Capacitance Detection in a Droplet Actuator

• US 20100194408A1
• Filed: 02/15/2008
• Published: 08/05/2010
• Est. Priority Date: 02/15/2007
• Status: Active Grant

• Current Assignee: Advanced Liquid Logic, Inc. (Illumina Incorporated)
• Original Assignee: Advanced Liquid Logic, Inc. (Illumina Incorporated)
• Inventors: Paik, Philip Y., Pamula, Vamsee K., Srinivasan, Vijay, Sturmer, Ryan A., Pollack, Michael G.
• Granted Patent: US 8,872,527 B2
• Time in Patent Office: Days
• US Class Current: 324/664
• CPC Class Codes: B01L 2200/0605 Metering of fluids
• CPC Class Codes: B01L 2300/0627 Sensor or part of a sensor is integrated
• CPC Class Codes: B01L 2300/0636 Integrated biosensor, microarrays
• CPC Class Codes: B01L 2300/0645 Electrodes
• CPC Class Codes: B01L 2300/0809 rectangular shaped
• CPC Class Codes: B01L 2400/0427 Electrowetting
• CPC Class Codes: B01L 3/502715 characterised by interfacing components, e.g. fluidic, electrical, optical or mechanical interfaces
• CPC Class Codes: B01L 3/50273 characterised by the means or forces applied to move the fluids micropumps F04B19/006, of the membrane type F04B43/043
• CPC Class Codes: B01L 3/502784 specially adapted for droplet or plug flow, e.g. digital microfluidics automatic analysis using a stream of discrete samples in a tube system G01N35/08
• CPC Class Codes: B01L 3/502792 for moving individual droplets on a plate, e.g. by locally altering surface tension
• CPC Class Codes: G01N 2203/00 Investigating strength properties of solid materials by application of mechanical stress
• CPC Class Codes: G01N 27/22 by investigating capacitance
• CPC Class Codes: G01N 27/223 for determining moisture content, e.g. humidity rain detectors on vehicle windows B60S1/0825
• CPC Class Codes: G01N 27/227 Sensors changing capacitance upon adsorption or absorption of fluid components, e.g. electrolyte-insulator-semiconductor sensors, MOS capacitors G01N27/225 takes precedence
• CPC Class Codes: G01V 3/02 operating with propagation of electric current

Abstract

A method, circuit and apparatus for detecting capacitance on a droplet actuator, inter alia, for determining the presence, partial presence or absence of a droplet at an electrode on a droplet actuator by: (a) providing a droplet actuator comprising: (i) a substrate comprising electrodes arranged on the substrate for conducting droplet operations on a surface of the substrate; (ii) a capacitance detection circuit for detecting capacitance at the droplet operations surface at one or more of the electrodes; (b) detecting capacitance at the droplet operations surface at one or more of the electrodes; and (c) determining from the capacitance the presence, partial presence or absence of a droplet at the droplet operations surface at the electrode.

Claims

1. A method of determining the presence, partial presence or absence of a droplet at an electrode on a droplet actuator, the method comprising:

(a) providing a droplet actuator comprising;

(i) a substrate comprising electrodes arranged on the substrate for conducting droplet operations on a surface of the substrate;

(ii) a capacitance detection circuit for detecting capacitance at the droplet operations surface at one or more of the electrodes;

(b) detecting capacitance at the droplet operations surface at one or more of the electrodes; and

(c) determining from the capacitance the presence, partial presence or absence of a droplet at the droplet operations surface at the electrode.

2. The method of claim 1 wherein the electrodes arranged on the substrate for conducting droplet operations are configured for mediating droplet operations by a means comprising electrowetting.

3. The method of claim 1 wherein the electrodes arranged on the substrate for conducting droplet operations are not configured for mediating droplet operations by a means comprising dielectrophoresis.

4. The method of claim 2 comprising determining in real time the absence, presence, partial presence, and/or location of a droplet along a path of transport electrodes.

5. The method of claim 4 wherein the absence, presence, partial presence, and/or location of the droplet along the path of transport electrodes is determined without visualization of the droplet'"'"'s location.

6. The method of claim 2 comprising:

(a) determining among several electrodes which electrode has maximum capacitance relative to the others of the several electrodes; and

(b) concluding based on such determination that the droplet is at the electrode having the maximum capacitance.

7. The method of claim 6 further comprising conducting a subsequent droplet operation on the same or a different droplet based on the determined location of the droplet.

8. A method of transporting a droplet on a droplet actuator, the method comprising:

(a) conducting one or more droplet operations to transport a droplet on the droplet actuator; and

(b) executing the method of claim 1 to determine whether the droplet has been transported.

9. A method of identifying a faulty electrode on a droplet actuator, the method comprising:

(a) attempting a droplet transport operation and using the method of claim 2 to determine whether the droplet transport operation is successful; and

(b) where the droplet operation is not successful at a particular electrode, identifying the electrode as faulty.

10. A method of re-routing a droplet around a faulty electrode on a droplet actuator, the method comprising:

(a) identifying a faulty electrode according to the method of claim 9; and

(b) re-routing the droplet around the faulty electrode.

11. The method of claim 10 wherein the re-routing is controlled by a controller or processor.

12. The method of claim 10 further comprising re-routing a subsequent droplet around the target electrode.

13. A method of providing a sub-droplet from a droplet on a droplet actuator, the method comprising:

(a) providing a droplet actuator comprising;

(i) a substrate comprising electrodes arranged on the substrate for forming the sub-droplet from the droplet on a surface of the substrate; and

(ii) a capacitance detection circuit for detecting capacitance at the droplet operations surface at one or more of the electrodes;

(b) activating one or more electrodes to elongate some portion or all of the droplet while using the capacitance detection circuit to determine the location of one or more portions of the droplet; and

(c) when the location of one or more portions of the droplet reaches a predetermined position as determined by the capacitance detection circuit, deactivating an intermediate one of the one or more electrodes along the elongated droplet to form the sub-droplet.

14. A method of determining whether a droplet comprises one or more beads, the method comprising:

(a) providing a droplet on an electrode on a droplet actuator;

(b) determining capacitance at the electrode;

(c) comparing the capacitance with an expected capacitance in the presence of one or more beads; and

(d) determining whether one or more beads is present.

15. The method of claim 14 wherein the step of providing a droplet on an electrode on a droplet actuator is mediated by a means comprising electrowetting.

16. The method of claim 14 wherein the step of providing a droplet on an electrode on a droplet actuator is not mediated by a means comprising dielectrophoresis.

17. The method of claim 14 wherein the determination:

(a) follows a droplet splitting operation in a bead washing protocol; and

(b) identifies whether bead loss has occurred during the splitting operation.

18. A method of determining whether a droplet comprises one or more biological cells, the method comprising:

(a) providing a droplet on an electrode on a droplet actuator;

(b) determining capacitance at the electrode;

(c) comparing the capacitance with an expected capacitance in the presence of one or more cells; and

(d) determining whether one or more cells is present.

19. The method of claim 18 wherein the step of providing a droplet on an electrode on a droplet actuator is mediated by a means comprising electrowetting.

20. The method of claim 18 wherein the step of providing a droplet on an electrode on a droplet actuator is not mediated by a means comprising dielectrophoresis.

21. A method of quantifying or estimating the quantity of beads in a droplet, the method comprising:

(a) providing a droplet on an electrode on a droplet actuator;

(b) determining capacitance at the electrode;

(c) comparing the capacitance with a curve for expected capacitance relative to number of beads; and

(d) quantifying or estimating the quantity of beads in the droplet.

22. The method of claim 21 wherein the step of providing a droplet on an electrode on a droplet actuator is mediated by a means comprising electrowetting.

23. The method of claim 21 wherein the step of providing a droplet on an electrode on a droplet actuator is not mediated by a means comprising dielectrophoresis.

24. A method of quantifying or estimating cells in a droplet, the method comprising:

(a) providing a droplet on an electrode on a droplet actuator;

(b) determining capacitance at the electrode;

(c) comparing the capacitance with a curve for expected capacitance relative to number of cells; and

(d) quantifying or estimating cells in the droplet.

25. The method of claim 24 wherein the step of providing a droplet on an electrode on a droplet actuator is mediated by a means comprising electrowetting.

26. The method of claim 24 wherein the step of providing a droplet on an electrode on a droplet actuator is not mediated by a means comprising dielectrophoresis.

27. A method of assessing viability of cells in a droplet, the method comprising:

(a) providing a droplet on an electrode on a droplet actuator;

(b) determining capacitance at the electrode;

(c) comparing the capacitance expected capacitance in the presence of viable cells and/or non-viable cells; and

(d) assessing viability cells in the droplet.

28. The method of claim 27 wherein the step of providing a droplet on an electrode on a droplet actuator is mediated by a means comprising electrowetting.

29. The method of claim 27 wherein the step of providing a droplet on an electrode on a droplet actuator is not mediated by a means comprising dielectrophoresis.

30. A method of assessing apoptosis of cells in a droplet, the method comprising:

(a) providing a droplet on an electrode on a droplet actuator;

(b) determining capacitance at the electrode;

(c) comparing the capacitance expected in the presence of cells undergoing apoptosis and/or cells not undergoing apoptosis; and

(d) assessing apoptosis of cells in the droplet.

31. The method of claim 30 wherein the step of providing a droplet on an electrode on a droplet actuator is mediated by a means comprising electrowetting.

32. The method of claim 30 wherein the step of providing a droplet on an electrode on a droplet actuator is not mediated by a means comprising dielectrophoresis.

33. A method of identifying a bead type in a droplet, the method comprising:

(a) providing a droplet on a droplet actuator comprising multiple bead types, each bead type having a different capacitance;

(b) dispensing from the droplet onto an electrode a sub-droplet comprising one or a few beads;

(c) determining capacitance at the electrode;

(d) comparing capacitance at the electrode to expected capacitance in the presence some or all of the bead types; and

(e) identifying the bead type.

34. The method of claim 33 wherein the step of providing a droplet on an electrode on a droplet actuator is mediated by a means comprising electrowetting.

35. The method of claim 33 wherein the step of providing a droplet on an electrode on a droplet actuator is not mediated by a means comprising dielectrophoresis.

36. The method of claim 33 wherein the step of dispensing from the droplet onto an electrode a sub-droplet comprising one or a few beads is mediated by a means comprising electrowetting.

37. The method of claim 33 wherein the step of dispensing from the droplet onto an electrode a sub-droplet comprising one or a few beads is not mediated by a means comprising dielectrophoresis.

38. A method of identifying a bead type in a droplet, the method comprising:

(a) providing on an electrode on a dioplet actuator a droplet comprising a bead of a specific unknown type and having a unique capacitance;

(b) determining capacitance at the electrode;

(c) comparing capacitance at the electrode to expected capacitance in the presence one or more bead types; and

(d) identifying the bead type.

39. The method of claim 38 wherein the step of providing a droplet on an electrode on a droplet actuator is mediated by a means comprising electrowetting.

40. The method of claim 38 wherein the step of providing a droplet on an electrode on a droplet actuator is not mediated by a means comprising dielectrophoresis.

41. A method of providing a sub-droplet from a droplet on a droplet actuator, the method comprising determining the location of one or more portions of the droplet during a splitting or dispensing droplet operation by detecting capacitance at one or more electrodes on the droplet actuator and adjusting one or more parameters of the splitting or dispensing operation to influence the volume of the sub-droplet.

42. The method of claim 41 wherein the parameter is selected from the group consisting of reservoir volume, electrode bias voltage, electrode bias time, and any combinations thereof.

43. The method of claim 41 wherein the splitting or dispensing droplet operation is mediated by a means comprising electrowetting.

44. The method of claim 41 wherein the splitting or dispensing droplet operation is not mediated by a means comprising dielectrophoresis.

45. A method of determining the presence of air bubbles in a droplet actuator, the method comprising:

(a) attempting to fill some portion of the droplet actuator with a filler fluid to provide a filled portion of the droplet actuator;

(b) testing capacitance at electrodes in the filled portion of the droplet actuator; and

(c) identifying the presence of one or more air bubbles based on a capacitance at one or more of the electrodes that differs from the expected capacitance at the electrode in the presence of the filler fluid.

46. The method of claim 45 further comprising reloading the portion of the droplet actuator with the filler fluid.

47. The method of claim 45 wherein the electrodes are configured for mediating droplet operations by a means comprising electrowetting.

48. The method of claim 45 wherein the electrodes are configured for mediating droplet operations by a means not comprising dielectrophoresis.

49. A method of testing electrodes on a droplet actuator, the method comprising:

(a) filling at least a portion of the droplet actuator with a fluid having a known capacitance to provide a filled portion of the droplet actuator;

(b) testing capacitance at electrodes in the filled portion of the droplet actuator; and

(c) identifying faulty electrodes based on a capacitance that differs from the expected capacitance at the electrode in the presence of the fluid.

50. The method of claim 49 wherein the electrodes are configured for mediating droplet operations by a means comprising electrowetting.

51. The method of claim 49 wherein the electrodes are configured for mediating droplet operations by a means not comprising dielectrophoresis.

52. A droplet actuator for manipulating a fluid using an electrical field, comprising:

(a) an actuator electrode configured to communicate the electrical field to the fluid;

(b) a reference electrode in communication with the actuator electrode and in selective communication with at least one of a ground node, a high voltage node and a high impedance node having an intermediate potential;

(c) a capacitance detection circuit in communication with the reference electrode and configured to detect a capacitance of the fluid; and

(d) a processor in communication with the capacitor detection circuit and configured to determine a position of the fluid based on the detected capacitance of the fluid.

53. The droplet actuator of claim 52 wherein the actuator electrode is configured for mediating droplet operations by a means comprising electrowetting.

54. The droplet actuator of claim 52 wherein the actuator electrode is configured for mediating droplet operations by a means not comprising dielectrophoresis.

55. The droplet actuator of claim 52 further comprising a switch configured to selectively enable communication between the actuator and reference electrodes.

56. The droplet actuator of claim 52 further comprising a plurality of actuator nodes in communication with the reference electrode.

57. The droplet actuator of claim 56 wherein the plurality of actuator nodes sequentially communicate with the reference electrode.

58. The droplet actuator of claim 52 further comprising a voltage source in communication with the actuator electrode.

59. The droplet actuator of claim 58 further comprising a switch configured to enable selective communication between the voltage source and the actuator electrode.

60. The droplet actuator of claim 52 wherein the reference electrode further comprises another actuator electrode.

61. The droplet actuator of claim 52 further comprising a switch configured to selectively enable communication between the actuator electrode and at least one of the ground, high voltage and high impedance nodes.

62. The droplet actuator of claim 52 wherein the processor is further configured to conduct droplet processes while the actuator electrode is in communication with the ground node.

63. The droplet actuator of claim 52 wherein the processor is further configured to conduct droplet detection processes while the actuator electrode is in communication with the high impedance node.

64. The droplet actuator of claim 52 wherein the processor is further configured to enable communication between the actuator electrode and the voltage source while the actuator electrode is in communication with the high impedance node.

65. The droplet actuator of claim 52 wherein the capacitance detection circuit further comprises a protection circuit configured to regulate a voltage entering the capacitance detection circuit to minimize damage caused by a high voltage condition.

66. The droplet actuator of claim 65 wherein the capacitance detection circuit comprises at least one of a voltage divider and a capacitor.

67. The droplet actuator of claim 52 wherein the capacitance detection circuit comprises at least one of an operational amplifier, an integrating amplifier and an analog-to-digital converter.

68. The droplet actuator of claim 52 wherein the processor is further configured to determine that the fluid is not present at the actuator electrode in response to receiving an electrical value associated with the detected capacitance.

69. The droplet actuator of claim 52 wherein the processor is further configured to determine that the fluid is present at the actuator electrode in response to receiving an electrical value associated with the detected capacitance.

70. The droplet actuator of claim 52 wherein the fluid comprises a droplet.

71. The droplet actuator of claim 52 wherein the fluid comprises a droplet comprising beads.

72. The droplet actuator of claim 52 wherein the fluid comprises a droplet comprising cells.

73. The droplet actuator of claim 52 wherein the fluid comprises a filler fluid.

74. The droplet actuator of claim 52 wherein the fluid comprises a filler fluid comprising an oil.

75. The droplet actuator of claim 52 further comprising first and second substrates separated by the fluid, wherein the first substrate comprises the actuator electrode and the second substrate comprises the reference electrode.

76. The droplet actuator of claim 75 wherein the fluid comprises a droplet and the droplet is located over the actuator electrode.

77. The droplet actuator of claim 76 wherein the processor is further configured to initiate a measurement of an amount by which a portion of the droplet overlaps the actuator electrode.

78. The droplet actuator of claim 75 wherein the first substrate comprises a plurality of actuator electrodes and each actuator electrode is equidistant from the reference electrode.

79. The droplet actuator of claim 78 wherein the first substrate comprises a grid or array of actuator electrodes and the second substrate comprises a single reference electrode that is substantially equidistant from each of the actuator electrodes, wherein the measurement at each of the actuator electrodes in the grid or array may be determined based on the single reference electrode.

80. The droplet actuator of claim 78 wherein the first substrate comprises a grid or array of actuator electrodes and the detected capacitance measurement is based upon two of more of the actuator electrodes.

81. A method of determining fluid droplet overlap comprising:

(a) providing a droplet actuator comprising;

(i) an actuator electrode configured to communicate the electrical field to the fluid;

(ii) a reference electrode in communication with the actuator electrode and in selective communication with at least one of a ground node, a high voltage node and a high impedance node having an intermediate potential;

(iii) a capacitance detection circuit in communication with the reference electrode and configured to detect a capacitance of the fluid; and

(iv) a processor in communication with the capacitor detection circuit and configured to determine a position of the fluid based on the detected capacitance of the fluid;

(b) locating a fluid droplet over the actuator electrode; and

(c) initiating a processor measurement of an amount by which a portion of the droplet overlaps the actuator electrode.

82. A method of detecting fluid within a droplet actuator having a plurality of actuator electrodes each configured to communicate an electric field to the fluid, the method comprising:

(a) sensing a capacitance of the fluid at an actuator electrode of the plurality of actuator electrodes; and

(b) determining that the fluid is present at the actuator electrode using the sensed capacitance.

83. The method of claim 82 wherein the actuator electrode is configured for mediating droplet operations by a means comprising electrowetting.

84. The method of claim 82 wherein the actuator electrode is configured for mediating droplet operations by a means not comprising dielectrophoresis.

85. The method of claim 82 wherein sensing the capacitance further comprises selectively enabling communication between the actuator electrode and a reference electrode.

86. The method of claim 85 wherein sensing the capacitance further comprises selectively enabling communication between the reference electrode and at least one of a ground node, a high voltage node and a high impedance node having an intermediate potential.

87. The method of claim 82 wherein sensing the capacitance further comprises selectively enabling communication between the actuator and a voltage source.

88. The method of claim 85 or 86 wherein sensing the capacitance further comprises concurrently enabling communication between the reference electrode and the high impedance node, and between the actuator electrode and the voltage source.

89. The method of claim 85 wherein determining that the fluid is present further comprises sequentially sensing a capacitance at each of the plurality of electrodes.

90. The method of claim 85 wherein sensing the capacitance further comprises sensing the capacitance while the reference electrode is in communication with the high impedance node.

91. The method of claim 82 further comprising regulating a voltage entering a capacitance detection circuit of the droplet actuator to minimize damage caused by a high voltage condition.

92. The droplet actuator of claim 82 wherein the fluid comprises a droplet.

93. The droplet actuator of claim 82 wherein the fluid comprises a droplet comprising beads.

94. The droplet actuator of claim 82 wherein the fluid comprises a droplet comprising cells.

95. The droplet actuator of claim 82 wherein the fluid comprises a filler fluid.

96. The droplet actuator of claim 82 wherein the fluid comprises a filler fluid comprising an oil.

97. A capacitance detection circuit for detecting a capacitance of fluid present in a droplet actuator, comprising:

(a) an amplifier in communication with an electrode of the droplet actuator;

(b) an analog-to-digital converter in communication with the amplifier and a processor configured to determine a position of the fluid based on the detected capacitance of the fluid; and

(c) a protection circuit configured to regulate a voltage entering the capacitance detection circuit to minimize damage caused by a high voltage condition.

98. The capacitance detection circuit of claim 97 wherein the electrode of the droplet actuator is configured for mediating droplet operations by a means comprising electrowetting.

99. The capacitance detection circuit of claim 97 wherein the electrode of the droplet actuator is configured for mediating droplet operations by a means not comprising dielectrophoresis.

100. A method of determining the presence, partial presence or absence of a droplet at an electrode on a droplet actuator, the method comprising:

(a) providing a droplet actuator comprising;

(i) a substrate comprising droplet operations electrodes arranged on the substrate for conducting droplet operations on a surface of the substrate; and

(ii) an impedance detection circuit coupled to;

(1) one or more of the droplet operations electrodes; and

(2) a second electrode and arranged for detecting capacitance at the droplet operations surface at one or more of the electrodes;

(b) detecting impedance at the droplet operations surface at one or more of the electrodes; and

(c) determining from the impedance the presence, partial presence or absence of a droplet at the droplet operations surface at the electrode.

101. The method of claim 100 wherein the impedance detection circuit comprises a capacitance detection circuit.

102. The method of claim 100 wherein the impedance detection circuit comprises a resistance detection circuit.

103. The method of claim 100 wherein:

(a) at least a subset of the droplet operations electrodes comprises two or more of the droplet operations electrodes each coupled to an impedance detection circuit; and

(b) all of such impedance detection circuits are coupled to a common second electrode.

104. The method of claim 100 wherein:

(a) the droplet;

(i) has a location on the substrate; and

(ii) overlaps multiple droplet operations electrodes; and

(b) the method further comprises determining the location of the droplet by measuring the overlap of the droplet with each of the multiple droplet operations electrodes by measuring impedance at each of the multiple droplet operations electrodes.

105. The method of claim 100 wherein:

(a) the droplet;

(i) has a location on the substrate; and

(ii) overlaps multiple droplet operations electrodes; and

(b) the method further comprises;

(i) determining the location of the droplet by measuring the overlap of the droplet with each of the multiple droplet operations electrodes by measuring impedance at each of the multiple droplet operations electrodes; and

(ii) based on the overlap of the droplet with each of the multiple droplet operations electrodes, determining the volume of the droplet.

106. The method of claim 100 wherein:

(a) the droplet is smaller than an operations electrode; and

(b) the method comprises determining the size of the droplet based on the capacitance.

107. The method of claim 100 wherein the impedance detection circuit is coupled to electrodes in a substantially common plane.

108. A method of determining the velocity of deformation of a droplet, the method comprising detecting a change in impedance at a locus of a droplet actuator during a droplet operation that results in deformation of the droplet, where the locus is in impedance detecting proximity to the droplet.

109. The method of claim 108 wherein the droplet operation comprises a droplet transport operation.

110. A method of enhancing droplet transport rate on a droplet actuator, the method comprising:

(a) determining the velocity of droplet transport according to the method of claim 109; and

(b) adjusting droplet transport electrode switching rate and/or voltage of droplet transport electrodes to suit the droplet transport velocity.

111. The method of claim 110 comprising enhancing the droplet transport rate of two or more droplets on a single droplet actuator, the two or more droplets comprising at least two droplets each having different properties, and the method comprising independently adjusting droplet transport electrode switching rate and/or voltage of droplet transport electrodes to yield a suitable droplet transport velocity of each droplet.

112. A method of transporting a droplet onto a droplet transport electrode from an adjacent electrode on a droplet operations surface of a droplet actuator, the method comprising:

(a) monitoring impedance at the droplet transport electrode as a droplet is being transported on the droplet operations surface onto the adjacent electrode; and

(b) activating the droplet transport electrode when the impedance at the droplet transport electrode indicates that the droplet is in sufficient proximity with the droplet operations electrode to result in transport of the droplet from the adjacent electrode upon activation of the droplet transport electrode and deactivation of the adjacent electrode.

113. A method of transporting a droplet on a droplet operations surface of a droplet actuator, wherein:

(a) the droplet actuator comprises;

(i) a first electrode;

(ii) a second electrode adjacent to the first electrode; and

(iii) a third electrode adjacent to the second electrode; and

(b) the method comprises;

(i) monitoring impedance at the first electrode, the second electrode and/or the third electrode during a droplet operation comprising activating the second electrode to transport a droplet from the first electrode to the second electrode; and

(ii) sequentially or simultaneously activating the third electrode and deactivating the second electrode when at least one of the following conditions is satisfied;

(1) impedance at the first electrode indicates that the droplet is sufficiently on the second electrode to result in transport of the droplet from the second electrode to the third electrode upon activation of the third electrode and deactivation of the second electrode;

(2) impedance at the second electrode indicates that the droplet is sufficiently on the second electrode to result in transport of the droplet from the second electrode to the third electrode upon activation of the third electrode and deactivation of the second electrode; and

(3) impedance at the third electrode indicates that the droplet is sufficiently on the second electrode to result in transport of the droplet from the second electrode to the third electrode upon activation of the third electrode and deactivation of the second electrode.

114. A method of quality control for a droplet actuator, the method comprising:

(a) transporting a droplet on a droplet operations surface of the droplet actuator and determining velocity of transport across and/or onto one or more of the droplet operations electrodes; and

(b) making a quality determination based on deviations in transport velocity from a predetermined standard.

115. A method of determining a property of a droplet, the method comprising determining the velocity of deformation of a droplet during a droplet operation on a droplet actuator correlating the velocity with a property of the droplet.

116. The method of claim 115 wherein the property comprises viscosity.

117. The method of claim 115 wherein the property comprises surface tension.

118. The method of claim 115 wherein the droplet operation comprises a droplet transport operation wherein the droplet is transported from one electrode to another electrode.

119. A method of controlling droplet location on a droplet actuator, the method comprising determining a droplet footprint of the droplet on a droplet operations surface of the droplet actuator and activating and/or deactivating one or more electrodes of the droplet actuator in order reposition the droplet.

120. The method of claim 119 wherein the repositioning of the droplet improves droplet location relative to sensing by a sensor.

121. A method of determining the location of a droplet on a droplet actuator, the method comprising:

(a) providing a droplet actuator comprising;

(i) a substrate comprising droplet operations electrodes arranged on the substrate for conducting droplet operations on a surface of the substrate and one or more droplets arranged on one or more of the droplet operations electrodes; and

(ii) an impedance detection circuit coupled to;

(1) one or more of the droplet operations electrodes; and

/or

(2) one or more impedance detection electrodes; and

(b) detecting, using one or more of the impedance detection circuits, the location of a droplet on the droplet operations electrodes.

122. The method of claim 121 wherein the droplet operations electrodes are configured to mediate droplet operations by an electrowetting effect.