diff --git a/pylabrobot/liquid_handling/standard.py b/pylabrobot/liquid_handling/standard.py
index bc0e9580a50..fdc057b7c94 100644
--- a/pylabrobot/liquid_handling/standard.py
+++ b/pylabrobot/liquid_handling/standard.py
@@ -74,9 +74,20 @@ class SingleChannelDispense:
 
 @dataclass(frozen=True)
 class Mix:
+  """Repeated aspirate/dispense cycles to mix the liquid during a transfer.
+
+  Args:
+    volume: The volume drawn then expelled each cycle.
+    repetitions: The number of aspirate/dispense cycles.
+    flow_rate: The flow rate of the mix.
+    surface_following_distance: The distance (mm) the tip follows the liquid surface each cycle on
+      backends that support it (e.g. Hamilton STAR); others ignore it.
+  """
+
   volume: float
   repetitions: int
   flow_rate: float
+  surface_following_distance: Optional[float] = None
 
 
 @dataclass(frozen=True)