00054] In a preferred embodiment, the microrheological probe can have helical shape and the length of the micro or nanometer scale particles can range from 100 nm to 100 µm, preferably from 1 to 5 µm.
[00055] In an embodiment, the microrheological probe can exhibit ferromagnetic characteristics which can be imparted by evaporatively depositing a thin magnetic layer, preferably having a thickess in the range of 1nm to 100 µm onto the surface of the micro or nanometer scale particles
[00056] In an embodiment, micro or nanometer scale particles can be magnetized by coating a magnetic material that can preferably be selected from the group consisting of Iron, Cobalt, Neodymium, Samarium, Nickel, and alloys thereof, on the surfaces of the micro or nanometer scale particles.
[00057] In an alternative embodiment, the micro or nanometer scale particles can be made of a magnetic material.
[00058] In an embodiment, cobalt or nickel can be coated on micro or nanometer scale particles by thermal or electron beam evaporation to impart ferromagnetic characteristics to the probe.
[00059] In an embodiment, a thin layer of silver or titanium can be coated on micro or nanometer scale particles prior to coating the same with magnetic material in order to facilitate adhesion of magnetic material with the surface of the particles. The adhesion layer can have thickness in the range of from 0.1nm to 50nm, preferably from 0.1nm to 10 µm.
[00060] In another aspect, the present disclosure provides a method for fabricating a microrheological probe, wherein the method can include the steps of a) providing micro or nanometer scale particles using physical vapor deposition technique, wherein each of the particles have elongated structure and have a shape selected from the group consisting of helical, rod-like, disc and ellipsoid; b) evaporatively depositing an adhesive layer on the surface of micro or nanometer scale particles; and c) evaporatively depositing a ferromagnetic material on the adhesive layer.
[00061] In an embodiment, micro or nanometer scale particles can be fabricated using glancing angle deposition (GLAD) technique, and the deposition of adhesive layer and ferromagnetic material in steps (b) and (c) can be carried out by thermal or electron beam evaporation.
[00062] In an embodiment, the magnetic material can be bonded to the micro or nanometer scale particles using a using a technique preferably selected from chemical vapor deposition, physical vapor deposition and chemical functionalization.
[00063] In another embodiment, the adhesive layer can be deposited on the micro or nanometer scale particles using a technique preferably selected from sputtering, pulsed laser deposition, thermal evaporation and electron beam evaporation.
[00064] FIG. 1 shows scanning electron microscope (SEM) image of an exemplary nanofabricated helical probe in accordance with embodiments of the present disclosure. The nanofabricated helical probe can be fabricated using physical vapor deposition techniques such as glancing angle deposition (GLAD). The helical probe can be coated with a magnetic material such as cobalt or nickel and the thickness of the magnetic coating can range from 5nm to 100nm, preferably 40nm. A thin layer of silver or titanium can be coated on the helical probe prior to coating the same with magnetic material in order to facilitate adhesion of the magnetic material with the helical probe. The adhesion layer can have thickness in the range of from 0.5nm to 50nm, preferably from 5 to 10nm. The adhesion layer can be deposited on the helical probe using techniques such as thermal or electron beam evaporation. The nanofabricated helical probe can exhibit a variety of different dynamical configurations when dispersed in a fluid and subjected to uniform rotating magnetic field. Below a certain critical frequency of rotation of the magnetic field (O1), the probes can rotate about its own short axis in the plane of rotating magnetic field. At frequencies higher than O1, the probes may come out of the plane and start precessing about the direction of propulsion. As the frequency (OB) is increased to higher values, the precession angle can gradually decrease till it becomes almost zero, which corresponds to the helical motion.
[00065] The Euler equations for an elongated object such as rods, ellipsoids and helices, which has got two different rotational drag coefficients about its two axes (long and short), can be solved under the condition for Stokes' flow (Reynolds' number << 1). The following equations can be used to obtain the precession angle a and the critical frequency O1.

Where,
wherein, M and B can represent the permanent magnetic moment value of the object and the magnitude of the rotating magnetic field respectively. While ?s describes the rotational drag coefficient of the object about the short axis, ?m is the angle that the permanent magnetic moment makes with the short axis of the object. The schematic of propeller motion of a nanofabricated helical probe is shown in FIG. 2. ?s can be directly proportional to local viscosity (?) value of a medium. Thus, measurement of a can provide an estimate of local viscosity value when the other parameters are known. Further, a relative measurement of viscosity can also be obtained without the other parameter values using the below formula:

[00066] The externally controlled parameters viz. magnetic field and frequency can be useful for obtaining the relative viscosity values by measuring the precession angle. To measure the value of precession angle a, the helical probe motion can be captured using video microscopy with a high frame rate. The orientation of the nanofabricated helical probe obtained from 2D microscopy can be a sinusoidal function of time, where the amplitude of the sinusoids can give average precession angle over that period of time. FIG. 3 shows the plot of nanofabricated helical probe orientation as a function of time, when the helical probe precesses at a frequency of 6 Hz and the video is captured at 145 frames per second. As evident from the plot, the precession angle of the nanofabricated helical probe may be affected by thermal fluctuations, but the corresponding time scale of fluctuations can be much larger (~10 s) compared to the rotation time scale (~0.1 s or less) and hence there can be negligible effect of fluctuations on the measurement of precession angle a. Measurement of a over one or two cycles of rotation can thus be sufficient to obtain a fairly correct estimate of the local viscosity (or relative viscosity) values using the above equations. The extremely fast measurement time of ~ 0.1 s or less, which corresponds to a spatial resolution of around 0.1 µm (probe speed ~ 1 µm/s) can empower the present techniques with unprecedented spatial and temporal resolutions.
[00067] Referring to FIG. 4, there is shown an exemplary configuration of a microfluidic cell for obtaining rheological characteristics of heterogeneous fluids. The microfluidic cell can be constructed of glass slides and cover slips as shown in FIG. 4, and double sided sticky tapes can be used to make channels on the glass slide in such a way that the microfluidic cell forms boundaries for two fluids. The double sided sticky tapes can have thickness in the range of from 10 to 500 µm, preferably 100 µm. The cover slip can be disposed on top of sticky tapes in order to have closed cell configuration and thereby obtaining channels of desired thickness. In order to visually identify the boundaries of two fluids used, one of the two fluids can be colored using small quantity of coloring dye while the probes can be dispersed in the other fluid. The variation of viscosity values across the boundary can be measured by moving a probe from one fluid into the other fluid.
[00068] According to embodiments, the methods of the present disclosure which are used to measure elasticity can be extended to provide a large and fast rotation about the short axis (e.g. by an oscillating magnetic field) of an elongated probe and observe the dynamics as a function of time. FIGs. 11A-B illustrate two different levels of actuation; in the first one the probe can be made to rotate by half of a full rotation cycle and in the second one, the probe can be rotated by one fourth of a full rotation cycle. For a purely viscous medium at such low Reynolds numbers, the rise and fall of displacement, in response to such actuations can be instantaneous, while elastic effects can show up as slowing decaying/oscillating components in the temporal variation of the displacement. Thus, elastic properties of a fluid sample can be measured faster, in particular the elastic relaxation times with very high spatial resolution. Specifically, the probes of the present disclosure can facilitate rheological measurements in media with large viscosity as the torque required to rotate an elongated probe about its long axis is smaller than the torque required for rotation about the short axis.
[00069] According to embodiments, the elongated probe can be rotated about its short or long axis very quickly, and its angular trajectory can be followed as a function of time. Further, any slowly decaying or oscillatory component can be directly related to the elastic properties of the surrounding fluid.
[00070] In another aspect, the present disclosure provides a rheometer which can include at least one microrheological probe dispersed in a chamber filled with a fluid; at least one permanent magnet or electromagnetic coil to actuate the probe externally; an imaging system to capture dynamics and speed of the probe; and a means to receive and analyze the captured images from the imaging system.
[00071] According to embodiments of the present disclosure, the chamber of the rheometer can be a microfluidic chamber that can contain one or more microfluidic channels and the fluid inside the chamber can have a plurality of components. The microrheological probe can be controllably maneuvered in the chamber to obtain local rheological information. Further, the rheological properties of the fluid inside the chamber can vary as a function of time.
[00072] FIG. 12 illustrates schematic block diagram of an exemplary rheometer apparatus 100 in accordance with embodiments of the present disclosure. The main block 102 can be a microfluidic test chamber which can house a probe. A probe can be held in position using a magnetic/optical tweezer set up 104 that can provide gradient magnetic/optical forces. A probe can be actuated using magnetic coils 106. Upon injection of test fluid using the test fluid injection set up 108 into the microfluidic test chamber 100, a probe can be actuated and the dynamics/ speed of a probe can be captured using an imaging system 110 consisting of optical lenses and a camera. The image from the camera can be fed to an image analysis block 112 which can be a PC or an on board computing arrangement. Based on the analysis results, the final measurement values can be displayed on a monitor 114.
[00073] The micro or nanofabricated probe and the rheology measurement techniques using the same can be useful for measuring rheological characteristics of fluids during gelation and thereby enabling determination of gel point for determining the texture, spreadability, stability etc. of products in food processing, pharmaceuticals, polymer industries, cosmetics manufacturing etc. Further, the probes can facilitate real time monitoring of fluid flow rates in various processes. The methods of the present disclosure can be suitable for rheological measurements in systems where very quick changes in mechanical properties are encountered, like that in paints, cosmetics, polymerization reactions induced by temperature change, light and chemically, like that are found in photo resists. Also, fast measurement of viscosity of fluids using the present methods can facilitate high throughput measurements required in industrial process flows.
[00074] The methods of the present disclosure can be suitable for measuring rheological properties of fluids inside a microfluidic cell which in turn can be useful for measuring local temperature changes occurring due to microheating inside microfluidic devices by knowing the viscosity changes of the fluid in question, due to changes in temperature. Similarly, the probes can be useful to measure any physical parameter that causes local changes in viscosity of a test fluid. Further, the method can be used to measure dynamic changes of rheological properties in various biological systems such as inside of a cell. The method can also be used for diagnosis of diseases by measuring changes in rheological properties of a biological test fluid such as blood, serum, mucus, etc.
[00075] The rheology measurement techniques using the micro or nanofabricated probe require very small amount of test fluids and thus can particularly be suitable for fluids which are rare and difficult to obtain like proteins for example. The micro or nanofabricated probe of the present disclosure can be actuated by small, non invasive magnetic fields and hence it can be used for in vivo characterization of bodily fluids/tissues or disease diagnosis.

EXAMPLES
[00076] The present disclosure is further explained with help of following examples. However, it is to be understood that the foregoing examples are merely illustrative and are not to be taken as limitations upon the scope of the invention. Various changes and modifications to the disclosed embodiments will be apparent to those skilled in the art. Such changes and modifications may be made without departing from the scope of the invention.
Example 1: Measurement of viscosity using nanofabricated helical probe of the present disclosure: De-ionized (DI) water and aqueous dextrose solution (~ 40 % w/w or 50 % w/w) were used as fluids in the viscosity measurement experiment. Specifically, in this experiment de-ionized (DI) water and aqueous dextrose solution (~ 40 % w/w) were used. While water had a viscosity of 0.89 cP at room temperature, bulk rheology (Anton Paar rheometer; cone & plate rotation) provided a value of around 8.7 cP and 10 cP for the purely viscous dextrose solution and glycerol solution. In order to avoid sticking of the nanofabricated helical probes to the glass slides and cover slips which were used to make the microfluidic chambers, the glass surfaces were coated with protein Bovine Serum Albumin (BSA), dissolved in DI water (1mg/ml). As shown is FIG. 4, thin (100 µm) double sided sticky tapes were used to make the channels on the glass slide. On top of the tapes the cover slip was placed to have a closed cell resulting in channel thicknesses of around 100 µm. This ensured low Reynolds number flow conditions under which fluids do not mix easily. In order to visually identify the boundary, one of the two fluids used was colored using a small quantity of methylene blue dye and the nanofabricated helical probe was dispersed in the other solution.
[00077] Two different experiments were carried out using the microfluidic devices. The first experimental scheme is shown in FIGs. 5A-C which consisted of moving the helical probe from one fluid into another fluid and measuring the variation of the viscosity values across the boundary. FIGs. 5A-C schematically depicting the measurement of viscosity across the boundaries of two fluids viz. water and aqueous dextrose solution.
[00078] The nanofabricated helical probe was moved from DI water into dextrose/glycerol solution and measurements of the relative viscosity values were obtained as the probe moved across the boundary. The results are shown in FIGs. 6A and 6B.
[00079] In another experiment, temporal evolution of viscosity of two fluids was measured at the stage the two fluids began to mix across the boundary. Equal volumes of de-ionized (DI) water and aqueous dextrose solution (~ 40 % w/w) were put inside the microfluidic chamber. The nanofabricated probe was kept moving inside the DI water very close to the boundary. As shown in FIGs. 7A and 7B, the two fluids mixed over a time scale of 10 to 15 minutes, and the change in relative viscosity values throughout the mixing process was monitored.
[00080] In order to make sure that the measured values were correct, the inventors carried out the measurements of viscosity using an alternate scheme in which the diffusion of the miscroscale propellers were captured using video microscopy at the same points at which the measurements were stopped into the solution of higher viscosity. The diffusion experiments were also carried out at the first point at which the measurement was started across the boundary. Thus the ratios of the diffusion coefficients (also represented by star symbols in FIGs. 6A and 6B, and FIGs. 7A and 7B) at the end and start points give an estimate of the local relative viscosity at the end point. The values obtained were found to be in very close agreement to the values measured by the propellers using precession angle measurement.
Example 2: Measurement of elasticity using nanofabricated helical probe of the present disclosure: Viscoelastic gel methyl cellulose was used in experiments to evaluate elasticity measuring capabilities of the probe of the present disclosure. Microfluidic cells as used in viscosity measurement experiments were used and the nanofabricated helical probe was moved in propulsion mode from water, on one side of the cell to methyl cellulose of various concentrations on the other side of the cell. A SEM image of the nanofabricated helical probe used in the elasticity measurement experiments is shown in FIG. 8. The nanofabricated helical probe used in the elasticity measurement was smaller than the probes used for viscosity measurements, which reduced the effect of adjacent wall on the probe speed, because of increased diffusion in the vertical direction.
[00081] Aqueous methyl cellulose solutions were prepared by the standard methods known in the art. The viscoelastic nature of the methyl cellulose solutions were confirmed by conventional passive microrheology in which 1 µm beads were kept dispersed in the solution and their thermal fluctuations measured. The thermal fluctuations gave a measure of the storage modulus G’(?) and loss modulus G’’(?) as well as the viscosity ? using Cox Merz rule.
[00082] The hydrodynamic pitch of the nanofabricated helical probe was described as the distance the probe moved along its helical axis for one complete rotation. It was measured by measuring the velocity of propulsion vp and dividing the same by the frequency of rotation OB. p denoted the pitch of the probe in viscoelastic methyl cellulose and p0, the pitch of the same probe in purely viscous medium water. FIG. 9 is a graph showing ratio of effective pitch of a nanofabricated helical probe as it is moved across the boundary from water to methyl cellulose of concentration 0.5% w/w. In the same experiment, the probe was intermittently kept precessing which helped to provide an estimate of the local viscosity as well. Both the viscosity and the pitch ratio obtained are plotted in FIG. 9 as the probe was moved from DI water into the methyl cellulose solution. It was observed that while the measured viscosity slowly increased to the final value of the fluid at the other end of the microfluidic chamber, the pitch decreased by almost 30% till some distance and then rose again to values almost similar to the pitch value in water.
[00083] The variation of the effective pitch as a function of concentration of methyl cellulose was obtained from the above experiment and plotted in FIG. 10. These experiments show that the speed (pitch) of the helical probe can be used as a measure of the elasticity of the media. Thus the speed of the probe was found to be very sensitive to the elasticity of the surrounding media and even the addition of extremely low concentrations of methyl cellulose could reduce the speed of the probe by a large amount which could be easily measured.
,CLAIMS:1. A microrheological probe comprising micro or nanometer scale particles having an elongated structure and comprising a magnetic material.

2. The microrheological probe according to claim 1, wherein the micro or nanometer scale particles have a shape selected from the group consisting of helical, rod-like, disc and ellipsoidal.

3. The microrheological probe according to claim 1, wherein the micro or nanometer scale particles have a length ranging from 100 nm to 100 µm.

4. The microrheological probe according to claim 1, wherein the magnetic material exhibits ferromagnetic characteristics.

5. The microrheological probe according to claim 1, wherein the magnetic material is selected from Iron, Cobalt, Neodymium, Samarium, Nickel, and alloys thereof.

6. The microrheological probe according to claim 1, wherein the micro or nanometer scale particles are made of a magnetic material.

7. The microrheological probe according to claim 1, wherein the micro or nanometer scale particles are made of a non-magnetic material with a magnetic material coated on their surface.

8. The microrheological probe according to claim 7, wherein thickness of the magnetic material based coating is ranging from 1 nm to 100 µm.

9. The microrheological probe according to claim 7, wherein the probe comprises an adhesive layer between a surface of the micro or nanometer scale particles and the magnetic material based coating.

10. The microrheological probe according to claim 9, wherein the adhesive layer has a thickness ranging from 0.1 nm to 10 µm.

11. The microrheological probe according to claim 1, wherein the probe is trackable.

12. The microrheological probe according to claim 1, wherein the probe has ability of being torqued or translated externally.

13. The microrheological probe according to claim 1, wherein the probe exhibits a variety of dynamic configurations when dispersed in a fluid and subjected to a rotating or oscillating magnetic field.

14. The microrheological probe according to claim 1, wherein the probe is controllably translated when dispersed in a fluid and subjected to a rotating or oscillating magnetic field.

15. A method for fabricating a microrheological probe comprising the steps of:
a) providing micro or nanometer scale particles using physical vapor deposition technique, wherein each of the particles have elongated structure and have a shape selected from the group consisting of helical, rod-like, disc and ellipsoidal;
b) evaporatively depositing an adhesive layer on the surface of micro or nanometer scale particles;
c) evaporatively depositing a magnetic material on the adhesive layer.
16. The method according to claim 15, where the magnetic material is attached to the micro and nanometer scale particles using chemical vapor deposition.

17. A method according to claim 15, where the magnetic material is attached to the micro and nanometer scale particles using chemical functionalization.

18. A method according to claim 15, where the micro and nanometer size particles are made of a magnetic material deposited using a physical vapor deposition technique.

19. The method according to claim 15, wherein the micro or nanometer scale particles are fabricated using glancing angle deposition (GLAD) technique.

20. The method according to claim 15, wherein deposition of the adhesive layer and magnetic material in steps (b) and (c) is carried out by any or a combination of sputtering, pulsed laser deposition, thermal evaporation or electron beam evaporation.

21. A rheometer comprising:
at least one microrheological probe dispersed in a chamber filled with a fluid;
at least one permanent magnet or electromagnetic coil to actuate the probe externally;
an imaging system to capture dynamics and speed of the probe; and
a means to receive and analyze captured images from the imaging system.

22. The rheometer according to claim 21, wherein the chamber is a microfluidic chamber having one or more microfluidic channels.

23. The rheometer according to claim 21, wherein the fluid inside the chamber has a plurality of components, and wherein the at least one microrheological probe is controllably maneuvered in the chamber to obtain local rheological information.

24. The rheometer according to claim 21, wherein the rheological properties of the fluid inside the chamber varies as a function of time.